Designing a 3D printed enclosure for my KiCad project in Blender

I recently built a small 6502-based computer on a custom PCB, which I designed in KiCad. This blog post is about the process of building a 3D-printed case to house this project, using Blender.

This is my first ever 3D print, so I had a few things to learn.

Quick background

My effort on this project has been focused on making a design which works, then transferring that design to a PCB. Since this is my first electronics project, I’m working sequentially, and have completely ignored the need for a case until now.

I could have saved some time if I positioned the ports and mounting holes to match pre-made electronics enclosures, but I’ll take that as a lesson for my next project.

The requirements for the enclosure are simple:

  • Ability to mount the PCB inside the case
  • Cut-out for power input
  • Cut-outs for cable routing to expansion ports
  • Cut-outs for power and reset buttons

My PCB design files are in KiCad, and I decided to design the case in Blender, since I already know how to use it. Blender is not a CAD tool, but it is excellent for general-purpose 3D work, and will do just fine for this task.

Getting scale

My first challenge was to get the PCB into Blender at the correct scale, so that I could build a model around it. This is mostly just juggling file formats, but also note that Blender works in metres by default. I’ve set it to millimetres, with a scale of 0.001. There are guides on the web about how to set this.

I tried two different methods.

Method 1: Image import

In KiCad, I added two “dimensions” to show the distance between the centre of the mounting holes, then plotted the front copper layer as an SVG.

I used Inkscape to convert from SVG to PNG. I then used GIMP to add cross-hairs to the centre of the mounting holes.

In Blender, I added a single vertex at the origin, then extruded it on the X axis to match the measured dimension. I then added a reference image (the PNG file exported above), and scaled/moved the reference image until the cross-hairs lined up with the vertex.

Method 2: Model import

KiCad can export its 3D model to a STEP file. I imported this file into FreeCAD, then exported it to STL.

Lastly, I imported the STL into Blender.

The scale is correct with both of these methods, because I can align the image and model.

The second method produces much larger .blend files, but has less room to make undetected errors, so I’ll be using it in future.

Building a box

I modeled out the case as a lid and base with 3mm walls, and used boolean modifiers to cut out spaces for buttons and cables.

One challenge was rounding the corners without the two pieces intersecting. I built the box as a square, then beveled it in wire-frame view. The spaces are larger than necessary, and I will try using modifiers slot the pieces together with a fixed tolerance if I need to do this again.

The result was two pieces which sit together but do not attach, with rounded corners and no sharp overhangs, for ease of manufacturing.

Manufacturing and test

I exported the STL files, and sent them to a local manufacturer, since I don’t have my own 3D printer. The print is black PLA, printed with fused deposition modeling, with an 0.2mm layer height and 30% infill.

When I received the parts, I first checked that the two pieces fitted together, then placed a blank PCB over the base to check that the holes lined up. PLA shrinks as it cools, but this aligned perfectly, which is either good luck or correct calibration.

The computer’s board will be installed on standoffs, which are secured by a nut on the other side. I like the idea of using heat-set inserts instead, but I don’t want to try too many new things at once, so I’ve skipped the idea for now.

Next steps

At the time of writing, I’m still waiting for some parts to assemble this computer with a power/reset button, which will be the end of the hardware side of this project.

I’m quite happy with how this turned out as my first ever 3D print. If I need to make enclosures for future projects, I’ll take another look at open source parametric CAD tools such as OpenSCAD and FreeCAD, which are more targeted to this kind of work.

The STL files for this case are available on GitHub at mike42/6502-computer, critique is welcome.

Building a hardware interrupt controller

I’ve recently been adding simple hardware devices to my home-built 6502 computer, and ran into a problem.

The 6502 has two active-low interrupt inputs (IRQ and NMI), both of which are used in my design. If I add any devices which can trigger their own interrupts, I will need a way to combine multiple interrupt signals into one.

Choosing an approach

I researched some retro computer designs to see how this problem was handled, and found some very simple approaches.

Most commonly, multiple I/O chips would be connected to the 6502’s IRQ line, which is level-triggered and active-low. As long as all connected chips had an open-drain IRQ output, they could be connected in a so-called wired-or configuration:

In software, each interrupt source would be checked in priority order.

I can’t use something so straightforward to add hardware to my design, for two main reasons:

  • I would need to use logic gates to combine the inputs instead. The I/O chip which is connected to the CPU’s IRQ input at the moment is a WDC 65C22S, which does not have an open-drain IRQ output.
  • It’s not going to be practical to check every I/O device from an interrupt service routine. I’m planning to add devices which are accessed over SPI, and will take many clock cycles to return their status.

Over in the PC world, programmable interrupt controllers such as the Intel 8259 were used. Among other features, these chips combine multiple interrupt sources into one, and can report the interrupt source on the data bus.

Rather than use out-of-production retro parts, I decided to program an ATF22V10 programmable logic device with these functions. A PLD is cheaper, and I’ve just figured out how to program them, so I might as well put those skills to use.

Creating the interrupt controller

I am using galette to program these PLD’s, and went through 5 revisions of the PLD source file before landing on something which could complete these functions.

