Unusual 74HC595 circuit design

[karan279]

Hi all ,

here is a circuit i traced from a machine spare part for Looming , so it has this 74hc595 with its 2 pins the storage clock and shift clock connected via a resistor and cap , and all i can think of is maybe its done to save a pin , but could anyone tell who may have worked on such a design and how i could probably test it .
 

[dannyf]

maybe its done to save a pin

You are right. It is one of the oldest tricks in the book: short pulses will trigger one pin and low pulses another.

In the extreme, you could trigger a shift register like HC595/164 with one single pin.

[karan279]

so i guess short pulses should trigger both the clocks 

[dannyf]

Short pulses will trigger the one it is directly connected to. Long pulses will trigger both.

It relies on the rc constant of the r/c network: quick pulses doesn't change the voltage on the capacitor.