nesdev.org

kevtris wrote:

horseboy wrote:

82atari5200 wrote:You have to use a 27256 instead of the 27128. and double up the file.

Why is that? That is strange because it worked just fine on my Yobo and Retro Mini. I am not doubting you here, just curious.

Also, on a side note, is there any reason you can't use a 27c512 for the PRG? The pinouts seemed like it should work just fine, but when I tried it it didn't work. Then I doubled it up and put it on a 27c1001 and it worked just fine.

if there's only 16K of CHR, you can indeed use a 27128. You do not need a 27256. Be that as it may, a 27128 has a pin on it that is most likely causing the problem. This is pin #27, the /PGM pin. A 27256 has A14 here.

So, the fix is to cut the route going to pin 27, and solder pin 27 to pin 28 (VCC). This will get the chip out of programming mode and probably make it work properly.

Pulling /PGM low will not erase or reprogram the part, however, unless you have 12.5V or whatever it needs to program on pin 1. It just won't work right until you pull it up to 5V.

Incidentally, you should always pull VPP (pin 1 on a 2764, 128, or 256) to 5V also. Some EPROMs do weird stuff if you do not pull VPP up. So if you pull /PGM and VPP to 5V, a 27128 SHOULD work.

The only other caveat that might be causing an issue is the speed of the CHR bus. Make sure your EPROM is rated for 150ns or faster to be sure.

(I was thinking this because the clone system might not multiplex the CHR bus either at all, or in the same way a real PPU does.)

as a final note, whatever happens when /PGM and VPP are not properly connected tends to be brand-specific. Like, an intel EPROM might behave different from a TI one. The only sure way to have them all work the same is to pull both of those pins high.