Is it possible to trigger CPU wakeup upon SARA-R4 modem wakeup?

The ArduinoLowPower library has some helpful examples for putting the CPU to sleep and waking it up by triggering interrupts in a few different ways - via the internal clock, analog pins or digital pins.

Triggering an interrupt on a digital pin is easily done externally, as described in the ExternalWakeup example. However, I would like to be able to do it via the UART communication between SARA-R4 and the CPU. Ideally, doing it when a specific URC is sent.

The reason for this is to coordinate network-coordinated modem sleep with CPU sleep. I would like to be able to wake the CPU up when the modem wakes up. Doing this in any way would be helpful, but I mentioned UART/URC specifically because that was the first thing that came to my mind since a URC is sent when the modem wakes up (assuming AT+UPSMR=1 has been set). I would not like to rely on putting the CPU to sleep on a timer based on the TAU, as that is unreliable.

Any help would be appreciated!

Very good question! I have no solution just some thoughts. The wakeup interrupt must come over a wired connection.
As i see in the schematics, it could be TX RX CTS DTR. DTR (Pin9 at the modem) is wired to GPIO1 of the modem.
I think it's neccesary to modify the ArduinoLowPower library and check the behaviour of the modem on these pins.
This may be a path to a solution and a interesting new function for the ArduinoLowPower lib ?!? Happy RTFM.. :grinning:

PS : Arduino engineers, wire the I2S on the MKR1500!!!