Cannot write lock byte on Arduino Pro Micro

Question

I have an Arduino Pro Micro, with atmega32u4 chip and a caterine bootloader. However, every time I (successfully) flash some program over the USB port, the /dev/ttyACM0 device disappears, and I cannot flash it any more over USB.

So I connect it to my stk500v2-compatible ISP programmer (MISO/MOSI/SCK go to pins 14,16 and 15 on the board; VCC,GND,RST to their counterparts). With this setup, I can flash a new Caterina bootloader onto it. The problem with flashing over the board's USB port stays, though.

So I tried to set the lock byte with the ISP programmer, which writes successfully, but then fails at verification:

> avrdude -p m32u4 -P /dev/ttyUSB0 -c stk500v2 -b 115200 -U lock:w:0x0F:m
...
avrdude: verification error, first mismatch at byte 0x0000
 0xcf != 0x0f

Is my assumption correct that the disappearing bootloader is caused by the wrongly set (un-)lock bits? If yes, why can't I set the lock byte via ISP?

Answer 1

Meanwhile, I managed to solve this problem. The reason for the verification error is that the two most significant bits of the Lock Byte are read-only on my variant of the Arduino Pro Micro. Which means that, if you (try to) write a value of 0x00 = 0b0000 0000, it will result in a value of 0b1100 0000 = 0xC0 on the hardware. The solution, then, was to simply ignore those two bits.

> avrdude -p m32u4 -P /dev/ttyUSB0 -c stk500v2 -b 115200 -U lock:w:0xCF:m