Quote:
Originally Posted by Mark1960
I think parasitic diodes in the output of U1 would still pull the chip select down when 5v is turned off.
|
I'm going to have to get a 74HCT02 and test it before pushing the button on the PCBs! In any case, this iteration of the PCB is meant to contain "experimental" features so I can see how it works in practice. But it won't be too hard to get a 74HCT02, not power it and measure the voltage on the output when it is pulled up via a resistor. This will also tell me how much current is being pulled through the resistor and if that is excessive and likely to cause problems for the IC or battery. I already have a couple of memory chips so I can wire those up to see the effect of R1,2,3 on battery consumption too.
Quote:
Originally Posted by Mark1960
R1, R2 and R3 are also probably going to be draining the battery when 5v is off.
I think R1 and R3 could pull up to 5v instead of the battery supply.
|
They are deliberately large values, so this is mitigated somewhat. They are intended to pull up to whatever the Vcc pin of the memory chip is, whether it is from the PSU or battery.
Quote:
Originally Posted by Mark1960
You could power U1 from the battery supply and remove R2, but then you probably don’t want to use the spare gate as an inverter for the address buffer enable.
The 74hc02 might not work with address input levels from the SCMP, and might be better to use 74hct02. I wasn’t sure if you deliberately picked the hc or if that was just a package that was available. Then the 74hct02 probably shouldn’t be powered from the battery as it might not behave itself at battery supply voltages.
|
The HC part was what came up tbh. I would probably use the HCT part in practice. I will amend the diagram appropriately.
Quote:
Originally Posted by Mark1960
If you have 74hct4070 instead of the 4070, or 74hct86, you could use the spare gate as an inverter. I don’t think a cd4070 would be fast enough for the address buffer circuit. But this would mean the prbs generator is not completely separate from the rest of the board.
|
The address buffer uses the spare gate from the 74HC(T)02, which previously was inverting the TOP signal as I could no longer remember why I wanted to do this.