Hi all,

I'm thinking about my first 6502 project. I would like to use programmable logic for address decoding so I can change the memory map on the fly, and also make the hardware more simple. But I'm going round in circles trying to find the best solution.

I've been considering GALs, such as a 22V10 or CPLDs, such as an ATF150. But I have also read comments asking why someone would want to use ancient technology, and why not use something modern that is JTAG programmable. But I don't know what they are suggesting, other than FPGAs. GALs and CPLDs seem such a good idea; something you can program once (and reprogram) and is non volatile. What is the modern alternative? One suggestion I saw was a Cypress PSoC, but using an device with an Arm Cortex processor to replace 3 or 4 TTL chips seems like insanity to me!

So I guess what I am asking is what is the best solution for address decoding on a small 8-bit 5V design, with a low-cost programmer and simple toolchain? I would prefer the tools to run on Linux. Buying another computer to run Windows is going to cost more than a programmer.

Altera USB Blaster programmer is availabel on eBay for around $5. The Quartus development tool is free download and run on Linux. The big hurdle is getting working used EPM7032S or EPM7064S (PLCC44 package). I was successful get several batches of CPLD on eBay, but that source has dried up. UTSource.net seems only place I can get them reliably. Used EPM7032S there is around $1.50. CPLD is so useful that once I've identified a reliable source I'll buy 50 or even 100 of them and be good for a long time. Just-in-time is not a viable approach with retro computing, especially now.
Bill

CPLDs are also programmed via JTAG, plus both the ATF22V10 and ATF150x series are still being produced to this day so i wouldn't call them "ancient"... as in you don't HAVE to buy old-stock part.
buying them brand new can be more expensive, but that's ultimately your choice.


if you have a very simply memory map and almost no IO Devices then you could probably get away with just using 1 or 2 22V10's. (which can be programmed with a TL866 II Plus, which is a Programmer i highly recommend if you don't already have an alternative as it supports a crap-ton of Chips, from Microcontrollers, to ROM, FLASH, PLD/GALs, it can even test 74xx Logic ICs)
Though if you plan on making more complicated systems in the future then getting into CPLDs now seems like a good idea.
for the Programmer i personally use the ATDH1150USB-K, and while it works pretty good it's unnecessarily expensive so also recommend going with the Altera USB Blaster.

also the choice of what specific Chip to use is almost always about the amount of pins you need, rather than the Macrocells of the PLD/CPLD.
for example my simple SBC uses an ATF1504, and while the circuit only takes up around 10 out of 64 Macrocells i use almost all of the pins (34/36).

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Unicorn Electronics also lists them as potentially in stock on their website for a fair price.

http://unicornelectronics.com/IC/GAL.html

But their stock is variable.

Well, there are numerous devices out there for PLD, CPLD, FPGA, etc., along with many different tool requirements, both hardware and software.

As you noted, a small 8-bit 5-volt design, that doesn't tell us much. However, I've been using the ATF22V10C with good results. A single one of these can provide full address decoding (aka single glue logic) for RAM, ROM and 5- I/O selects (at 32 bytes wide in my implementation) and clock 2 qualified Read and Write signals for memory and non-65xx I/O devices.

If you can elaborate on what your intended small 8-bit design is, that might help. Also, what tools do you currently have that might be able to program a PLD as well as a EPROM/EEPROM or other devices?

_________________
Regards, KM
https://github.com/floobydust

Thanks for everyone's advice. This is all very new to me, so I'm starting slowly. I did initially post this in 'Newbies' because I'm right on step 1.

My first project will probably be more or less a Grant Searl 6502 copy. I would like to work towards making a 'headless' Acorn Atom. So, a 6502 board with the same memory map and I/O capabilities but communicating over serial. I want it to run Atom BASIC. I'll have to modify the Atom ROM to redefine OSWRCH and OSRDCH to read from a UART that I will put on the board. I will probably opt for a 65C02 and CMOS components, but it does need to be 5V and be IO compatible with a real ATOM (I have a lot to learn here. I'm a software person and still have a lot to learn with electronics). I'll use a single RAM chip of course, not stacks of 2114s like my real Atom. So the address decoding is really to select RAM / ROM, a couple of 6522s and a UART. But I'd like to be able to (for example) switch to the Atom BBC BASIC ROM memory map or anything else I want to test and play with.

But step 1 is to see if I can make the most basic 6502 board with programmable logic for address decoding, and running Grant Searl's Microsoft basic. It's essentially a 'hello world' of 6502 design, and I'll take it from there.

I have a USB Blaster and a TL866 II+. I also have a G540 programmer somewhere, which I never managed to get to work. I would really prefer working with components that are still being manufactured and tools that are still available and reasonably 'current'.

Thanks for the advise.

Well, you can certainly start with a single glue chip and use the ATF22V10C. My C02 Pocket uses one and carves out the I/O section starting at $FE00. Daryl Rictor also has a 22V10 single glue logic showing on website as well. You don't need to reinvent the wheel on this one... my github page has all of the details for my implementation. You can easily modify the WinCUPL source to create your own memory map.

_________________
Regards, KM
https://github.com/floobydust

I think this is probably exactly what I want. Thank you for your advise. I might try building one of your CO2 Pocket boards. All helps with the learning process.

Another Forum member (edzard) has built a C02 Pocket SBC... he might have a PCB leftover. I also have gerber files for the PCB, so you can buy them from any PCB house... both edzard requested them when he ordered the PCBs from ExpressPCB and BDD also converted the original file to Gerbers. Here's a link to the that thread (gerbers are there):

viewtopic.php?f=12&t=6641&hilit=c02+pocket#p84427

If all else fails... I've got a few of them already built with RTC/CF-Card adapters... I can always make a couple available.

_________________
Regards, KM
https://github.com/floobydust

Thanks a lot. I'll let you know how I get on.