  • combine multiple interrupt lines into one.
  • allow the CPU to quickly identify the highest-priority interrupt to service.

For this section, I’ll briefly describe each part of the PLD definition is doing.

The file starts with the name of the target device, and a name for the chip.


Next pin definitions are listed. I’m using the ATF22V10, which has 24 pins. The first row is pins 1-12, while the second row is pins 13-24. Numbers go left-to-right in both rows, unlike a physical microchip!

Clock    /IRQ0 /IRQ1 IRQ2  /IRQ3 /IRQ4 IRQ5  /IRQ6 /IRQ7 /IRQ8 /IRQ9  GND
/CS      D7    D6    D5    D4    D3    D2    D1    D0    /IRQ  /WE    VCC

For combining the interrupts, I simply use a big OR function.

IRQ = IRQ0 + IRQ1 + IRQ2 + IRQ3 + IRQ4 + IRQ5 + IRQ6 + IRQ7 + IRQ8 + IRQ9

This reads “IRQ is active if IRQ0 is active or IRQ1 is active or IRQ2 is active, etc.”. All logic is the positive case, so “IRQ0” means “IRQ0 is active”. Whether “active” means 1 or 0 at the input depends on those pin definitions. The active-low inputs are inverted before being fed to this expression, and the result will be inverted if the output is active-low. This confused me at first, because I thought that “/IRQ0” is just a pin name.

The expressions for the data bus come next.

  • These are all ‘registered’ outputs (indicated with the .R suffix). This runs the output through a D-type flip-flop, so that it will only change on clock transitions, rather than asynchronously. This is to avoid garbage output if an interrupt triggers while we are reading from the controller.
  • D1..D4 is a priority encoder, encoding the number 0 to 10 to identify the lowest-numbered interrupt source, or 11 if there is no interrupt.
  • D0 is always 0. This multiples the output by 2, which makes things easier for the software.
D0.R = GND
D1.R = IRQ1*/IRQ0 + IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ9*/IRQ8*/IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0
D2.R = IRQ2*/IRQ1*/IRQ0 + IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + /IRQ9*/IRQ8*/IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0
D3.R = IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0 + IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0
D4.R = /IRQ7*/IRQ6*/IRQ5*/IRQ4*/IRQ3*/IRQ2*/IRQ1*/IRQ0
D5.R = GND
D6.R = GND
D7.R = GND

For the 22V10, not all pins support the same number of product terms. As I built up to 10 interrupt sources, I needed to re-arrange the pins a few times.

Lastly, I needed to tri-state the output when the chip was not being read. This is a second definition for each pin (.E suffix). I used both a chip select (/CS) and write enable (/WE) input. The interrupt controller cannot be written to, this just allows me to avoid bus contention if somebody is attempting to.

D0.E = CS*/WE
D1.E = CS*/WE
D2.E = CS*/WE
D3.E = CS*/WE
D4.E = CS*/WE
D5.E = CS*/WE
D6.E = CS*/WE
D7.E = CS*/WE

Test circuit and software

After testing everything on breadboard, I connected the new interrupt controller to my 6502 computer. All of the required signals are exposed via pin headers on my 6502 computer, more information about that can be found on previous blog posts.

This is how it looks. It’s not the neatest, but at least I don’t have the whole computer on breadboards anymore.

I then started writing some 6502 assembly code to test this out. I’ve been working on a shell which runs named commands, so I added a command called irqtest. This code references other parts of the ROM, which can be found in the GitHub repository for this project.

This code sets up a timer to trigger an interrupt on the 65C22 VIA, then uses the wai instruction to wait for interrupts. When the interrupt service routine completes, the code resumes. I’ve assigned two addresses (DEBUG_LAST_INTERRUPT_INDEX and DEBUG_INTERRUPT_COUNT) so that we can check that the correct interrupt routines are running.

; Set up a timer to trigger an IRQ

; Some values to help us debug

  lda #$ff              ; set interrupt index to dummy value (so we can see if it's not being overridden)
  lda #$00              ; reset interrupt counter
  sta $01
  ; setup for via
  lda #%00000000        ; set ACR. first two bits = 00 is one-shot for T1
  sta VIA_ACR
  lda #%11000000        ; enable VIA interrupt for T1
  sta VIA_IER
  sei                   ; enable IRQ at CPU - normally off in this code
  ; set up a timer at ~65535 clock pulses.
  lda #$ff              ; set T1 low-order counter
  sta VIA_T1C_L
  lda #$ff              ; set T1 high-order counter
  sta VIA_T1C_H
  wai                   ; wait for interrupt
  ; reset for via
  cli                   ; disable IRQ at CPU - normally off in this code
  lda #%01000000        ; disable VIA interrupt for T1
  ; Print out which interrupt was used, should be 02 if irq1_isr ran
  jsr hex_print_byte
  jsr shell_newline
  ; print number of times interrupt ran, should be 01 if it only ran once
  jsr hex_print_byte
  jsr shell_newline
  lda #0
  jmp sys_exit

I set my interrupt service routine read from the interrupt controller, then jump to the correct routine.

