Switch ONLY when powered off

I do not know if there is any switch that will do what you need, but you should look into Hot Swap Controllers.

Thanks, will look into that.

To clearify. The only way to switch to the other chip so to speak is to power off the device, and then toggle a switch and last power on again.

I want it to just "check" upon power on.

I would say the best way for implementing this is through a circuit loop for sensing connection and a transistor to to turn power on/off. Then have a section of code that looks for the sense loops during start up.

You could use an DPST switch to select which chip gets power on power-up. The DPST switch state gets latched on power-up by a D-flip-flop. The D-flip-flop clock can be a simple capacitor to the power rail, with a pull down resistor on the clock pin (creating a pulse on power-up). The flip-flop output turns on power to one chip or the other depending on its state (using transistors).

Hi atmn,

Perhaps you could use an SPDT switch as a programming switch to a microprocessor. The uP reads the switch position once at power up and pulls the corresponding output High to power your IC "A" or IC "B". Since the program switch is only read once in the setup of the uP program nothing would happen if you switched it while the unit is powered up. You would have to power down reset the program switch and power up to move to the other IC.

John

This sounds good.

Are there any schematics for a curcuit like this? This must have been done before I would assume.

The easier solution the better.

OK, so a slightly more complicated approach would be to use a very low cost microcontroller (ATTiny or similar), have two power transistors, one for each unit and a button. When you press the button, the micro will power off the first device, wait a period of time, then power on the other. this approach can be applied to more than two devies in this case.

Yes you can do it with simple ttl chips and passives but it will take just as long and less fun. A clunky switch will give no time for one to power off before the other starts powering up. You could also include a GPIO signal to the target device to tell it to go to sleep or some other pre-defined condition prior to the power being removed.

This is a quick example that doesn't need any chips...

Q1 and Q3 are P channel FETs like BSS84, Q2 and Q4 are N channel FETs like BSS138.

With the switch in the capacitor position, Q2 will turn on Q1 and turn off Q4 upon power up. Q3 stays off because R4 pulls the Q3 gate up. The circuit will then latch in this state as R3 will hold Q1 on. Switching the switch will not change the state.

With the switch in the grounded position at power on, Q2 and Q1 never turn on since Q2 gate is always grounded. This Allows R2 to pull up the Q4 gate, turning on Q4 which turns on Q3. Again the switch has no effect after initial power up.

Looks great!

I really wanted to go analogue with my solution so this looks doable.

Gonna order some components and try it out!, again thanks!