doc: add bootloader

README.rst

@@ -93,9 +93,11 @@ firmware
 
 The devices is based on a [STM32F042F6P](https://www.st.com/en/microcontrollers-microprocessors/stm32f042f6.html) micro-controller.
 The firmware uses the [TinyUSB](https://github.com/hathach/tinyusb) USB stack.
-The device comes locked, preventing it to be re-flashed.
-I connected a DAPlink programmer to flash it, using the SWD test points on the back.
-To compile and flash it:
+
+If you got the passkey from CuVoodoo, it comes configured with a manufacturer key and locked.
+You can also request for a blank devices, and manufacturer it and flash the firmware yourself.
+
+To compile the firmware, you will need and ARM GCC toolchain and run these commands:
 
 ~~~
 git clone https://git.cuvoodoo.info/kingkevin/passkey_fw
@@ -104,5 +106,34 @@ git checkout passkey
 cd examples/device/hid_cdc_passkey
 make BOARD=stm32f042passkey get-deps
 make BOARD=stm32f042passkey
-make flash
+~~~
+
+When no firmware has been programmed on the passkey, the STM32 bootloader will allow to flash the firmware over USB using dfu-util:
+
+~~~
+make dfu
+~~~
+
+Re-plug the dongle in the USB port and the passkey will boot the flashed firmware.
+
+Using the serial terminal you can then enter the manufacturer key by pressing 'K'.
+Finally, you can lock the firmware by pressing 'L'.
+This it not revocable.
+Once locked, you can re-flash the device, change de manufacturer key, or debug the device.
+
+Before it is locked, you can erase the firmware using 'E'.
+Re-plugging the dongle will start the bootloader again, allowing to re-flash a firmware.
+You can also connect an SWD programmer, such as a DAPlink, on the test points.
+The pinout is a follow:
+
+- G: ground
+- C: SWCLK
+- D: SWDIO
+- T: the UART TX used to print debug messages
+
+You can then flash and debug the firmware.
+
+~~~
+make flash
+make debug
 ~~~