  phx                       ; push x for later
  inc DEBUG_INTERRUPT_COUNT ; count how many times this runs..
  ldx IRQ_CONTROLLER        ; read interrupt controller to find highest-priority interrupt to service
  jmp (isr_jump_table, X)   ; jump to matching service routine

  plx                       ; restore x

The interrupt controller can return 11 possible values, so I made an 11-entry table, with the address for each interrupt handler (2 bytes each). I have connected the VIA to IRQ1, so I set the second entry to a subroutine called irq1_isr.

isr_jump_table:              ; 10 possible interrupt sources
.word nop_isr
.word irq1_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr
.word nop_isr               ; 11th option for when no source is triggering the interrupt (phantom interrupt)

Most of the interrupt routines map to nothing at all. If this were a real OS, an interrupt from an unknown source would need to be a fatal error, since it can’t be individually masked/ignored on this hardware.

nop_isr:                         ; interrupt routine for anything else
  stx DEBUG_LAST_INTERRUPT_INDEX ; store interrupt index for debugging
  jmp irq_return

When IRQ1 is triggered though, this routine will clear the interrupt on the VIA. If we fail to do this, then the CPU will become stuck, processing interrupts forever.

irq1_isr:                        ; interrupt routine for VIA
  stx DEBUG_LAST_INTERRUPT_INDEX ; store interrupt index for debugging
  ldx VIA_T1C_L                  ; clear IFR bit 6 on VIA (side-effect of reading T1 low-order counter)
  jmp irq_return

Once I fixed some bugs, I was able to get the expected output, which is 02 (the index we are using to jump into isr_jump_table), followed by 01 (the number of times the interrupt service routine runs).

While I was researching this, I also found that online 6502 assembly guides often suggest clearing the interrupt flag on I/O chips near the start of the interrupt service routine, apparently to avoid running the routine twice. From this test, I know that the interrupt service routine is only running once, so I am happily disregarding that advice. Thinking about it, I can only see this applying to open-drain IRQ outputs.

Note about KiCad symbols

Since my last blog post about PLD’s, I’ve started to create separate symbols in KiCad for each programmed chip. You can see one of these in the schematic above.

I’m producing these automatically. Galette has a .pin file as one of its outputs. The file for this chip is as follows:

 Pin # | Name     | Pin Type
   1   | Clock    | Clock/Input
   2   | /IRQ0    | Input
   3   | /IRQ1    | Input
   4   | IRQ2     | Input
   5   | /IRQ3    | Input
   6   | /IRQ4    | Input
   7   | IRQ5     | Input
   8   | /IRQ6    | Input
   9   | /IRQ7    | Input
  10   | /IRQ8    | Input
  11   | /IRQ9    | Input
  12   | GND      | GND
  13   | /CS      | Input
  14   | D7       | Output
  15   | D6       | Output
  16   | D5       | Output
  17   | D4       | Output
  18   | D3       | Output
  19   | D2       | Output
  20   | D1       | Output
  21   | D0       | Output
  22   | /IRQ     | Output
  23   | /WE      | Input
  24   | VCC      | VCC

To produce schematic symbols, I wrote a small Python script to convert this text format to a CSV file, suitable for KiPart.



KiPart ships with profiles for FPGA chips, but the generic input worked fine for the ATF22V10 PLD.

./KiPart/venv/bin/kipart -r generic --overwrite irq_controller.csv

The output is a .lib file, which I can include in any KiCad project.


This approach works, and introduces far less overhead compared with checking each device in the interrupt service routine. In particular, it will allow me to test interrupts from slow-to-access SPI devices.

I will be disconnecting this this for now, but may include it in an expansion board or updated computer design if I ever make one!

Re-creating the world’s worst sound card

I recently built a computer from scratch, and wanted to add some basic sound output. I am not attempting to produce any 8-bit music, but I would like to be able to write programs use beeps and clicks to provide feedback, instead of only text.

I did a bit of digging into the subject, and found quite possibly the world’s worst sound card, in the Apple II. This is my attempt to re-draw the relevant part of the schematic, which I found in a 1983 publication called The Apple II Circuit Description.

The linked book does a good job of explaining the details, but the important part for this blog post is simply that Z3 is an address decoding output. It is normally high, but is low for half a cycle each time that memory address $C030 is accessed. This is fed to a flip-flop, which will toggle each time this happens, driving a speaker.

This should be easy for me to implement, since I have spare address decoding outputs on my computer.

First attempt

I decided to use a piezo buzzer, which can be driven directly from an IC pin, simplifying things greatly.

I first connected the buzzer directly to one of the spare address decoding outputs of my computer.

This is so simple that it is almost not worth diagramming, but here goes:

The input is IO2, which works out to address $8800 in my computer’s memory map. I wrote a bit about in an earlier blog post.

From BASIC (yes, this computer runs BASIC), I can read from this address and produce a click.


Second attempt

To produce a beep, I wanted a square wave with a 50% duty cycle. The Apple II used a 74LS74 dual D flip flop IC for this, which I don’t have in my inventory. Instead, I used a 74AC163 4-bit counter.

