To put the module into UART programming mode, tie MISO to 0 V while releasing
RESET. This causes the device to read zeros back from the Flash memory during
initialization and, as a result, to enter programming mode. If you are using the module in
conjunction with a serial interface then the signal PGM is available to put the device into
programming mode. To allow this signal to operate correctly from the Flash programming
What is the signal PGM?