MMC5 Reproduction

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roundy
Posts: 9
Joined: Tue May 09, 2023 1:41 pm

MMC5 Reproduction

Post by roundy »

Hi there

I have a faulty MMC5 game (Gunsight on Famicom) so rather than it go to waste I was hoping to repurpose it as a PAL Castlevania III

I am, however finding conflicting information on the MMC5 PRG and CHR pinouts to wire my EPROM up to.

So in this page from HVC-Man (viewtopic.php?p=257334) the following notes are given

MMC5 (mapper 5):
PRG socket:
Bend up pins 1, 2, 24, 30 and 31
Solder Pin 1 to Hole 30 (A19)
Solder pin 2 to hole 24 (A16)
Solder pin 24 to hole 31 (/CE)
Solder pin 30 to hole 1 (A17)
Solder pin 31 to hole 2 (A18)
CHR socket:
bend up pins 1, 2, 22, 24, 30 and 31
Solder Pin 1 to Hole 30 (A19)
Solder pin 2 to hole 24 (A16)
Solder pin 22 to hole 31 (/CE)
Solder pin 24 to hole 22 (A13)
Solder pin 30 to hole 1 (A17)
Solder pin 31 to hole 2 (A18)

But on the Mask Rom Pinout page (https://www.nesdev.org/wiki/Mask_ROM_pinout) I can see MMC5 has the same pinout for CHR and PRG and using the above example on PRG "Solder pin 24 to hole 31 (/CE)" the mask rom pinout states that Pin 24 is /OE and hole 31 is /CE, as far as I can see shouldn't Pin 22 (/CE) go to hole 31 (/CE) ?

Another page advises this is the correct way to use the MMC5 pinout (https://moddingfridays.bleu255.com/Metal_Slader_Glory)

Furthermore I've found this post (https://www.tumblr.com/hackitup/3271320 ... s-properly) which shows another way of wiring up MMC5 CHR and PRG carts and to my mind looks to match up to the Mask Rom pinout guide, but at this point I'm just confused!

Can somebody advise me which is the correct way to wire up MMC5 CHR and PRG's please?
roundy
Posts: 9
Joined: Tue May 09, 2023 1:41 pm

Re: MMC5 Reproduction

Post by roundy »

I'm using AM27C020 (max address is A17) EPROMs so if my reckoning is right is the below the correct configuration?

Pin 1 Lifted
Pin 31 Lifted

Pin 2 -> Hole 24 (A16)
Pin 30 -> Hole 1 (A17)
Pin 24 -> Pin 16 (OE to GND)

Pin 22 left down (Mask Rom pinout suggests this is the same on MMC5 as CE?)
roundy
Posts: 9
Joined: Tue May 09, 2023 1:41 pm

Re: MMC5 Reproduction

Post by roundy »

Okay so I've cracked it

Pin 1 Lifted
Pin 31 Lifted

Pin 2 -> Hole 24 (A16)
Pin 30 -> Hole 1 (A17)
Pin 24 -> Hole 31

All I'm struggling to understand is why Pin 24 goes to hole 31, on all other boards it seems to need to go to ground/Pin 16 so I can only assume by moving it to pin 31 interfaces it with the MMC5 chip but I don't know for what purpose.

Either way, it's all working :D
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Memblers
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Re: MMC5 Reproduction

Post by Memblers »

Glad to hear you got it working. You didn't say what you did with pins 1 and 31 though (VPP and /PGM), and you don't want to leave any inputs floating (unconnected). the M27C010 datasheet says for read mode, pin 1 (/PGM) can be connected to VCC (pin 32) or ground (pin 16). Pin 31 is a "don't care" input, and may be connected easily to pin 32 or even pin 30.
roundy wrote: Sun May 14, 2023 6:58 am All I'm struggling to understand is why Pin 24 goes to hole 31, on all other boards it seems to need to go to ground/Pin 16 so I can only assume by moving it to pin 31 interfaces it with the MMC5 chip but I don't know for what purpose.
I don't know why they use a different ROM pinout for MMC5 games. I just did a comparison, and MMC5 32-pin maskROM has the same pinout as SNES 32-pin maskROM. So, it's like they switched over at some point. Maybe it was slightly cheaper/easier to make them that way.
roundy
Posts: 9
Joined: Tue May 09, 2023 1:41 pm

Re: MMC5 Reproduction

Post by roundy »

Memblers wrote: Mon May 15, 2023 7:51 pm Glad to hear you got it working. You didn't say what you did with pins 1 and 31 though (VPP and /PGM), and you don't want to leave any inputs floating (unconnected). the M27C010 datasheet says for read mode, pin 1 (/PGM) can be connected to VCC (pin 32) or ground (pin 16). Pin 31 is a "don't care" input, and may be connected easily to pin 32 or even pin 30.
So this is what I did, however I do see all sorts of conflicting information around this also.

I've seen various people mention leaving pins floating is not an issue, whereas just as many others insist on always tying them to something. I guess the question is, what does leaving floating pins do in a negative manner that would impact how the game runs/plays?
tepples
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Re: MMC5 Reproduction

Post by tepples »

The answer is "It depends." You're supposed to leave unused outputs floating, leave pins documented as "no connection" (N.C.) floating, and tie unused inputs to an appropriate rail (power or ground).

Unused outputs must be left floating to avoid excess current when two parts try to drive the same wire to different voltages.

Unused inputs must be tied to a rail to avoid rapidly crossing the gap between voltages detected as low and voltages detected as high. Staying in this gap causes excess switching, which causes things within the chip to draw excess current.
roundy
Posts: 9
Joined: Tue May 09, 2023 1:41 pm

Re: MMC5 Reproduction

Post by roundy »

tepples wrote: Tue May 23, 2023 9:47 am The answer is "It depends." You're supposed to leave unused outputs floating, leave pins documented as "no connection" (N.C.) floating, and tie unused inputs to an appropriate rail (power or ground).

Unused outputs must be left floating to avoid excess current when two parts try to drive the same wire to different voltages.

Unused inputs must be tied to a rail to avoid rapidly crossing the gap between voltages detected as low and voltages detected as high. Staying in this gap causes excess switching, which causes things within the chip to draw excess current.
Thank you, that’s really helpful.

I find a lot of the guides online say “do this” but it’s never actually covered off as to why things are or aren’t done. I know some of my repros certainly have pins left floating as per whatever guide I followed at the time. Now I have a better understanding of the hardware I’ll certainly tie the pins going forward.
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