The circuit on the breadboard is wired up like this:

I was then able to produce a beeping sound by reading a memory address in a loop, just as you could on the Apple II.

10 A=PEEK($8800)
20 GOTO 10

Mission accomplished.

Alternatives and next steps

I’ve already started adding clicks and beeps to assembly code while testing, and my immediate plan is to add a separate startup beep for each of the two boot ROM’s. If I ever do a second revision of this computer board, I’ll most likely add a speaker as an on-board peripheral, using a 74LS74 instead of the counter.

There are two other methods which considered for adding beeps to this computer. The first was to utilise its 65C22 chip, which can be set to output a square wave on its PB7 pin. I have other plans for that output, which is the reason I haven’t used it for sound.

The second idea I looked into was sampled audio, which would require me to latch 8-bit audio samples a few thousand times per second, and then use some resistors to build a digital-to-analog converter. There are a few ways that I could achieve this with parts which I have on-hand, such as a SN74AC573 latch, or the 65C22.

6502 computer – from breadboard to PCB

I’ve recently been working on building a 6502-based computer on breadboards as a learning project. After making a few revisions, and porting some software to run on it, I was confident that my computer worked well enough to connect up permanently on a circuit board.

This blog post about the process that I went through to convert my working breadboard prototype to a PCB. As somebody who is not trained in electronics, this involved some fresh challenges for me.

Schematic capture

As a first step, I needed to draw up a proper schematic in a CAD tool. I’ve already been learning to use KiCad’s Eeschema for schematic capture, so I went ahead and drew up the whole thing.

I decided to put all of the components on one crowded page, just because I haven’t learned to manage multiple pages yet.

Mostly, this was just replicating what I’d already built, but I also needed to:

  • create a symbol for the DS-1813, which I could not find in the libraries I’m using
  • decide on a pinout for expansion ports
  • add a jumper block, so that the two interrupt lines (NMI and IRQ) can be disconnected from the on-board chips

I avoided changing the scope of the board to include important features (storage, audio), and instead added a header with all of the important buses and signals. My goal is to have a stable base system to work with, and I can always do a second board once I’ve figured out how these should work.

I did not use KiCad’s inbuilt BOM plugins, but instead manually wrote a parts list for future reference. The only components which I needed to order were sockets and pin headers, everything else was being lifted from the prototype.

Footprint assignment

Once the Eeschema electrical rules check was passing, I moved on to selecting footprints for everything. I spent quite a bit of time checking the datasheets for my planned components against the KiCad library to choose footprints that would fit.

I could not find anything which matched the footprint of the mini SPDT switches that I’m using, so I made my own.

I had not yet chosen a ZIF socket for the EEPROM, so I selected a footprint for a standard socket instead. This was a risky move, because I had to guess how much space to leave when laying out the board.

PCB design

I had already selected a board manufacturer, so I set up my track widths and via sizes according to their capabilities.

Next, I imported the netlist and placed the components. I attempted to fit everything in 10cm x 10cm, 2-layer board to save on manufacturing costs, but it was very crowded. I was worried about having enough space for traces, fitting an (unmeasured) ZIF socket, and adding some mounting holes, so I spaced it out to fit on a 10cm x 11.6cm board instead.

I also added footprints for M3-sized mounting holes, and edge cuts with rounded corners.

It took me 4 attempts to successfully route all of the traces. The result breaks every PCB layout best practice which I had read about, but I was happy just to have everything connected by that point.

In hindsight, I could have made this task easier by splitting up the expansion header, and building an accurate footprint for a ZIF socket. I also could have placed the decoupling capacitors closer to the IC power pins, and avoided interrupting the ground plane.

The silkscreen setup was more time-consuming than expected. I wanted to make the board as self-documenting as possible, so that I would not need to open the design files on a computer when writing code and adding hardware peripherals. I labelled every switch, button, light and IC, as well as every expansion header pin (74 of them!). I also added some simple assembly hints such as resistor values, and +/- signs to indicate the polarity of the lone electrolytic capacitor.

The 3D render view in KiCad shows how the finished board would look.

Components did not display on the board at first, but there is a menu option to download the 3D models. After setting this up, and setting the solder mask colour to black, I was able to render the board properly. This looks very similar to the assembled board, with just a few parts missing.


I only recently discovered that low-volume PCB manufacturing is accessible to hobbyists. Ordering the boards was really easy. I exported gerber and drill files according to their instructions, loaded them into a zip file, and uploaded them to a web portal.

I am glad that an online gerber viewer was available, because it allowed me to spot an error which I had not noticed in KiCad, where I had two pins labelled PA1.

The default settings are reasonable, but I chose black solder mask, a lead-free surface finish, and chose the option to exclude the manufacturer’s order number from the board.

The gerber viewer also has a tab which shows checks against their manufacturing rules.

I placed the order, and eagerly checked each day as the boards progressed through the manufacturing steps. From placing the order to getting the boards on my desk, the whole process took 7 days.

Assembly & test

Once all of the sockets arrived in the mail, I started assembling.

The standard advice I’ve read for through-hole assembly is to solder low-profile components first. Instead, I added enough components to light up the power LED.

