PRG RAM circuit
The iNES format implies 8 KiB of PRG RAM at $6000-$7FFF, which may or may not be battery backed, even for discrete boards such as NROM and UxROM that never actually had RAM there. This inspired some people on the nesdev.com BBS to come up with circuits to add PRG RAM to the original boards, so that games relying on it can run on an NES. The primary problem is in producing the enable signals for a 6264 or 62256 static RAM or compatible PSRAM.
On the forum, kyuusaku and Bregalad discussed PRG RAM decoder circuits built from 7400 series parts to approximate this behavior in an NES cartridge board. The first tries took two chips or had possible timing problems. They settled on the following circuits:
kyuusaku suggested a circuit based on a 74HC10 (triple three-input NAND) stick a pulldown on CE2 to take advantage of Phi2 going high-impedance during reset in order to "offer some write protection".
,-------------- ROM /CE | ____ /ROMSEL --+--| `-. | \ A14 ---------| )o-- RAM /CE | / A13 ---------|____,-' ____ +5V ------+--| `-. | | \ `--| )o-- ROM /OE | / R/W ------+--|____,-' | `--------------- RAM /WE Phi2 ---------+----------- RAM CE2 | < < "big R" < | GND ----------+----------- RAM /OE
He also suggested a circuit based on a 74HC20 (double 4-input NAND), which appears to be the same one in Family BASIC:
- Or you could just use a NAND4 to decode any active low memory, also using the /WE priority method. If this is done with a two gate 7420, the second gate could be used to invert r/w to prevent bus conflicts as in the circuit above. This is probably the *final* best way unless you happen to need the extra AND3 from the 7410 and have a positive CE.
- A = Phi2
- B = /ROMSEL
- C = A14
- D = A13
- Y = PRG RAM /CE
- PRG RAM /OE = GND
- PRG RAM /WE = Vcc or R//W, depending on the Family BASIC cart's write-protect switch
Kevin Horton suggested the same circuit.
You could also use the other gate to invert R//W for /OE on the ROM to prevent bus conflicts.
If you don't need bus conflict prevention, you can use a 74HC139 (double 2-to-4 decoder), which may be cheaper or have better timing than a 74HC20. This circuit resembles the decoder in Jaleco's discrete mappers (87 and 140), which uses a 74139 to decode a single mapper register to $6000-$7FFF.
- 1/E = GND
- 1A0 = M2
- 1A1 = A14
- 2/E = 1/Y3
- 2A0 = A13
- 2A1 = PRG /CE
- PRG RAM /CE = 2/Y3
The PlayChoice version of Mike Tyson's Punch-Out!! uses an extra IC to add battery-backed RAM. The digits in existing photos are hard to read, but it is believed to be a 74HC139. Its wiring has not been traced.
PRG /CE delay
One thing that can complicate adding PRG RAM to a board is the fact that PRG /CE and M2, used together to decode $6000-$7FFF, do not change at the same time. PRG /CE is the logical NAND of M2 and PRG A15. This is accomplished by sending M2 and PRG A15 into a 74LS139 two-to-four line decoder on the NES main board. This introduces a small delay of up to 33 ns between the time M2 rises and the time PRG /CE rises.
If this delay is too long it can cause unintentional writes to PRG RAM when writing to mapper registers $E000-$FFFF.
This is not a problem for the original cartridge hardware because the RAM chips used require a /WE (Write Enable) pulse of at least 50ns to 70ns depending on the chip. This means that the spurious /WE signal generated by this delay (MAX 33ns) will not be sufficient to trigger a write on the RAM chip. The circuits above give even more head-room as they tie PRG RAM /OE to ground and decode to /CE. The /CE to end of write timing is typically longer than the minimum /WE pulse width.
If your RAMs are faster than these timing specifications, your decoding logic must delay M2 by about 33 ns to match the PRG /CE delay, as in the 74139-based circuit shown above.