Hello everybody.
I cant seem to find any information about how to add a WRAM/SRAM to NROM cart. Are there any docs anywhere on how to do this? Im trying to make a Family Basic(V1) cardridge, but do not know how to decode 2kb ram(WE, OE, CE) and two prg_rom’s only one 74HC20.
You should wire SRAM /CE to NOT(A13 AND A14 AND M2 AND ROMSEL).
If you use a 8kb SRAM you will have a CE pin and don't need to invert the signal, so wire /CE to ground and use a 74HC08 to and the signals. If you use a 2kb (or 32kb) SRAM you will only have a /CE signal so you'll need a 74HC20 quad input nand gate to handle that. This should work (the SRAM is mapped at $6000-$7fff right ? If it's not then what I said does not apply).
Also instead of two ROMs, you can just use one 32K one decoded using /ROMSEL (or use the other NAND gate to make an inverter so that A14 and /A14 select which ROM /OEs)
BTW, the cart also uses /WE priority of the RAM to minimize gates, connect R/W to /WE and ground /OE.
I try make Family Basic devcart... But doesnt work.
Schematics of that cart:
Any ideas?
Does the ROM at least work? The signals are all connected OK, but to save power you may want to connect ROM /CE to /ROMSEL and ROM /OE to GND.
Have you tried directly connecting the 7420 output to the RAM /CE? That's all I can think of, just recheck your wiring.