I know that the power LED dims if anything is drawing too much current (eg. a short circuit). By getting this working first, I will know I’ve made a mistake (and can switch off the board quickly) if it doesn’t light up later.

I don’t think I’ve ever soldered more than a few pins or wires at a time, and this board had 323 pins to solder.

After a about 2 hours, I had a fully-populated board. It’s smaller and looks better than the breadboards, but it was time to move all the chips across to see if it actually worked.

Much to my surprise, the computer booted up to EhBASIC on the first attempt.


This has been quite a fun project. It has proven to me that open source tools are perfectly sufficient for this type of hardware development. I’m also very happy to have reached a point where can call this project “done”, since I have a permanent home-built computer.

My plans for learning about low-level computing are not done yet, of course. I’ve kept everything at a low 1.8 MHz, so that I can continue to prototype hardware peripherals on a breadboard. I’ve also left a switch to select which half of ROM to boot from, so that I don’t brick my computer every time I make an error in an assembly language program.

I’ve uploaded the full parts list, design files, and firmware to GitHub at mike42/6502-computer.

Porting BASIC to my 6502 computer

I’ve recently been building my own 6502-based computer. After a lot of work on the hardware side, I decided that it’s time to move past “Hello World” programs, and port a BASIC interpreter.


This computer is architecturally similar to a 1980’s home computer, where BASIC was a widely-used interpreted language. Porting an existing interpreter will be a fast way to get a command-prompt, and it will also add the ability to load arbitrary software, by typing out a program.

I heard about an interpreter called EhBASIC (“Enhanced BASIC”) from on this YouTube video by Chris Bird. It’s source-available (free for non-commercial use only), and seems to be well-regarded by 6502 enthusiasts.

EhBASIC was written by the late Lee Davidson. I based my port on the version v2.22, hosted on Klaus Dormann’s GitHub. I used Hans Otten’s mirror of Lee’s website as a reference, plus the EhBASIC section of the forum.

Porting process

EhBASIC was a breeze to port.

I started with the ‘patched’ code from Klaus2m5/6502_EhBASIC_V2.22, which I placed in a local git repository, so that I could track the changes.

I first changed some of the syntax so that the code would assemble with ca65, since that is the assembler I’m using for everything else. There are other ca65 ports around, though they do not include the same patches.

The --feature labels_without_colons setting in ca65 was useful here, since the original source does not include colons after label names. The output was 10.5KiB of machine code, which will fit in the 16KiB of ROM space which I have available in my home-built computer.

I also confirmed that the code did not depend on using undocumented 6502 CPU opcodes, which would have been incompatible with my newer-generation 65C02 CPU.

Next, I dropped in the correct routines to read and write characters. I already had something similar from an earlier blog post. I needed to make some small changes to use the carry bit (SEC / CLC opcodes) to indicate whether a character had been received, and to set up the 6551 ACIA on startup. I programmed the code to an EEPROM, and it worked the first time.

All of my changes against the original code, including the Makefile and ca65 config file are in this commit. The memory addresses line up with the decoding scheme described here.


BASIC is an interesting language, if only for historical reasons. Classic BASIC feels very clunky by modern standards, but the user experience is not that different to the modern Python REPL. Or at least, it is more similar than you might expect given how far computers have advanced.

I’m running EhBASIC on an 8-bit CPU, where the main alternative is plain 6502 assembly language. BASIC has allowed me to hit the ground running, and gives me access to floating point maths and user-loadable programs, on a computer that does not have an operating system or removable storage.

My home-built computer has a switch for selecting between two ROM’s. My plan is to write my own code in assembly language, but keep this port of EhBASIC in the secondary ROM, so that the computer can always boot to something that works.

Upgrades and improvements to my 6502 computer

For the past few weeks, I’ve been building a 6502-based retro computer, based originally on a tutorial and design by Ben Eater. My first major change was to add a serial port, so that I can write programs which accept text as input.

This blog post is a list of things which I’ve changed since, to try to make a computer which is a bit more suited to my intended use. My aim at the moment is to lock in a simple base system, so that I can use it as permanent platform for 6502 assembly and hardware experiments.

Re-visiting address decoding

The computer has a 32 KiB RAM chip, and a 32 KiB ROM chip. Due to the way that the glue logic was set up, only 16 KiB of the RAM can be used, but the full ROM space is available.

I wanted to flip this around, since I’m planning to run most programs from RAM, with only a loader or interpreter in the ROM. The idea would be to add a switch to select between the upper and lower half of ROM, map the full RAM in, and leave a large space available for I/O and other experiments.

I planned out a coarse, 2-chip address decoder to achieve this.

The resulting memory map is:

Address Maps to
C000-FFFF ROM – 16 KiB
A000-BFFF Not decoded – 8 KiB
8000-9FFF I/O – 8 KiB
0000-7FFF RAM

This is partly based on the info in Garth Wilson’s address decoding primer, and also Daryl Richtor’s SBC-2 computer.

I re-wired every chip select, modified my test program to use the new memory map, and also added a switch to select between the two halves of the ROM.

