updating RH bootloader (only from main version 16.19)
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IMPORTANT: *** Do not copy the bootloader code ``RH_stm32_bl_*.bin`` to USB until RH main software has been updated to 16.19 since the older bootloaders do not differentiate the name difference between bootloader and main firmware ***

Perform following steps only after updating RH main software to version 16.17 and verifying it on DH

1. copy the RH bootloader code ``RH_stm32_bl_*.bin`` onto a USB stick and plug this into the RH
2. boot the unit whilst pressing the ``Fn`` button, wait for the downloading process to complete.

 - progress is shown on the ``100Mb/s`` LED and ``layer3`` LEDs
 - solid green on the ``100Mb/s`` LED indicates completion, the process is reasonable quick (<10s) normally


note
++++
 - an external PSU should also be plugged in to perform a firmware upgrade
