I recently spent some time re-building my 65C816 computer project to run at 3.3 volts, instead of 5 volts which it used previously. This blog post covers some of things I needed to take into consideration for the change.

It involved making use of options which I added when I designed this test board, as well as re-visiting all of the mistakes in that design.

Re-cap: Why 3.3 V is useful

One of the goals of this project is to build a modern computer which uses a 65C816 CPU, using only in-production parts.

A lot of interesting retro chips run at 5 V, but it’s much easier to find modern components which run at 3.3 V. I can make use of these options without adding level-shifting if I can switch important buses and control signals to 3.3 V.

ROM

The ROM chip is the most visible change, because the chip has a different footprint. When assembling this board for 5 V use, I used used an AT28C256 Parallel EEPROM, in a PDIP-28 ZIF socket. I couldn’t find 3.3 V drop-in replacement for this, so I added a footprint for a SST39LF010 flash chip, which has a similar-enough interface.

This was the first time I’ve soldered a surface-mount PLCC socket. I attempted to solder this with hot air, which was unsuccessful, so I instead cut the center of the socket out so that I could use a soldering iron. I then added a small square of 1000 GSM card (approx 1mm thick) as a spacer under the chip.

I also added a new make target to build the system ROM for this chip, which involves padding the file to a larger size, and invoking minipro with a different option so that it could write the file. I’m using a TL866II+ for programming, and adapter boards are available for programming PLCC-packaged chips.

$ make flashrom_sst39lf010
cp -f rom.bin rom_sst39lf010.bin
truncate -s 131072 rom_sst39lf010.bin
minipro -p "SST39LF010@PLCC32" -w rom_sst39lf010.bin
Found TL866II+ 04.2.126 (0x27e)
Warning: Firmware is newer than expected.
  Expected  04.2.123 (0x27b)
  Found     04.2.126 (0x27e)
Chip ID OK: 0xBFD5
Erasing... 0.40Sec OK
Writing Code...  8.38Sec  OK
Reading Code...  1.18Sec  OK
Verification OK

UART

The UART chip is an NXP SC16C752B, which is 3.3 V or 5 V compatible. I rescued one of these from an adapter board (previous experiment). This was my first time de-soldering a component with hot air gun. Hopefully it still works!

My FTDI-based USB/UART adapter has a configuration jumper which I adjusted to 3.3 V.

Clock, reset and address decode

I again used a MIC2775 supervisory IC for power-on reset, though the exact part was different. I previously used a MIC2275-46YM5 (4.6 V threshold), where the re-build used a MIC2275-31YM5 (3.1 V threshold).

I needed a 1.8432 MHz oscillator for the UART. The 3.3 V surface-mount part I substituted in had a different footprint, which I had prepared for.

I also prepared for this change by using an ATF22LV10 PLD for address decoding, which is both 3.3 V and 5 V compatible. The ATF22V10 which I used for earlier experiments works at 5 V only.

SD card

I installed the DEV-13743 SD card module more securely this time by soldering it in place and adding some double-sided tape as a spacer. This module is compatible with 3.3 V or 5 V.

The level-shifting and voltage regulation on this module is now superfluous, so I could probably simplify things by adding an SD card slot and some resistors directly in a later revision.

Modifications

There are three errors in the board which I know about (all described in my earlier blog post). This is my second time fixing them, so I tried to make sure the modifications were neat and reliable this time.

Firstly, the flip-flop used as a clock divider is wired up incorrectly, so I cut a trace and run a wire under the board.

Second, the reset button pin assignments are incorrect. Simply rotating the button worked on the previous board, but it wasn’t fitted securely. This this time I cut one of the legs and ran a short wire under the board, and the modification is barely noticeable.

Lastly, the address bus is wired incorrectly into the chip which selects I/O devices. I previously worked around this in software, but this time I cut two traces and ran two wires (the orange wires in the picture). I made an equivalent modification to the 5 V board, so that I could update the software and test it.

The wire I was using for these mods is far too thick and inflexible. I’ve added some 30 AWG silicon-insulated wire to my inventory for the next time I need to do this type of work, which should be more suitable.