Implementing power-on reset

In Ben Eater’s 6502 computer, the reset line is connected to +5v through a resistor, and grounded when a momentary switch is pressed. At power-on, I needed to press the reset button before the computer would do anything useful.

I removed the pull-up resistor from the reset line, and added a DS1813-5+ supervisory IC instead. This holds the reset line low for 150ms at power-on, or when reset is pressed. This suggestion was from the forums.

There was no KiCad symbol for the DS1813-5 in the default library, so I needed to create one.

Interrupt lines

I found that I had made an error when connecting the interrupt lines.

The 6502 has two interrupt inputs: IRQ and NMI. On my 6502 computer, the interrupt output of the 65C22S is connected to the IRQ input of the CPU, while the interrupt output of the 65C51S is connected to the NMI input of the CPU.

The 65C51N uses an open-drain IRQ output, so this configuration left the CPU’s NMI input floating most of the time, which caused many spurious interrupts. This was easily fixed by connecting the NMI line to +5v through a 3.3k resistor.

I also implemented some tips from Garth Wilson’s 6502 primer, and connected RDY and BE to +5v through 3.3k resistors, where they had previously been connected to +5v directly.


Up until this point, I had been powering my computer with a breadboard power supply. I’m hoping to build something permanent, so I designed a circuit to power it through a DC barrel jack.

This takes a DC voltage, and steps it down to 5v through a voltage regulator. The other components are a power switch, plus a diode to avoid damaging anything if I connect a plug with the wrong polarity.

This computer does not draw much current, so it’s not necessary to add a heat-sink on the voltage regulator.

Getting my wires crossed

My next change was to remove the LCD screen. This had been useful for debugging, but I would prefer to free up breadboard space and I/O connections for other parts of this project.

I did this in four steps, testing after each one. First I moved the LCD off to its own breadboard, then updated the computer’s test program to stop using the LCD, ran the computer without the LCD connected, and finally re-located the UART chip into the newly-vacant space.

After the last step, the computer started producing the wrong text (eg. “H” became “D”). I had mixed up two bits in the data bus, and by some luck the UART setup code was not affected. The orange and red wires on the left of this photo are the ones which were swapped.

I was able narrow down the source of the problem quickly, since I was breaking everything down into small steps, which were easy to individually verify.

Next steps

For all this effort, not much has changed. I still just have an 8-bit computer which says “Hello” when it starts. The next part of this project will involve porting across some non-trivial software.

On the hardware side, I’ve made a lot of progress learning how to use KiCad. As long as everything works, my plan is to migrate this design from breadboard to a PCB in the near future.

Adding a serial port to my 6502 computer

In my last blog post, I wrote about the 8-bit computer which I’ve been building, using an existing design by Ben Eater. The I/O capabilities of the original design are rather limited, so one of the first enhancements I’m making is to add a serial port.


The main chip I am adding is the WDC 65C51N ACIA, which is a modern version of the MOS 6551. Versions of this chip have been on the market for around 40 years, and lots of classic computer designs use some version of it for serial output. I bought this one new, and the date code indicates that it was manufactured 11 years ago, so it’s a fair guess that they are not selling as fast as they used to.

I also needed a 1.8432 MHz oscillator, which I used to clock both the computer and the UART.

Lastly, I used a USB/UART module to interface with a modern computer. This module hosts a FT232RL chip, and the pins on this one are DTR, RX, TX, VCC, CTS and GND.

Address decoding

The 65C02 CPU in my computer uses memory-mapped I/O, so I needed to fit this new I/O chip into the memory map before I could start using it.

The original design uses a single-chip solution for address decoding, where the select lines for the ROM, RAM, and a 65C22 VIA chip are connected via 3 NAND gates.

This leaves an unused space between address 4000 and address 5FFF.

Address Maps to
6000-7FFF I/O – 65C22 VIA
4000-5FFF Not decoded
0000-3FFF RAM

The 65C51 ACIA has two chip select inputs: one active-high and one active-low, much the same as the 65C22 VIA. All I needed to do was invert A13, and there was an unused NAND gate in the existing design which I could use for it.

This places the 65C51 in the unused address space. It’s not exactly efficient to assign 8KB of address space to a device which needs 4 bytes, but it does work.

Address Maps to
6000-7FFF I/O – 65C22 VIA.
4000-5FFF UART – 65C51 ACIA
0000-3FFF RAM

While editing this blog post, I also re-read Garth Wilson’s address decoding guide for the 6502, which shows some alternative schemes for achieving this.


I’m using the following wiring between the 65C51 and the USB/UART module.

Note: The the two clock inputs are connected to the 1.8432 MHz oscillator, which is not shown correctly here.


This particular revision of the 6551 has some hardware bugs, though they are well-documented and can be worked around in software. Most of the excellent example code online is aimed at older (less buggy) revisions.

After four attempts, I was able to write an assembly-language program which could produce some output. The information on this thread, and this comment were the most accurate for my hardware setup.

I’m using this code to set up the ACIA for 8-N-1 communication at 19,200 bytes per second, with no interrupts.

