6502 : Add RAM and VIA

 My 6502 system now has a working screen but as yet no RAM.  I find the idea of a working computer with no RAM strangely intruiging, but it is time to move on.  On the face of it, adding RAM is easy as I have done it before following the Ben Eater tutorial and its setup is very similar to the ROM.

RAM

Indeed 62256 memory connection is mostly straightforward.  The chip is located adjacent to the ROM and address/data pin connections are the same.  I rewired two existing NAND gates so they were in the same configuration as Ben's tutorial and used an extra NAND for RAM address wire setup as Ben did.

All worked well, with the proviso that it took a long time.  I found it vital to update the wiring schematic (now Hardware revison 3) before starting and to list out all the changes to existing wiring before I started.  As the circuit becomes more complex, any faults introduced can be very hard to identify.  Loose wires coming out are quite difficult to diagnose but an incorrect wiring change is even harder.

Initially I just added the RAM and wrote a small program to test reading/writing addresses and using the stack in a subroutine call.  I then moved theACIA serial chip from address $0000 where it conflicted with RAM to $6000, which Bens tutorial had previously made accessible for the VIA chip.

It feels very good indeed to have a computer with RAM, ROM, screen input / output which I can program easily in assembler language.  

VIA

To complete my basic computer I need to connect the Versatile Interface Adapter (VIA) so that I can access other peripherals.  I am hoping that this will be my last device attached to the 6502 directly as I have concerns I could mess it up or break it if I do too much tweaking.

Although Ben's initial LCD connection to VIA is not required, it does provide a useful test and confirm whether VIA is working so I connect / test the LCD as previously.

VIA connections are not complex, 8 data pins, 6 address related and 4 control pins.  I need an extra logic gate, and I connected a HCF4069 hex invertor for this.  I moved the A13 address pin from ACIA to VIA so that it uses address $6000.  I then inverted A13 as input to ACIA which is now at $4000.

I found that updating my schematic was vital before making connections as it is VERY easy to confuse yourself.  It is also sensible to write down each connection you need to make beforehand.  Thirdly, testing regularly that the original functions still work provides confidence that important existing pins have not been dislodged by the work.

On completion of the connections I could check that ACIA worked at its new address and then connect an LCD to the VIA as per Ben's instructions and check it work.  All is good, our computer now has RAM and I/O capabilities :) :) :)