Wrap-up

I transferred components from the old board, and attempted to boot it every time I made change. The power-on self test routine built in to the ROM showed that each new chip was being detected, and I soon had a working 65C816 test board again, now running at 3.3 V. I’m using my tiny linear power supply module to power it.

I can now interface to a variety of chips which only run at 3.3 V. This opens up some interesting possibilities for adding peripherals, which I hope to explore in future blog posts.

It will be more difficult to remove and re-program the ROM chip going forward though. This is hopefully not a problem, since can now run code from an SD card or serial connection as well (part 1, part 2).

In the last few months, I’ve been learning about the 65C816 processor, and trying to build a working computer which uses it. My latest breadboard-based prototype was not reliable, and I decided to convert it to a PCB to hopefully eliminate the problem, or to at least identify it.

Quick goals

I was aiming to make a debug-friendly 4-layer PCB, the size of two standard breadboards. This will be my first time designing a 4-layer board, and also my first time using KiCad 6 to create a PCB.

I didn’t have a working prototype, so I built in test points for connecting an oscilloscope or logic analyser to troubleshoot. In case of errors, I made it possible to leave some components unpopulated, and instead drive signals externally.

I also left some extra footprints which I might use for future improvements, or can fall back to if some of my ideas don’t work out.

The clock

Some components require the inverse of the CPU clock, and there was previously a small delay from inverting the signal. I don’t think this is a problem on its own, but I introduced a D-type flip-flop to create a proper two-phase clock.

This does halve the CPU clock speed, so I added jumpers to allow an alternative clock source to be selected for the CPU, where previously the UART and CPU clocks needed to be the same. Note that there is an error in the wiring here, which I only discovered later.

The oscillator I’m using on the breadboard prototype is a DIP-8 package. A larger variety of oscillators are available as a 5x7mm QFN package, where the four corner pins have the same function as the DIP-8 version. I added an alternative footprint, which fits without taking up any extra board space. I expect that I’ll only use this if I attempt to run the board at 3.3V later.

Some 6-pin QFN oscillators provide an inverted clock output on one of the pins, and it would be an idea to use one of those in a future design to reduce the component count.

New reset controller

Several components on the board require a reset input. These are mostly active-low inputs, but one component has an active-high reset. I discovered the tiny MIC2775 supervisory IC, which provides both.

I haven’t used this part before, so in case of problems, I also made it possible to remove this part and use a DS1813, which generates an active-low reset on my prototype.

These parts work by sensing voltage, and drop-in alternatives exist for either part if I move to 3.3V.

ROM

I am currently using a parallel EEPROM to store code for my prototype, and will add this to the PCB for this test board. As with the the clock and reset controller, I’m working with 5V components today, but considering what I would need to change to run the whole board at 3.3V.

The few EEPROMs which are available at 3.3V use a different package, and are quite slow (200ns or worse). Parallel flash chips seem to be the most promising alternative, since they have a similar interface, although I haven’t confirmed that I can program them yet. I added a footprint for the SST39LF010, which also has a 5V variant with the same pin-out.

Other footprints

I also added footprints so that I could connect the SD card module which I used on my 6502 computer, a piezo buzzer, and some electrolytic capacitors if needed.

Design lessons learned

Although my first attempt at making a PCB was a success, I learned a few things which I’m using here.

For that previous project, I found that machined-pin sockets are not very tolerant of used chips with bent legs, so I’ve used cheaper stamped pin sockets for this test board. I’ve also left spacing at the ends of each socketed IC, so that they can be pulled out more easily.

I made an effort to keep unrelated traces away from points which I need to solder, because I had some problems with this on my last board.

I also avoided creating one large expansion header, and instead exposed signals in places which are easy to route. For example, the I/O device select signals are exposed on a pin header right beside the chip which generates them.

PCB layout

The standard process is to make a schematic, then place components, then route traces.

