Electrical – Cascade shift registers driven by single-cycle microcontroller


I'm trying to make a unit in which 4 digits can be updated in a small amount of time from a single-cycle microcontroller (preferably all digits updated within 5uS)

My circuit is setup in the following way, (I excluded 7 segment displays and resistors for simplicity). The shift registers are 74HC595 and the multivibrator on left is the CD4538 but I think I need to upgrade that to 74HC4538 (someone confirm this?)

I have a multivibrator because the shift registers need the register clock toggled to display the new data that is shifted in, so by adding one, I eliminated a need of a 3rd wire from the micro that only updates the value.

The resistor is 10K and the capacitor is 1nF.

If I use a lower speed micro like the AT89S52 that can only update GPIO pins at best of 1/2 a microsecond (with 22Mhz crystal), then this circuit has no problem. But if I start to use the high speed single-cycle microcontroller, then GPIO pins could update as fast as 20nS.

What can I do to improve this circuit without rearranging parts? Should I just upgrade the multivibrator and lower capacitor value?

shift register timing

Best Answer

Be aware that the 595 used in this way has a problem: The daisy chain output changes at the same clock edge as the input of the next chip samples. A chip that is really designed for this kind of use (IIRC there is one in the CD series) has a delayed daisy chain output.

You can mitigate this problem by:

  • delaying the daisy chain output data a tiny bit (RC)
  • applying the clock in reverse: wire it first to the last chip in the chain

I don't understand why you are using that 4538. If you just want to always have the last (shifting!) data on the outputs, you can just wire SCK and SLK together. You will get some ghosting, depending on your update/idle ratio.