I am designing a atmega 328 based irrigation syatem, my circuit contains a gsm module(sim800). Now no io pins available for driving powerkey of the gsm module(my gsm module has no breakout board). I found some details in the datasheet says "PWRKEY should be pulled low at least 1 second and then released to power on/down the module." how can set module power on when power supply is ON. please give me a solution.sorry,my English is poor

 

1. Upgrade the processor to one with more pins.
2. Create an I/O expander with a shift register.
3. If it does not need to be on all the time, turn it on periodically.

You know your design better than we do, what do you think might be a good solution?