For this test board, I instead chose the physical dimensions of the board first, and added components incrementally (particularly test points, expansion headers and alternative footprints) until I was making good use of the available space. KiCad has an item in the “Tools” menu to “Update PCB from Schematic”, which I used extensively.

Just to see how it would look, I also added the letters “65C816” to the back copper layer, which is visible in the top-right here.

I put a lot of time into labeling different parts of the board to help with debug/assembly, and used the 3D view to check that it wasn’t too crowded.

Before sending the files to a manufacturer, I printed it at 1:1 scale for a reality check.

Among other things, this confirmed that the ROM sockets had enough clearance from other components.

It also confirmed that either DIP-8 or QFN-packaged oscillators would fit.

Assembly

I ordered the boards from a manufacturer which I’ve used before. They are large-ish, lead-free 4-layer boards, but I’m not optimising for cost. I’ve left a lot of options in the board, so I’m hoping to make use of several of these PCBs in different configurations, depending on which direction this project goes.

I assembled the board incrementally, starting with the power LED. Most of the passive components on this board are 0603 (imperial) size surface-mount parts, and I’ve used footprints with long pads for hand soldering.

The first problem I found was with the reset button – I had switched to using a footprint with the correct pin spacing, but had assigned the pins incorrectly, so I needed to cut/bend some legs and install it rotated 90 degrees.

I next found a problem with the clock, where I had wired up the flip-flop incorrectly in the schematic – the /Q output should go to D. I could recover from this by cutting a trace and running a short wire. Of course the board is mirrored when flipped upside-down, so I cut the wrong trace first and needed to repair that too.

I added enough components to get the CPU to run NOP instructions from ROM, then built up to a running some test programs which I’ve blogged about here. The final mistake I discovered was a mix-up with some of the lines used for selecting I/O devices, which means that devices are not mapped to their intended addresses. I can work around this in software.

This is already an improvement over the breadboard prototype, because I can quickly swap the ROM chip without accidentally disconnecting anything.

The board looked fairly complete at this point, and the only major component missing was the UART chip. This was the part which did not work reliably in my prototype, so I was prepared to do some debugging here. Note that I’ve got orange test points all over the board to connect important signals to an oscilloscope, with a few black test points for GND connections. All of the 74-series chips are in the 74AC logic family, and I sourced 74HC versions as well in case I needed to switch any to see the difference.

However, I was able to run more or less the same test program I used before, and it now works reliably. This is captured through a Cypress FX2-based logic analyser using Sigrok.

It’s great that this works, but I don’t know for sure why this was so unreliable on my previous prototype. Several possible causes were eliminated through this process, since I used a new UART chip and freshly programmed address decode PLD, and eliminated a possible timing issue. On a PCB, I’m also able to make better electrical connections, and add a ground plane, which is an immediate advantage over breadboards.

The design as it stands

Now that I am back to having a working prototype, I’ll take this chance to post some updated schematics.

Just a note of caution: this is snapshot of a work-in-progress learning project. I’m absolutely aware that there are errors in here, and that the layout is quite messy. Still, I hope that this is useful to anybody else who is attempting to use this relatively obscure CPU.

The design no longer fits on one page, so I’ve split it into 3 sheets.

CPU

The CPU sheet contains the circuitry for de-multiplexing the data bus and bank address byte. All power, reset, and clock components are in here as well, along with pin headers for the address & data bus, and all those test points.

There are a lot of “just in case” components as well, such as pull-up resistors on the data bus, which I have not fitted.

Memory

The memory section is quite straightforward. I’ve got a PLD generating chip-selects for ROM, RAM0, or RAM1, with some extra components to add some flexibility.

I/O

In case I/O is selected, there are three possible choices: the 65C22 VIA, or one of the two UART interfaces. Most of the other components on this sheet are optional footprints or external ports. Note that the clock going into the UART is mis-labelled on this sheet.

Next steps

I’m going to spend some time using this board as a development platform for low-level software which targets the 65C816 CPU. I’ll most likely also use the emulator which I put together a few weeks ago to speed up development, since I’ve been able to confirm that the code I’m writing works on real hardware.

