I made a circuit that is supposed to detect when all 8 output lines are logic high. I connected the inputs of a 74HC30 (8-input NAND) to the data lines of an AT28C256 (ROM). After a while, I checked the datasheet of both of these devices, and it states that for AT28C256:
output low voltage max: 0.45V
output high voltage min: 2.4V
And for 74HC30, it states:
high level input voltage min: about 3.15V
low level input voltage max: about 1.8V
For 74HCT30, it states:
high level input voltage min: 2V
low level input voltage max: 0.8V
I've already built the circuit with everything soldered in place. Rather than go through 1/3 a spool of solder wick and replacing all my 74HC's with 74HCT's, would I be able to add pull-up or pull-down resistors to all the inputs of the 74HC30 to make it function like a 74HCT30 without using excessive current?
Best Answer
You may be able to add pull-up resistors to the AT28C256 outputs to get its High output voltage closer to Vcc, and acceptable to the 74HC part.
The actual switching threshold for the 74HC parts may be close to 2.5 volts, and the AT28C256 output High is probably well above 2.4 volts, so your circuit may work fine, with no modification - but I wouldn't trust it in a production situation.