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Using a W65C02 in a Vic-20

L

ast May my Vicky Twenty (Vic-20 clone) was progressing nicely. I claimed that all parts were new. At that point I had fitted a W65C02. A CMOS version of the 6502 which is still made today.

The computer didn't work when I first switched it on, and it turns out that the W65C02 is not a drop-in replacement for the 6502 in a Vic-20*. A couple of connections are different, a pull-up resistor is required on one line, and most importantly, the logic levels on the Vic-20 board aren't high enough for the CMOS processor which requires a good strong high.

The Byte Attic has worked on this problem and developed a circuit. He has been very open with this and it's now tried and tested.

His board uses through-hole components, so it's large, needs sockets to raise it and then overhangs the Kernal ROM. I was proud of the looks of my Vicky Twenty and wished for something more discreet. I found that the transceiver was available in a surface-mount package and wondered whether the components would fit below the processor. 

It turned out to be possible, and I'm very happy with the result.
It hangs just a few mm to the left. I've soldered the W65C02 directly to the adaptor board in order to keep the height to a minimum and not foul the Vic's keyboard.


Here it is in Vicky. The processor looks much better than the Kernal ROM I made for this machine. I have an EPROM adaptor board in progress, which will match the W65C02 adaptor.

One fun aspect of this processor is that it has some new instructions. These include push and pull of x and y, branch always, a new indexed addressing mode and some bit manipulation instructions. Here I'm trying out a few of them.


I've also tried the adaptor in an Electron. I'd like to try it in a BBC, but my 'B' and my Master both have the processor soldered directly to the board, and while they're working it's wrong to do unnecessary work.

Here it is in the Elk. I used it for a while, running a few games, it was rock steady. 
I've also run it for long periods of time in an original (2-prong) Vic20.

I've made and tested a few and have them (with Byte Attic's blessing) for sale here: 
https://www.ebay.co.uk/itm/284654961552


* Note that the WDC version of the 6522 VIA, the 65C22, does drop straight into a Vic, and I'm using a couple of those in Vicky.





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  1. Barnok1 September 2022 at 12:49

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  2. Barnok1 September 2022 at 12:50

    Great work on the board! Are you going to be selling any more of them? My Vicki Twenty would really look good with it. I made the same leap of logic that the 65C02 was pin compatible..... :)

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    Replies
    1. peahen1 September 2022 at 14:02

      The 65C02S is a successor to the 6502 and it seems very fitting to use a brand new processor in a Vicky Twenty. I've been making those adaptors with and without the transceiver and to be honest I think it's better without. (I stopped selling them when I realised that I was selling them for about what the components cost me!)
      So to go the 'bufferless' route, all you need to do is to change a couple of connections, add a pullup resistor. It's possible to make an adaptor using a socket, as Byte Attic himself demonstrates in this video (with timecode) https://youtu.be/BtxW8AWfh9s?t=1527

      Delete
    2. Barnok30 September 2022 at 07:47

      Thanks for the reply! Now I have a project for this weekend :)

      Delete

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