I'm designing a system powered by an ATX PSU. I'd like it to be able to be switched on and off by a momentary switch but also to be able to be turned off by another input.
The latch circuit is powered by the 5 V standby power rail from the PSU. The rest of the system is powered by the main 5 volt power rail.
My current circuit works fine with the momentary button, I'm now considering how to design the other power-off circuit.
Current working circuit.
Suggested modification:
Add a resistor after the first inverter and connect a tristate buffer after this resistor. This buffer, when its output is enabled would pull low the line to the second inverter, latching off the circuit (an ATX PSU is turned on by pulling low the power on line) Since the output-enable is inverted, I put another inverter in front of that. So when the U25D inputs are low (as they would be when the main 5 V is powered off) the output is into /EN is high, so the output is in high-z and not affecting the latch circuit.
Only when the register output controlling SW_PWR_OFF becomes high, will U25D drive /EN low, enabling U27A output, which pulls the line into U15B low and latches the circuit into high output mode (turning off the main 5v supply, ATX supplies turn on when ATX_PW_ON is pulled low).
Would this approach work?