ACIA_RX = $4000
ACIA_TX = $4000

    ; ... other stuff
    ; ACIA setup
    lda #$00
    lda #$0b
    lda #$1f
    ; ... other stuff

To send, I needed to add a delay between bytes, since the hardware bug prevents the transmit bit in the status register from operating correctly. I found some code with nested loops, but it only worked after increasing the delay far beyond what should have been necessary. An alternative work-around is to generate a timed interrupt from the 65C22 VIA, which I’m hoping to try later.

; print A register to ACIA
; Based on
  sta ACIA_TX
  jsr delay_6551

  ldy #6 ; inflated from numbers in original code.
  ldx #$68
  bne delay_1
  bne minidly

I am using this routine to receive characters. It will block until the next character is received, and I will most likely need to replace this with something interrupt-driven once I start to add more complex programs.

; hang until we have a character, return it via A register.
  and #$08
  beq recv_char_acia
  lda ACIA_RX

I ended up with a program which prints “Hello” to both the LCD and serial port when the computer resets, then accepts text input. Any characters received over serial are then printed back to the terminal, and also to the LCD.

Mistakes were made

Since this is a learning project, I’m keeping a log of mistakes that I’m making along the way. Today’s lesson is to check everything, because it’s very difficult to debug multiple problems at once. When I ran my first test program, there were four faults.

  • I interfaced the USB/UART module to the 65C51 by matching up pin names, which does not work – RX on one side of the serial connection should go to TX on the other.
  • I incorrectly calculated the memory map, so the test program was writing to address 8000 while the 65C51 was mapped to address 4000.
  • I based my code on examples which do not work on this chip revision, because of the hardware bug noted above.
  • I also miscalculated the baud rate, so even if I didn’t have the other faults, my settings for minicom would not have worked.


It’s very straightforward to modify Ben Eater’s 6502 computer design by adding a 65C51 ACIA. This upgrade will allow me to write (or port) software which uses text I/O. I’m planning a few more changes to this design before I port anything too serious though.

This is also the first time I’ve included (pieces of) schematics in a blog post. I’m drawing these with KiCad, and using a slightly modified version of Nicholas Parks Young’s 6502 KiCad library, which has saved me a bit of time.

Building a 6502 computer

I’ve been using 6502 assembly for some hobby projects recently, but only testing in an emulator. It’s about time to target some real hardware, so for the past few weeks I’ve been following Ben Eater’s 6502 computer tutorial.

I am a complete beginner when it comes to electronics, so I spent a bit of time making useless circuits to toggle LED’s, then jumped right in to building the simplest possible circuit exercise a 65c02 CPU, known as a NOP generator. I used a 555 timer and an inverter to get a 3Hz clock, which is quite a bit slower than my desktop PC.

I then extended the circuit to run NOP instructions from a ROM. I generated a ROM filled with the 6502 NOP instruction by printing a character to a file, then concatenating the file to itself times to fill the ROM.

printf '\xEA' > rom-original.bin
dd if=rom-original.bin of=rom-original.bin bs=1 seek=1 count=32768

I am using an open-source tool called minipro with a TL866II+ programmer to burn the ROM.

$ minipro -p AT28C256 -w rom-original.bin
Found TL866II+ 04.2.86 (0x256)
Warning: Firmware is out of date.
  Expected  04.2.123 (0x27b)
  Found     04.2.86 (0x256)
Erasing... 0.02Sec OK
Protect off...OK
Writing Code...  7.44Sec  OK
Reading Code...  0.77Sec  OK
Verification OK
Protect on...OK

Next, I tried to extend the circuit to blink some LED’s based on a programmed sequence. When I ran the program, the 65c22 I/O chip warmed up, and my row of LED lights did not blink. It turns out that I had mixed up the meaning of VCC and VSS, and applied a reverse voltage to the chip. I found a post from somebody else who had made the same mistake, and corrected it before the chip was damaged.

This program worked well initially, but the computer would sometimes crash when running at this slow speed, so I started running it with a 1MHz or 1.8MHz oscillator instead. I now know that this is because I had plugged the 65c02 and 65c22 clock inputs into the 555 timer output, when I should have been running it through the inverter first. The rising-edge and falling-edge times of the 555 timer are apparently not fast enough to clock these chips reliably.

The next step was to add an LCD and some RAM. My first attempt did not work, and it took me a few hours of troubleshooting to rule out any hardware problems. In the end it was a simple programming error, where I had used a jmp instruction instead of jsr in my test program.

So it’s not much, but it works! Based on some of the problems that I had while building this, it was definitely a good idea to start with a known-good design on a breadboard.

I’ve got a few ideas (and components) for extending this computer already, and I’m hoping to learn a thing or two about hardware and software along the way.

IntelliJ plugin for 6502 assembly language

I’ve been writing some 6502 assembly code in the past year, and have found the developer experience for this language to be lacking some modern conveniences.

In my last blog post, I described the development tools that I used to implement a simple NES game on Linux. I used text editor to write the code, and it couldn’t do much more than syntax highlighting.

