6502 : Move to a processor board

 It is great to have a working circuit outputting to a terminal screen and we need to work on it, primarily to speed it up using a real clock and to do proper input/output.

My initial attempts to speed the circuit up were unreliable necessitating wiring checks amongst other tests.  In fact I am not sure why the circuit was unreliable, it could have been wiring, or my Arduino nano clock or the ACIA/terminal session hanging.  At any rate the reason for using soldered boards is to prevent wiring issues and this led me to build a second board for the CPU, ACIA and control logic.

For the new board I chose an alternative strip board which provided me with 4 connection holes available on each side of the 6502 / ACIA  chips.  This makes CPU/ACIA connections much easier to specify.

The only downside is that I dont have +5V and GND rails provided for me so I have to "daisy chain" power connections.  I underestimated the work involved with these and had to amend the design during soldering.  In future I should allocate some spare tracks for power wires.  Other than this problem the soldering is easier than the memory board as a lot of address wires are not required on the board.  Off board connections are provided from the CPU to the memory board.  There is also a spare row so that I can add a lower row of chips in the spare space at the bottom of the board and connect them to the CPU.

Once the board was soldered I could check the quality as usual by testing sdjacent shorts and circuit testing each board connection to make sure there was conductivity.  Annoyingly there was one short which I missed and I later had to take everything apart to check it again later.

With soldering finished I could connect the new processor board to the memory board using dupont ribbon cables and test functionality using the same Arduino Mega sketches which I used to develop the breadboard version.  Having a structured set of tests makes the process a lot easier.  As mentioned above there was a shorted connection which meant the read-write pin was always in write-mode stopping the CPU from running.  Once this was corrected and I had a working board I could attach my boards to their perspex backing board, hopefully making this a more permanent board.

The board isn't nearly as elegant as I thought it would be but I am hoping it will be more reliable.  On the picture you can see that the memory board is above the processor board and there is more space below for other components / boards.

At this stage the Mega is still connected.  It is needed to download programs into ROM.  Eventually I hope we will have a stable monitor program in ROM which is capable of downloading programs to RAM for testing.

It can also be seen on the picture that the clock is separate.  Initially I have used Ben Eaters 555 timer based clock but it only runs upto 500Hz.  The W65C02 is capable of running at 1 MHz or more so I have a 1 MHz clock chip.  However if I just substitute the 1 MHz clock for the Ben clock it doesn't work.  I need to be able to slow down the 1 MHz clock to see how fast I can run it.  To do this I purchased some CD4020 binary counter chips.  Using one of these chips I can reduce the clock speed by a factor of 32768 to about 30Hz.  Using a second counter I can reduce the speed further.

My breadboard version of the circuit worked upto about 5000Hz, first tests on the new processor board indicate that it will work upto 50KHz or more which is what I really wanted to see from this exercise.

Postscript 

[November 2022]The story didn't have a happy ending!  I couldn't get the ACIA serial interface to work reliably and progress on writing software slowed to a halt.  DON is now on hold.  The next step may possibly be careful debugging, but is more likely to be a replacement board.