WHat is the deep sleep shield?
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Obviously you didn't look hard enough: lhttps://docs.pycom.io/datasheets/boards/deepsleep/
It's right there in the docs
Its a fudge to solve a design problem with earlier Pycom units.
Effectively it is another micro (i think a PIC) that responds to commands from the Pycom device to pull the power and reapply it again after a period of time or on some event.