Question for RG (or anyone else) re CMOS FX switcher octal latch

[Ben N]

I'd like to implement a "radio switch" for a 3-or-4-channel preamp based on a simplified version of Geofex Programmable FX Switcher, but the 74C373 Octal Latch is obsolete and hard/impossible to find. On a recent swing through the Tayda website, I noticed that there is a "74HC374 74374 OCTAL D-TYPE FLIP-FLOP IC" for the princely sum of $0.36, but from the datasheet I couldn't tell if this is interchangeable. Can it be used for this purpose? In the same topology?

And I know, I know: PICs. But still.

Thanks.

[R.G.]

It can't, really, without adding a bunch of other stuff to it.

The 74C373 is/was a special case in several ways that made it work in this application.
- it was a "C" family part, not an "ACT" "AC" "HC" or "HCT" the difference being that "C" family parts can run at 9V supplies, not the 5-6V max of the others.
- it was a "transparent" latch, meaning that once you clocked the data at the data inputs, it flowed through the latch to the output with no song and dance needed on the clocks or timing to get it to the outputs.
- it had/has a negative hold time; that is, the clock can go away once it's been asserted, and the data that -was- valid at the input when the clock's significant edge happened will still be transferred to the latch. In other versions, the hold time was positive, meaning that the input data had to be held valid until after the clock went inactive. This little song and dance is what lets you use the data changes AS the clock and still get the data clocked in correctly.

There are tricks you can play with diodes, resistors and capacitors on the 74AC373, ACT, HC, and HCT versions to make the mostly work OK, but it gets complicated tuning delays with resistor/capacitor networks. There are additional logic networks you can add to do the timing properly without so many RC delay networks.

The overall result is that you can force it to work mostly if you are willing to design enough stuff to go around it and tinker with it till it does work, but it's not close to a drop in.

[Ben N]

Thanks. Oh, hey! Futurlec has them! Cool.