Deepsleep shield appears to limit the LoPy's minimum supply voltage?
The LoPy is rated to work from 3.5V to 5.5V. How should the deepsleep shield affect this?
When I supply the deepsleep board with 5V on pin 28, comms between the PIC and the LoPy are fine, and they continue to work down to about 4.6V. Lower than that and the comms fail when the deepsleep class initialises.
When IO0_H is pulled low, its voltage toggles from 3.3V to 0V and IO0_L toggles from my supply voltage to 0V. It looks as though the reply data from the PIC toggles IO0_L between the supply voltage and about 1V, while IO0_H toggles between 3.3V and 1.56V. I'm guessing this difference in the lower voltages is the drop across the drain-source diode in Q2, but I'm not sure why IO0_L isn't getting nearer to 0V. Perhaps my deepsleep shields are broken.
As the supply voltage drops, the low voltage driven by the PIC gradually increases. At about 4.6V it reaches 1.2V on IO0_L, the voltage at IO0_H is 1.68V and comms fail even though the PIC seems to be trying to transmit a reply.
By 4.4V, the reply doesn't appear at all and the value of IO0_L sticks at about 1.28V after the initial bytes from the LoPy. By 4.1V on the supply, the voltage sticks at 1.4V.
The PIC itself is rated down to 2.3V, so it shouldn't be a problem there.
Looks like we have got a similar issue