I write most of my other code in IntelliJ, so I decided to take a look at what would be required to write a plugin for 6502 assembly support. I managed to together something that mostly works, which I published to the JetBrains Marketplace last weekend.

You can find it by searching “6502 Assembly” in most JetBrains IDE’s.


Firstly, it supports syntax highlighting. I limited the scope of the plugin to ca65 assembler syntax only, since it’s the assembler that I know best.

You can navigate to any label with Ctrl+Click. This will not yet work for other types of symbols, such as constants, macros, and imports.

If the plugin sees a jump or branch statement, and can figure out where the jump goes, then it will show a gutter icon which navigates to the target.

You can find the usages of a label.

In ca65, you can use nested scopes. The plugin shows code folding buttons to collapse these scopes.

You can navigate to a symbol by name using the “Go To Symbol” action.

The plugin allows for commenting and un-commenting blocks of code, though there is no formatter, so indentation sometimes still has to be fixed manually.

It also allows you to rename a label and its usages, which is a great time-saver.


One limitation is that the plugin does not fully parse expressions, so it will not detect errors from mis-matched brackets.

Secondly, the plugin does not understand the project structure. This means that if you re-use the same label name in different places in your project, the “Rename” and “Find Usages” function will match all of them at once, because it is not smart enough to follow imports and apply scope rules.

Future improvements

A lot of things can be done with an IDE plugin, but I’m planning to use this initial implementation for a while before attempting to add any big-ticket features.

A reasonable goal might be to have “Hello World” project skeletons for common 6502 systems, and to support launching with some common emulators. I would like to be able to set breakpoints and debug 6502 programs in an IDE, but external debugging interfaces are not commonly available for retro emulators, so this is probably not a reasonable goal.

The code is on GitHub under an MIT license. If you are using this plugin and would like to help improve it, then pull requests are welcome.

Building my first NES game: A retrospective

Last year, I spent a fair amount of time learning how to make games for the Nintendo Entertainment System. The homebrew scene for this system is very much alive, and I was quite proud that I was able to make a simple, working video game which runs on a NES emulator.

So, I present to you: 8-bit Table Tennis.

This blog post just a few notes about the tools and resources that I used for NES development on Linux, plus a few things that I learned along the way.

Development tools

The CPU in the NES is a derivative of the once-ubiquitous MOS 6502, and games for it are mostly written in 6502 assembly. I already do a fair amount of programming, but quickly found that my usual editors, compilers and debuggers were useless for this platform.

The main resources I used were:

To build my code, I used the cc65 toolchain, which includes a cross-assembler. I did most of my testing in the Nestopia emulator, and most of my editing in Gedit. All three of these are available in the official Debian repositories.

Gedit setup

Out of the box, Gedit can’t syntax highlight 6502 assmebly. I found a language spec file for it on the the forums, and edited it slightly before using it.

At the time of writing, new language specs can be installed on Debian like this:

sudo cp asm6502.lang /usr/share/gtksourceview-4/language-specs/asm6502.lang && sudo chmod 0644 /usr/share/gtksourceview-4/language-specs/asm6502.lang

It possible to add a console and git support to with gedit, but this is still a long way from a full IDE.


Geany is a programming text editor, and is also available in the official Debian repositories.

I wasn’t able to get it to recognise 6502 assembly, but it does have good x86 assembly (NASM) support, which is similar enough for it to allow navigation through the source code using labels.

Geany can also set up projects with build scripts (such as a Makefile), which could allow for quicker testing.


FCEUX is a NES emulator which has an in-built debugger, and it runs well under WINE. I was also able to build and run it natively on Linux, but the debugger is only present in the Windows build.

I also briefly experimented with using the FCEUX Lua interface to pause/resume the emulator over a TCP socket for debugging, since that interface is available in the Linux build. I could only pause the emulator between frames, so I decided to abandon this.


I created the graphics in the GNU Image Manipulation Program, and manually broke the title page down into tiles.

I also wrote a custom program to convert a 4-colour PNG file into the native NES CHR graphics format. I run this conversion as part of the build process, so that the graphics files can be stored in a modern format.

Things I learned

Programming for the NES is quite simple once the project is up and running, but even simple tasks can be a real hassle. I spent more time than I expected on mundane tasks such as collision detection, and certainly did a poor job of implementing game physics. The 6502 CPU has no built-in way to multiply, divide, or perform floating point operations. The version used for the NES additionally lacks any way to perform a binary-to-decimal conversion, which would have been very useful for displaying the player scores! In hindsight, I would have been able to improve the physics by pre-computing some lookup tables.

On the other hand, I expected it to be difficult to work within 2KiB of RAM, but due to the simplicity of the game, I only used around 25% of it, and had plenty of CHR ROM space leftover as well.

It took me three weekends to make this project, without having ever written a line of 6502 assembly before. I think that development would be a lot faster if I were able to set breakpoints from my code editor, so I will definitely be looking at other debugging emulators before attempting my next 6502 assembly project.


8-bit Table Tennis is available as an iNES ROM on GitHub.

I have only been running it with an emulator, so if any readers of this blog own a flash cartridge for the NES (such as an Everdrive), then please let me know if it works on the real hardware!