Chip speed
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Chip speed
When choosing chips (EPROM or Flash) what speed would you suggest? When ordering from this site I noticed that they have chips with speeds from 150ns to 45ns. What should I get?
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OH! This probably explains why many of my devcarts didn't work ever! So, I'm a little confused here. Tokumaru says a chip can't be too fast, but you want anything Less than or equal to 200ns. Just to be clear, I can read, but do you want something Less than or equal to, or Greater than or equal to 200ns?
The speed rating of a chip is the maximum time until the data is valid under worst stated conditions. Like overclocking processors, the chips might perform better in some circumstances; one batch of seemingly too slow chips might work, another might fail. If you want reliable operation, pick parts whose specs meet your requirement.
I don't know... I think the memory devices CAN be too fast. 250ns is good for PRG, and 150ns is good for CHR usually. I wouldn't go below 90 or 100ns if you can help it (for either).Celius wrote:OH! This probably explains why many of my devcarts didn't work ever! So, I'm a little confused here. Tokumaru says a chip can't be too fast, but you want anything Less than or equal to 200ns. Just to be clear, I can read, but do you want something Less than or equal to, or Greater than or equal to 200ns?
Bad things can start happening, like bus ringing and such (due to the beefier pin drivers on the faster chips), and they can suck more power, and make larger spikes which need more decoupling.
The worst thing that can happen would be some ground bounce, which could totally trash the data in a bunch of RAM locations if it happened when the address bus was changing.
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The reason faster chips have higher power requirements is that they need faster transitions between logic levels. A slow chip can have a more gradual waveform since it doesn't need to be able to change as often, while a faster one needs a sharper one:
I've read that faster chips might also expect shorter transitions on inputs, so feeding them signals meant for slower chips might register as false transitions.
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|--------| longer transition
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|---| shorter transition