Electronic – way to prevent the random state of a shift register when power is applied


I have a project with two seperate output shift registers and they are working fine. They both have LEDs at the moment but one shift register will eventually be controlling relays and then solenoid valves to control water flow. The issue I have is when I turn the power on, the 8 channels of the shift registers are rarely in the off state. Normally some are on which is no good having the solenoid valves randomly turn on then off when power is turned on. Is there are way I can force the registers to be off when power is applied? The shift registers I am using are both 74HC595N.


The green LEDs light up before the MC can initialise the shift register.
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Best Answer

The chip you are using has an output enable pin that allow you to keep the shift register outputs in a high impedance state until you shift in known data. You would probably want to use a microcontroller for this.

From NXP Datasheet:

Output enable

Additionally you can use the master reset pin to clear shift register contents if you want them to be all 0. If you use this method it's probably doable to set up a resistor and capacitor to keep the pin low for a certain duration of time after power up.