Smallest eeprom for MMC5 carts?

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Septr
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Re: Smallest eeprom for MMC5 carts?

Post by Septr » Sun Feb 23, 2014 10:10 pm

I think the implication was
as long as you have the socket wired this way, it will only allow 32k

Cause some dev carts that require rewiring like this aren't exactly very fun to keep opening up and rewiring every time a new size of memory is desired. I'm using ribbon cable to move the socket locations so it'll be a bit easer for me to just desolder and swap a wire or two later down the line. but I think a dip switch will be a good idea in my case.

Septr
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Re: Smallest eeprom for MMC5 carts?

Post by Septr » Mon Feb 24, 2014 12:48 pm

Ok got one more question. You say to ground the higher numbered A pins for using it as a smaller eprom. what if I use a 28 pin eprom like a 27c256, are those floating pins on my socket going to be an obstacle, or will the mapper even care that they are floating? I think you mean that when grounding thos pins I would be lifting the eprom pins away from the socket and grounding them leaving the vias on the circuit board floating, but then I have started second guessing myself and maybe you mean ground the pin on the actual board. Just need to be sure before I got on soldering the rest of my board up.

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FARID
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Re: Smallest eeprom for MMC5 carts?

Post by FARID » Mon Feb 24, 2014 1:11 pm

Whenever you want to make the EPROM smaller you must bend the high address lines and tie them to either GND or 5V
For example :
27C080 is 1024KB EPROM :
A19 --> GND (0) --> you can use its first half (512KB)
A19 --> 5V (1) --> you can use its second half (512KB)

A19 A18
0 0 --> 256KB
0 1 --> 256KB
1 0 --> 256KB
1 1 --> 256KB

A19 A18 A17
0 0 0 --> 128KB
0 0 1 --> 128KB
0 1 0 --> 128KB
0 1 1 --> 128KB
1 0 0 --> 128KB
1 0 1 --> 128KB
1 1 0 --> 128KB
1 1 1 --> 128KB

and so on ...
1 is 5V and 0 is GND

lidnariq
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Re: Smallest eeprom for MMC5 carts?

Post by lidnariq » Mon Feb 24, 2014 1:19 pm

Septr wrote:what if I use a 28 pin eprom like a 27c256, are those floating pins on my socket going to be an obstacle, or will the mapper even care that they are floating? I think you mean that when grounding thos pins I would be lifting the eprom pins away from the socket and grounding them leaving the vias on the circuit board floating
When putting a square peg into a round hole:
* Leave outputs floating (such as the higher address lines out of the MMC5)
* Tie inputs to ground or Vcc (unused high address lines going into the ROM)

Failure to do the former will damage the MMC5; failure to do the latter will result in erratic computation (and software crashes).

Septr
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Re: Smallest eeprom for MMC5 carts?

Post by Septr » Mon Feb 24, 2014 1:42 pm

Thanks for trying to help, but I don't quite understand. What is needed to be done to pins 1,2,3, 30, 31 and 32 on my socket if I want to use a 28pin eprom or even just a larger one with grounded A pins? Are those the inputs and output you speak of? Do I ground the A pin and leave the socket floating or ground the socket location?

I looked up the mmc5 pinout but all the labels are named stuff like CHR A12, PPU A1, and so on. How do I know what is input and what is output in regards to your instruction?


EDIT: Ok, I think something just clicked. Tell me if this is right....
The outputs you speak of that I'm sposed to leave floating are my socket pins, and the inputs you mentioned are the pins on the rom chip itself that need to be tied to either high or low depending on the desired half of the memory I want to access.

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l_oliveira
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Re: Smallest eeprom for MMC5 carts?

Post by l_oliveira » Sat Mar 15, 2014 3:51 pm

You only need to think about this:

The address lines are just a binary representation of a numeric address in the memory range.

Therefore while people are bombarding you with a lot of useful suggestions, in fact all you have to do is decide what you want to do with your memory chip.

Do you want to partition it ? Do you want to use just the beginning of it ? Want to use just the end of it ?

So, all the address lines you don't use must be connected to either +5v ( logic 1) or to GND (logic 0). lindariq just mentioned you should not leave them disconnected (floating) as they can receive static electricity and detect it as if it was a state change, crashing your program as the ROM will change what is being served to the CPU without the CPU requesting the said change.

Hope this is of help. :)

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