I'm looking to try to add SRAM to a MMC5 board (ELROM) along with a battery to make it basically a EKROM board.
Looking at the pinouts for the DIP28 RAM I've got from dead cart pulls, it's pinout, and the pinout of the PRG ROM it looks like I can piggy back a lot of signals for RAM pins 2-14, 15-19, 21, and 23-25 right from PRG ROM. It looks like I can leave RAM pin 1 NC.
RAM pin 22 looks like it needs to go to a ground source.
Any suggestions on where route RAM pins 20, 26 and 27 to (WRAM /CE, WRAM /WE and WRAM /CE) per the pinout here
http://nesdev.com/NES%20ROM%20Pinouts.txt to?
Any suggestions on what circuit to use for the battery backup for the SRAM? Hopefully someone has a schematic.
Thanks! It appears the text doc I was using and linked in my post was a bit out of date/incorrect.
It looks like on MMC5 PROG ROM pin 30 which I was going to use for +5V for Pin 28 of the RAM is A19 so I'll have to pull 5V from the cart connector/battery.
RAM pin 27 I think I can pull from the cart connector pin 56 which should be WRAM /WE, correct?
RAM pin 26, which is /cs2 (A13) with what you linked it looks like I can piggy back right off of PROG ROM pin 28 so that's a plus.
I'm still a bit stuck on what I'd run RAM pin 20 (/CE1) to. I didn't see /CE1 on the MMC5 pinout you linked. Would it just be MMC5 pin 71 which is labeled PRG RAM 0 /CE?
Any tips on what circuit to use for the battery backup that I'd connect to RAM pin 28 and also 5V from the cart connector?
RAM pin 26 is +CS2... Double checking, you're making EKROM, not EWROM, right? And so you're piggy-backing an 8 KiB RAM, not a 32 KiB RAM?
If so, it looks like the MMC5 generates all the signals you need; you don't need to synthesize anything.
Also, remember with RAMs you can exchange all the address lines amongst themselves, and all the data lines, with no loss.
So the only funny signals left on a RAM are /CE, /OE, /WE, +CE, and +Vbb, and those are probably connected to "PRG RAM 0 /CE", ground, "PRG RAM /WE", "PRG RAM +CE", and "PRG RAM VCC" respectively.
Yup just trying to do EKROM.
I only need 8k for it to work but I've got chip pulls of different chips like Panasonic MN4464-08LL and NEC 64S20. My understanding is they both share the same standardized pinout.
I only need 8k SRAM for the save game function for what I want to make.
Is it possible that the MMC5 chip handles the battery/VCC handoff?
The reason I ask is RAM pin 28 on a famicom cartridge (EWROM) has direct continuity with pin 56 of the MMC5 chip which is labeled PRG RAM VCC. MMC5 pin 57 runs through a 1k resistor and to the Vbb+. With the cartridge out of the system I'm getting roughly battery voltage at RAM pin 28. So if the MMC5 chip handles the voltage going to the RAM it would make my battery circuit much easier.
Also here's what I've got for those signals for the RAM double double checked with my DMM on this famicom cart.
RAM pin 28 = RAM V = MMC5 56
RAM pin 27 = /WE = MMC5 76
RAM pin 26 = +CS2 = +5V = Pin 56 NES
RAM pin 22 = /OE = GND, connect to RAM pin 14
RAM pin 20 = /CS1 = MMC5 71
AaronE wrote:
Is it possible that the MMC5 chip handles the battery/VCC handoff?
Yes, that's why the pinout has "PRG RAM VCC" and "+batt[ery]" inputs.
Quote:
RAM pin 26 = +CS2 = +5V = Pin 56 NES
Odd. I wonder what MMC5 pin 83 was intended for, then?
Ah good call on using pin 83. I was also using some other references for making final fantasy III where they put CHR RAM in place of CHR ROM and rewired it a bit.
Well, maybe pin 83 is something else funny?
EWROM shouldn't connect PRG RAM pin 26 to either MMC5 pin 83 or to Vcc ... it should be MMC5 PRG RAM A13 (pin 69).