I am teaching myself how to use a 74HC595 shift register, and am fairly new to proper digital circuit design. I ran into a question. I am using a simple circuit involving a few LEDs, a switch for data, and a couple of pushbuttons to pulse the register clock and the latch clock. Basically, I can clock in the data, but it won't update the output LEDs until I pulse the latch clock.
I would like to get rid of the latch pushbutton and have it automatically latch pulse after each clock pulse high trigger. I am wondering what the easiest way is to "echo" a pulse from one clock to the next using simple parts? I.e., when I send a Low-to-high on the clock pin, I want another low-to-high-to-low to immediately follow on the latch signal. The latch signal just needs to stay high long enough for the register to recognize it, not necessarily for the same duration as the original pulse. Half a microsecond should be sufficient for the delay and the duration of the following pulse.
It is not simple enough to tie the clock and latch to the same signal. This results in the display always being one pulse behind (as described in the data sheet).
___/-\_________/----\________ Leader clock
____/-\_________/-\__________ Follower latch
… or perhaps some sort of delay line…
___/-\_________/----\________ Leader clock
____/-\_________/----\_______ Follower latch
Could I clone the clock pin to the latch using some sort of MOSFET with a capacitor for delay?
Best Answer
You did not supply a schematic, and that image thing is incomplete .. but anyway.. the maximum rise rate for the clock input is about 100ns/volt with a 5V supply, and setup time is only about 24ns, so a simple circuit can be used- something like this:
simulate this circuit – Schematic created using CircuitLab
If you don't properly debounce the pushbutton switches you will have issues.