Hi ,
I have Pixhawk (PX4FMU 2.4.6) with old bootloader protocol v4 .
I want to upgrade the bootloader to the last .
I try upgrade it with Mission Planner but it failed,I have try also with dfu after connect BOOT to 3.3 v but it also failed .and I try also to connect it with st-link with st-link utility the program post that there is read out protection
Good day,
just check the correct pinout for the swd and try use st link utility under Windows environment
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I have checked the pinout connection with st-link with st-link utility but i cann’t program the fmu IC.
i have connect (3.3v,gnd,swdio,swclk,cwo)
the problem solved when i connect (3.3v,gnd,swdio,swclk,cwo) and USB cable 
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Good… Happy fly… Shalom