It begins! (Wren Prototype build log)

[Paganini]

gfoot wrote:

Rather than using ZIF sockets, I make little straps out of paper and selotape and put one around each ROM IC, so that I can more easily pull them out:

This works *really* well - so much better than those chip-puller tongs things. But I always forget to do it and end up messing up my pins anyway!

[Paganini]

John West wrote:

It's a pattern that we see all too often in computing. The final design makes no sense in itself, but only as the end-product of a series of short-term decisions. With each chip, the new functions are squeezed in wherever they will fit, with no concern for what's going to happen on the next one.

This pinout hodge-podge has driven me crazy ever since I started in this hobby. Thanks for your explanation; it makes a lot of sense!

[barnacle]

Making it up as they went along, I think, but trying to maintain backwards compatibility as far as possible. And in that I think they mostly succeeded until you get to the 1MB and bigger chips which have, as observed, pins all over the place.

Neil

[barnacle]

Paganini wrote:

But I always forget to do it and end up messing up my pins anyway!

My homebrew programmer has a zif socket, but my eeprom just has a socket permanently attached. The pins haven't fallen off that yet, and a PO#1 screwdriver is a convenient lever

Neil

[allisonlastname]

barnacle wrote:

My homebrew programmer has a zif socket,

That's the other reason I bought them. I already made a post about my programmer, and I should probably update it or make a new one when I move it to stripboard.

[allisonlastname]

Day 2: address wiring is finished!

I ended up swapping A10 and A11 on the RAM, because it made the wiring easier and doesn't affect operation at all. Next is data bus, then a few days of waiting around for parts to arrive.

[allisonlastname]

Decided to flip the VIA round because all the IO pins were on the side closest to the processor and the control/address/data lines were closest to the edge of the board. This way, the wiring is neater and it's easier to rig stuff on another board to the IO pins.

[allisonlastname]

Also, I've just noticed that the data pins are in the same place on the 6502 and 6522. Not sure when this would be useful, but it's interesting

[barnacle]

allisonlastname wrote:

Also, I've just noticed that the data pins are in the same place on the 6502 and 6522. Not sure when this would be useful, but it's interesting

Anytime you're dropping a data bus onto a PCB layout... parallel pins really are the easiest to manage. (It's unfortunate that the data and signal pins on a DIP 6502 don't map onto a JEDEC memory (eeprom/ram) chip, but we can't have everything in life.)

Neil

[allisonlastname]

Day 3! Didn't get as much done on Wren itself because I spent all afternoon messing about with AVR programmers and shift registers. However, the parts I was waiting on did arrive, so I should be able to get a fair amount done over the weekend. I've already wired up the clock and caught a mistake in my schematic (/MRW was being generated as not /MRD instead of /MRD nand phi2).

[BillO]

allisonlastname wrote:

Day 2: address wiring is finished!

I ended up swapping A10 and A11 on the RAM, because it made the wiring easier and doesn't affect operation at all.

Good observation. You can mix up address and data lines on RAM 'till the cows come home. It's sometimes a handy thing for efficiently laying out PCBs.

Not so much with ROM unless you are willing to shuffle the code around too.

[allisonlastname]

Day 5. Wren itself is pretty much done, and all that's left to do is rebuild the flash programmer. After that I can start the inevitable debugging.

[RegH]

barnacle wrote:

Paganini wrote:

The pins haven't fallen off that yet, and a PO#1 screwdriver is a convenient lever
Neil

I wonder how many got the PO#1 reference - quite a few I suspect

[RegH]

I haven't come across the Wren prototype (eponymous surname??) but the RC6502 pcb that I build had many
power supply decoupling capacitors. Admit to not having done one on a breadboard but I assume you will need them.
Apologies if they are awaiting placement.
From memory they were 100nF but could be wrong; hopefully someone more knowledgeable than me can help with quantity/values.

Good luck with the project.

[and3rson]

gfoot wrote:

My wife is very good at sewing, and I originally asked her to make these things for me out of ribbon, but she laughed and told me to just use paper instead, so here we are!

You can ask her to make a cute little sweater for the chip to double as a pulling handle.