The basic functionality is now a lot more stable than what I had before, so this test board will allow me to prototype some different hardware options once I’ve got some simple text-based programs up and running.

The hardware design, software and emulator for this computer can be found in the GitHub repository for this project. I’m updating the repository as I make progress with this project, and the version used for this blog post is here.

Back in February, I blogged about my 65816 computer prototype on breadboards. I recently spent some time re-building this in an attempt to add some improvements.

The new prototype didn’t work particularly well, so I’ve left out quite a lot of detail from this update.

Power delivery

I soldered stranded wire to pin-headers to deliver a reliable ground and +5V power connection to each breadboard. This corrected some problems from my previous prototype, which used a long chain of unreliable connections.

Adding more RAM

I added a second 512 KiB RAM chip, and extended the address decoding scheme to provide a chip-select signal for it. The computer now uses the planned 20-bit address bus, though the wiring around these chips became very dense.

Adding a UART chip

I previously tested an NXP SC16C752 UART, and added it to this computer to provide text I/O. This is a small surface-mount chip, which I am connecting via a break-out board.

My previous prototype only supported one I/O device, so I also extended the address decoding scheme with a 74AC138 decoder.

Problems

I could get some simple test programs running, but text I/O did not work reliably at all. With my limited debugging tools (just a logic analyser), I was only able to conclude that data being written by the CPU was different to what was being received by the UART.

This seemed like it could be a timing or signal integrity issue, but I couldn’t be certain from looking at digital output with a low sample-rate. This image shows some control signals from when I was troubleshooting this.

Next steps

After hitting a dead-end, I made a list of possible causes, and decided to re-build this prototype on a debug-friendly PCB, and to source an oscilloscope for troubleshooting.

More on that in my next post!

I am currently working on building a 65C816-based computer from scratch. I have some ideas for how I want to do this, but my first task is to put together a working prototype. I’ll be extending this in different ways to work towards the goals outlined in my last blog post.

This is about the simplest computer I can come up with which uses the 65C816 processor, but there is still quite a lot to go though.

Parts

All of the integrated circuits on this prototype are through-hole packages, suitable for use on a breadboard.

• WDC W65C816S CPU
• WDC W65C22S VIA I/O chip.
• Alliance AS6C4008-55PCN 512k x 8 55ns SRAM
• Atmel AT28C256 32k x 8 EEPROM
• Atmel ATF22LV10 PLD for address decoding
• 74AC04, 74AC245, 74AC573 for handling the multiplexed data bus and bank address bits
• 1.8432 MHz oscillator
• DS1813-5+ supervisory circuit

I also used the following equipment to program the chips, and view the result of the test program:

• TL-866II+ EEPROM programmer
• 8-channel Cypress FX2-based logic analyser

I’m working on Linux, and also use the following software tools:

• ca65 – for assembling code for the CPU
• minipro – for programming the EEPROM and PLD.
• galette – for assembling the logic definitions for the PLD.
• sigrok – for working with the logic analyser

Bank address latching

The 65C816 has a 24-bit address bus, and the top 8 bits are multiplexed onto the data bus. This is not a big problem, but it does mean that I need some extra glue logic compared to a plain 6502 system. A diagram in the 65C816 data sheet shows a suggestion for how to do this, and I’m starting with that implementation.

I used a 74AC04 to invert the clock, 74AC245 bus transceiver for the data bus, and a 74AC573 latch for the bank address (upper 8 bits of the address bus). This first page also shows the DS1813-5+ supervisory circuit on the reset line, which resets all of the components on power-up.

I’m aware that I should be generating a two-phase clock (there is some discussion about that here), though I can’t see any problems arising from the the delay on the inverted clock on this particular circuit.

Memory map and address decoding

The memory map for this computer is very simple. All of this will change, but I did need to start with something to get some code running.

Only the lower 19 bits of the address space are properly decoded, and mapped to one of three chips.

I used a programmable logic device (PLD) to implement this. It’s easy enough to do this with discrete gates, but a PLD will make it much easier to keep chip count and propagation delays under control as I add more features. I’ve written about programming PLD’s in previous blog posts (here, here and here), and I plan to use them to implement several features.

