The implementation internally uses sector erase to wipe everything except
the sector(s) that mboot lives in (by erasing starting from
APPLICATION_ADDR).
The erase command can take some time (eg an STM32F765 with 2MB of flash
takes 8 to 10 seconds). This time is normally enough to make pydfu.py fail
with a timeout. The DFU standard includes a mechanism for the DFU device
to request a longer timeout as part of the get-status response just before
starting an operation. This timeout functionality has been implemented
here.
Andrew Leech