A lot of interesting articles at 1
Short instructions
- Upload serial-passthrough to your MCU
- Connect to the bluetooth with Phone/another HC-05 to make sure that communication works
- Put PIO11 high. Device will stop to reponse on serial link
- Put RESET pin low
- Put RESET pin high
- Now you are in AT mode. Configure HC-05
- Put PIO11 low
- Reset chip again
Seems to work but with delays.