9 years, 6 months ago.

It is useful to mention which device you are using.

But I can do a guess: In normal run-mode almost all mbed device use a PLL (or FLL) to increase the crystal/oscillator frequency to a higher frequency. Together with everything else this is disabled in deepsleep. On some devices (most/all LPCs, although that is not according to their user manual, so it depends on the implementation) they automatically start using their PLL again on wake from deepsleep. On the others they switch to their default clock (generally an internal RC oscillator) and you have to set the clock manually to the PLL again.

Now the good news is: You don't have to do this manually: The mbed library does this for you. So directly after the sleep code, there is code to put the clock back to normal. However the interrupt happens before that code can run, which means your printf happens with wrong clock setup.

Solution: Don't use the callback for that, but instead just put it in your user program right after you call deepsleep. Then the clock should be correct again and it should print correctly.

This does all depend a bit on the implementation: The KL25 for example has its Serial which is connected to USBTX/USBRX directly clocked from the crystal, so there your program would work fine. On the other hand the LPC1114 is clocked down by wakeup to something like 10kHz: just running your printf will take ages.