Hey i just want to know what speed should an eprom be for nes and for snes?
Thanks!
Thanks!
I always understood that a ROM for a 6502 family CPU needs to return data in a half cycle. In that case, you take 500 divided by the system's clock speed. That's about 250 ns for the NES (1.8 MHz) and 120 ns for the Super NES (3.6 MHz).
Thanks for writing back.
I just went on to ebay to buy some 27c040 eprom and i asked them to be at 250ns and they said that doesn't exist!
Whats that about?
I just went on to ebay to buy some 27c040 eprom and i asked them to be at 250ns and they said that doesn't exist!
Whats that about?
well, 250ns is really slow these days.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
hyarion wrote:
well, 250ns is really slow these days.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
Ok well then what eprom speed is not to old to get but still works fine on a nes?
pivotman wrote:
hyarion wrote:
well, 250ns is really slow these days.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
you don't need it to be exactly 250ns, a lower "address to output delay" is preferable.
Ok well then what eprom speed is not to old to get but still works fine on a nes?
I would think any faster eprom work...
Like what 120ns, 100ns, 80ns.
Which one?
Which one?
Any should work I would think. I used some 150ns EPROMs with NES before and they worked fine. 120ns probably will too. I think the slowest you might find around these days are 200ns.
Don't you have to use faster EPROMs for the CHR ROMs? I'm pretty sure you can't get away with anything much slower than 150ns with those.
Data should be latched at the end of every other PPU cycle (186ns), and the ROM should already be decoded (/CE asserted) shortly after the start of the first cycle. That gives the ROM ~350ns to grab the word, and at worst ~90ns for the output buffer (/OE), but I think /RD is active for a complete cycle though. Like any ROM should do.
great thanks for your help guys!