Trouble with Mother CHR ROM

[Arkanix38]

I'm trying to stick an Earth Bound rom set in my Mother Cart. The PRG rom is working fine, but appears to be having difficuly addressing my CHR rom (garbled graphics, mostly blank). Are there any differences in the pinout that I need to take into account? Is the ROM not compatible with the MMC3A mapper chip?

[muckyfingers]

The pinout is different between CHR and PRG, for MMC3 here is a diagram

[Arkanix38]

Thanks! Rewiring #OE and #CE got it going! I find it strange that it would make a difference as I had #OE already wired to GND and pin 22 (#CE) is already pulled to gnd on the board. Weird...
Post Merge: October 22, 2015, 02:19:55 am
 

[jensma]

Ahh, nice you got it working. Mother was my first repro for NES. It's such a great game 

[Arkanix38]

yeah it's pretty enjoyable, need to do a little levelling before I proceed to the graveyard but I should be there soon

[famifan]

I had #OE already wired to GND and pin 22 (#CE) is already pulled to gnd on the board. Weird...

that's a basics of 3-state bus application. Sometimes IC should be disabled to let other h/w work with the bus. Otherwise it deemed to bus conflicts.

weird thing is that freaks from nintendo thought that 1 wire was enough to do that.

however in real world applications

#OE goes for output enable. When disabled, outputs go to Hi-Z state (3rd state), kinda virtually disconnected from the bus. Just hiding the outputs.
#CE chip enable. Kinda power on/off trigger for IC.

Sometimes transition between power on/off (#CE) is relatively higher than just toggling the outputs with #OE on already enabled chip. Also, old ICs could show that difference in a more noticeable way than newer one. It depends on how it was manufactured. More details about transition times for the particular IC could be found in its datasheet.

So, famicom should trigger #OE for CHR chip, not a #CE. #CE should be hardwired to GND, so the chip stays always enabled.

[Arkanix38]

weird, its fine for the PRG rom #OE to be pulled low constantly