The particular part I am programming here is an ATF22LV10. These are still produced the time of writing, and operate at either 3.3 volts or 5 volts, which is useful for this project. They can be also programmed with the common TL-866II+ chip programmer, which I also use for programming EEPROMs.

The definitions for decoding this are below. I called this file address_decode.pld.

GAL22V10
AddrDecode

Clock  RW     NC     NC     A19    A18    A17    A16    A15    A14    A13    GND
NC     /RAMCS /ROMCS /IOCS  /RD    /WR    NC     NC     NC     NC     NC     VCC

; ROM is the top 8 KiB of bank 0.
; prefix is 0000 111xxxxx xxxxxxxx
ROMCS = /A19*/A18*/A17*/A16*A15*A14*A13

; IO addresses are mapped below ROM in bank 0.
; prefix is 0000 110xxxxx xxxxxxxx
IOCS = /A19*/A18*/A17*/A16*A15*A14*/A13

; RAM is selected for any address does *not* match 0000 11xxxxxx xxxxxxxx
RAMCS = A19 + A18 + A17 + A16 + /A15 + /A14

; Qualify writes with clock
RD = RW
WR = Clock*/RW

DESCRIPTION

Address decode logic for 65C816 computer.

ROM is top 8K of bank 0, I/O is 8k below that, everything else is RAM.

I assembled this with galette:

galette address_decode.pld

This generates a .jed file, which can be used to program the chip. The ATF22LV10 is not on the compatibility list for my programmer, but it works just fine if I identify it as an ATF22V10CQZ.

minipro -p ATF22V10CQZ -w address_decode.jed

Firmware and testing

I came up with a simple test program which would require working ROM, RAM, and I/O. The CPU boots to 6502 emulation mode, and this test program does not access memory outside bank 0.

I created a linker configuration file which mostly reflects the system’s memory map, and called it system.cfg.

MEMORY {
    ZP:     start = $00,    size = $0100, type = rw, file = "";
    RAM:    start = $0100,  size = $7e00, type = rw, file = "";
    PRG:    start = $e000,  size = $2000, type = ro, file = %O, fill = yes, fillval = $00;
}

SEGMENTS {
    ZEROPAGE: load = ZP,  type = zp;
    BSS:      load = RAM, type = bss;
    CODE:     load = PRG, type = ro,  start = $e000;
    VECTORS:  load = PRG, type = ro,  start = $fffa;
}

I then wrote this test program, main.s. The program runs from ROM, and writes a hard-coded value to one of the output ports. It then stores a 0 in RAM, then increments it and stores that to an output port in an infinite loop, which will show a counting pattern if RAM can be read and written.

PORTB = $c000
DDRB = $c002

SOME_ADDRESS_IN_RAM = $beef

.segment "CODE"

; Write 42 to VIA port B
reset:
  lda #%11111111 ; Set all pins to output
  sta DDRB

  lda #42
  sta PORTB

; Start at 0, and increment a value in RAM
  lda #00
  sta SOME_ADDRESS_IN_RAM

loop:
  inc SOME_ADDRESS_IN_RAM
  lda SOME_ADDRESS_IN_RAM
  sta PORTB         ; Output the value
  jmp loop

nmi:
  rti

irq:
  rti

.segment "VECTORS"
.word nmi
.word reset
.word irq

I assembled this with ca65, then linked it with ld65. As far as the assembler knows, it is generating code for a 6502 CPU, not a 65C816. I am not using any advanced features, so this should be fine for this test.

ca65 main.s
ld65 -o rom.bin -C system.cfg main.o

This outputs an 8 KiB binary file, which is the amount of ROM space mapped in the computer’s memory. The chip is actually 32 KiB though, so I filled the rest with zeroes.

truncate -s 32768 rom.bin

I then wrote the program to the EEPROM using minipro.

minipro -p AT28C256 -w rom.bin

I ran this program, with a logic analyser connected to port B of the 65C22 VIA. The capture below shows the initial hard-coded number, followed by the counting pattern, which indicates that everything is working as expected.

