In a test circuit I have just built I use two cascaded 74HC595 chips to control 16 relays.

The outputs of the '595 chips are low on powerup most of the time. But sometimes a lot of relays turn on, making the supply voltage drop and making the 80C31 controlling the circuit go bananas...

So I guess I need a reset circuit for the '595s. The 'SRCLR pin (pin 10) has to be held low for 24ns to reset the chips (according to the TI datasheet). Assuming no load a 1k resistor and a 24pF cap should work, but in the real world there is a load. How large a cap should I use to get a reliable reset? My goal is to get the reset time as short as possible, as I have to switch on a relay on powerup, and the user has to keep a button activated while this happens...

Best regards,

Mikkel C. Simonsen