6502.org

You should convert “all those wires” project into pc boards. Somewhere on this forum are several PLCC-to-DIP boards for CPLD. I myself have something like 8 “all those wires” pcb designs and used them for various experiments.

CPLD is very powerful even with the modest 64-macrocell ATF1504. For an example, it can accommodate both a serial port and memory manager in one chip.
Bill

The pretty side: Wires: I have indeed taken care to connect all Gnd and all Vcc pins. Each with a 100nF of its own. You may be able to see that I put a square wire around the PLCC for each of the rails. Don’t think I’ll do it like that next time, it really uses too much real estate like that.

PCBs… yeah, I’ll try that at some point I guess. I don’t feel like messing with chemicals myself, and waiting to get them in the mail also sounds annoying. And I do enjoy some good old fashioned manual work.

Martin

Wishful thinking: I’m thinking about using a 1508 as a video controller. Maybe like the VGAtonic, with a separate video RAM, accessed by IO ports, MSX style.

I think the wired side is the “pretty” side. I really like the power and ground rings, you should have no troubles with ground bounces. Brush it with isopropyl alcohol and the wired side will look like an art piece. I agree there are something therapeutic about soldering carefully one wire at a time.
Bill

Not wishful thinking at all. With help of a dual port RAM, you can do video text display in VGA even with 1504.
Bill

I thought I might as well post an update on my CPLD adventures.

The first project was a programmable clock divider to control the baud rate of a SIO/2 dual serial IO. It just barely fits on a 1502 and uses the upper and lower nibbles of one IO port to select the two divider ratios. If I ever find I need more dividers (75 baud anyone?) then I can drop in a 1504 without changing the wiring. Nice!

I posted a picture of my "prototyping board" a few weeks ago with the two rings around the PLCC for ground and Vcc. I found this took up too much real estate on the perfboard, so for the SIO/2 board, I had two solder points in the centre leading to the 2x4 power supply pins. The decoupling caps were still on the outside. The result works but it’s pretty chaotic.

Yesterday I finished my take on the paged 512k RAM/512k Flash module. The page registers and logic around just barely don’t fit on a 1502, so a 1504 it was.

I’m a lot more happy with the wiring arrangement, let me share some pictures…
Oh, there’s a 10 pin header for JTAG reprogramming of the CPLD. In the end I decided that I’d rather have fewer wires around and remove the chip from the socket if I ever need to change its programming.

Wouldn’t have gotten this far without the advice on this forum

Martin

Beautiful!

In fact, I would probably try to avoid bending all the green and blue wires at right angles, and just run the buses with shortest path.

That's a good looking prototype board. The power and ground ring are best way of dealing with the relatively fast CPLD.
By the look of it, you are emulating RC2014's 512K RAM/ROM board. I do believe it is possible to fit the design in an ATF1502, but you need to have freedom in pin assignments for the fitter to pick the right fit.
Bill

Glad you like it But… it’s a computer! They’re meant to be all angular, no?

Kidding aside, there were several reasons for not using shortest paths here. 1. approaching pins with a minimum risk of shorting to neighbouring pins. In particular since many pins on the DIP sockets and the bus have two leads, above each other or opposite. 2. I want to avoid running wires over holes that I might need to thread another wire through later on. The latter issue can often be avoided by working in the right order, but that requires more premeditation.

But point taken, I wouldn’t have needed to make actual angles there. The transients are terrible. You can see each edge of the clock signal on every output, even with twice as much decoupling You’re right. Spencer writes about his board "The circuit is largely a reproduction of the ROM/RAM part of Sergey Kiselevs Zeta 2, with some modifications inspired by Dr Scott Bakers ROM/RAM Module." Putting the MMU logic on a CPLD is not new either, I was inspired by https://github.com/skiselev/Z80-512K/ but redid the coding in Verilog instead of VHDL. The main reason for using a CPLD (apart from having an excuse to learn about programmable logic) is to avoid the otherwise commonly used 74HCT670 register file ICs which you would need two of, at a cost about 11USD a piece

Little twist: the official pinout of the flash and the RAM are a bit different for the most significant couple of address bits. That would have led to some awkward wire crossings. I stuck to the correct pinout of the flash so I would be reading what my external programmer wrote. But the address bits of the RAM are permuted to simplify the wiring. The only wire that does not run parallel is the write enable. Yes, it fits exactly, using 32 of 32 macrocells. I went for the more wasteful 1504 out of pure laziness, to simplify the point to point wiring and continuity testing, and reduce the number of differently coloured wire coils flying around on the work bench simultaneously.

Martin

Thanks, I've been trying to remember the part number for that register file. Mouser Europe have some in soic-16 (which suits me) for €1.50, or in dip16 for €2. Sadly, the part is way beyond not recommended for new designs; it's obsolete so while it's available, it won't be for long. (Digikey are plus the Rochester premium and probably minimum order as well).

I'm sure I've come across a 16x16 equivalent, though, once upon a time. Now that would be handy...

Neil

Spencer’s 512K RAM/ROM board is quite expensive, driven by high cost of 74hct670. ATF1502 is around $3, so it can significantly lower the cost had he chosen to go with CPLD, although Spencer may not consider CPLD retro enough.

512K RAM/ROM is mainly used by RomWBW, but only as 32KB banks. The CPLD design simplifies tremendously for 32KB banks which can free it for other functions resulting in tighter integration of processor, memory and IO.
Bill

Are you referring to the '670 4x4 register file? Happily -- and somewhat surprisingly -- the HC version of this chip is still listed as active and in stock at Digikey.

<sigh> And an utterly charming little chip it is! Years ago I posted about it. Too bad the AC version is no longer available...

-- Jeff

I always try to match the 'official' address and data pinning designations (for debugging and the archeologists), but it really does not matter... Sometimes you can make the layout work much better by picking whatever works.