How to Ensure State of Latch at Power On

Search POWER ON RESET (a simple RC circuit).

I'm using CMOS 4001 NOR gates, cross-connected. The inputs will have resistors to ground. I'm thinking maybe just a small cap to +12 on one of the inputs will do the job. I'll breadboard it tomorrow and report.

The cap will force the logic state for a power-up condition. The problem with this approach is that during a power glitch, the capacitor may not discharge all the way during the glitch and the system will not reset to the correct state. Usually a diode across the resistor that charges the cap will cause the cap to discharge rapidly allowing a good reset. There are still some glitch conditions that may escape this fix, but the diode significantly reduces the possibility. A true PowerOnReset circuit addresses this potential problem. For a memory refresh, I played around with LTspice, simulating the effect and saw a short power glitch cause the system to reset to the wrong state.

Well, it was too easy. Here's the circuit:

Adding a tiny 1nf cap causes pin 4 to reliably go high when power is applied. 10k pull down resistors. 12v power.

As long as your circuit won't start a fire, kill somebody, or cause other significant harm, this circuit will work well. There is a condition where a specific power up-down-up timing will cause this circuit to fail. If this is not a problem for your application, then continue with your current solution. If the condition occurs, and the user can just cycle power to regain control, that is fine. If your circuit must work under all conditions, then more work is needed.

Have fun!

Carl

Nah, just a trigger for an ICBM that'll probably never get used.