I'm getting confused by the datasheet for the Z84C00 CPU, while trying to work out if I can use it to drive 74HCxxx chips, or if I need TTL-compatible logic (i.e. either 74HCTxxx or 74LSxxx). It's clear that the low level voltages are compatible, but I can't work out what's going on with the high level voltages.

74HCxxx chips require between 3.15V (at VCC = 4.5V) and 4.2V (at VCC = 6V); I don't expect my voltage input to range this far (I plan on keeping it between 5.2V and 5.5V).

The output for the Z84C00 is specified in its datasheet (p34) as:

Symbol    Parameter                   Min  Max  Unit  Condition
VOH1      Output High Voltage         2.4        V   IOH = -1.6mA
VOH2      Output High Voltage       VCC-0.8      V   IOH = -250uA

This seems to suggest that the voltage varies dependent on the input current on the pin, but isn't the input current entirely dependent on the internal circuitry of the chip itself? How can I work out what the expected voltage of the output will be?

I'm also not sure what the subscript "1" and "2" in the symbol name here is used for. I can't see any reference to them elsewhere, and the equivalent NMOS parts don't use the subscripts, just giving a single line (with 2.4V output level). How should I interpret this?

  • \$\begingroup\$ Ensure the current you pull out of that pin is 250uA or less, and Zilog guarantees VOH >= VCC-0.8, i.e. 4,2V with a 5V supply. This is fine for either logic series. \$\endgroup\$ – Brian Drummond Jan 30 '18 at 11:23

I'm also not sure what the subscript "1" and "2" in the symbol name here is used for

Look at the end of each line - #1 is for a loading current of 1.6 mA and #2 is for a loading current of 0.25 mA. This means that if your 74HC chips don't take much current (you'll have to check) then you are probably going to be OK with a supply at 5.5 volts.

The input currents to HC series are much less that 250 uA but you will need to confirm this in the data sheets of the parts you intend to use.

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