Wrap-up

I’m starting simple here, so that I have a working base system to modify. Everything seems to be working fine on breadboards, so I’ll proceed with that for now. I may still need to re-build everything with shorter wires, since this is getting quite difficult to work with.

It is also worth noting that most of the components here should run at 3.3 volts, which is where this project is eventually headed. Only the clock, reset IC and EEPROM would need to be substituted at the moment.

This blog post is only a snapshot of the project. You can check the current status on GitHub here.

The 65C816 is an interesting processor from the 1980’s. I recently wired one up on a breadboard, and I’ll be blogging here about my attempts to build something useful with it.

How I got here

I spent a couple of months last year designing and building a computer based on the 65C02, an old 8 bit processor. I wanted to create a hardware platform for running 6502 assembly programs. This was successful, and I learned a lot about how computers work while building and programming it.

This has got me interested in the 65C816, which is a later extension of the same architecture. The only well-known systems which used it were the Super Nintendo and Apple IIGS. From a programmer’s point of view, this chip seems far more capable than its pre-decessor, though developer tools are a bit scarce.

New project goals

My plan is to build a modern computer which uses the 65C816 CPU, so that I can really learn how it works.

Over a few revisions, I am aiming to build up to a system with a serial connection for simple text I/O, a modest clock speed, and 1 megabyte of static RAM.

I’ve researched some existing designs, and settled on two simple constraints to give this project its own character.

1. Use only in-production parts.
2. Don’t use an FPGA or microcontroller to bootstrap the system.

This will ensure that I am learning what it takes to build a computer around this processor, and not just offloading the tricky parts to more powerful device with better tooling. I also hope this increases the accessibility of the design, since anybody could build it without needing to obtain obscure retro parts.

I am not avoiding programmable logic entirely though. Where classic designs often used custom chips, I will be using ATF22V10 PLD’s. I will also leave the door open adding video output via a microcontroller-based terminal emulator in the future.

Lastly, I’m selecting parts with a view to converting everything from 5 volts to 3.3 volts part-way through the project, which will open up many possibilities.

Other than the Apple IIGS and Super Nintendo, there are three hobbyist designs which I’m using for inspiration:

• BCS Technology’s proof of concept W65C816S computers.
• Gordon Henderson’s Ruby ‘816 project.
• Adrien Kohlbecker’s 65C816 project.

A quick note about open source

It’s never been a stated goal of this blog, but wherever possible, I use open source tools, and produce open source software. I expect that to be difficult for this project. The 65C816 not widely used, and most hobbyist code for the CPU is licensed as source-available freeware, with restrictions on commercial use.

I would like to be able to release my code under an OSI-approved open source license, and have the option of incorporating copyleft code later on. Unfortunately for me, it seems I will need to write a lot of low-level code from scratch to get this kind of licensing certainty. I will be working primarily in assembly language, though C code would be very useful, so I will certainly be spending some time investigating compiler options along the way.

A smoke test

This is the first iteration of the design, which will live for now on a series of solderless breadboards.

This is simply a 65C816 processor, being fed a hard-coded NOP (no-operation) command. When it runs, the lights show the address bus counting upwards, as the CPU program counter increases.

The RDY, ABORTB, and BE inputs are connected to +5v through resistors, while RESB, NMIB, IRQB are connected to +5v directly. The NOP opcode is fed to the CPU over the data bus, by making the pattern 11101010 (0xEA) with 1k resistors. The schematic for this is below.

The clock is an LM555 timer, fed through a 74AC04 inverter, because the rise and fall times on a 555 are not fast enough. The clock speed is 6.9Hz (that is not a typo), since R1 is 10 kΩ, and R2 is 100 kΩ, and the capacitor is 1 µF.

Next steps

This is going to be a long project, which I will develop incrementally. The next step will be adding ROM, RAM, glue logic and a 65C22 I/O chip.

I am also testing alternatives to the 65C51N UART chip, which I found quite limited when I used it for my last project.