3 ms·
because it's NMOS, only the pulldowns can work quickly. the pullups are resistors (depletion mode FETs in the IC) which take some time to charge the downstream
by tubetime 10y ago
because it's NMOS, only the pulldowns can work quickly. the pullups are resistors (depletion mode FETs in the IC) which take some time to charge the downstream gate capacitance. i wrote a quick and dirty python program to calculate the optimal value for each of the 1,019 pullup resistors to hit a target clock of 500khz. board bringup is ongoing so we'll see how fast it ends up being...
- sowbug 10y ago500kHz is not too far from the Apple ]['s 1MHz. Come on, you can do it!
- mozumder 10y agoSo then the only way to get it going faster is to raise voltage for the pullups?
- tubetime 10y agowell, it has to run on 5v (standard ttl voltage) so the only way to speed it up is to decrease the pullup resistor values, which also increases the current consumption. it's a tradeoff.
- deleted 10y ago[deleted]
- ta160516 10y agoIf the FETs can take it, you could run it from 12V or whatever and put level shifters on the I/Os...
- j1vms 10y agoAgreed, would seem to make sense too, however it would increase power consumption and heat, so might "age" the whole circuit a bit faster than keeping the voltage (and thus clock speed) down. Mind you, he could just pop in replace components at will :)