Data Sheet

Bluegiga – A Silicon Labs Company 14
4.4 Recovery mode
Pulling the BOOT0 pin high at reset sets the BT121 module’s internal microcontroller into a recovery mode,
which allows the Bluegiga DFU to be rewritten to the module using the DumoGUI software. The BOOT0 pin
should be connected to a header or test point to enable DFU recovery. The pin is internally connected to a 10
kohm pull-down resistor.
4.5 Clock signals
BT121 generates all the required clock signals internally. The clocks used by the internal microcontroller and
external peripherals are synchronized to an internal 32.768 kHz crystal connected to the internal RTC. The
micro power RTC is always kept running when the module is supplied with power. It will take approximately
two seconds for the RTC oscillator to stabilize after power is connected. To avoid this delay it is recommended
that the power supply feed to the BT121 is not switched off but instead the module can be set into the lowest
power mode providing the smallest current consumption.