RPI safe mode
EDIT: This feature is removed from default firmware as of 18th March 2014. NOOBS still supports a safe mode, using the same gpio pins.
This results in config.txt being ignored (except for avoid_safe_mode). A default cmdline.txt is applied, and kernel_emergency.img is loaded (if not available, then just kernel.img).
It provides a means of recovering from, e.g.
- too high an overclock
- a typo in cmdline.txt
- an incorrect kernel=<filename> parameter
- an invalid display mode
without requiring a PC. (Obviously it won't help if you've deleted start.elf...)
The busybox based kernel_emergency.img has /boot mounted, and vi installed so you can fix up config.txt or cmdline.txt. (You can also mount /dev/mmcblk0p2 and fix up the rootfs if you have problems there that stop you booting).
Connect pins 5 & 6 with a jumper or piece of wire. These pins are the 3rd row from the end of the P1 header closest to the edge of the board.
Be very careful with where you insert the jumper. Some pins carry power, so shorting those would not be a good idea. Wire insulation stripped off of 20 AWG wire can be used to cover the power pins and help prevent accidents.
If you are using these pins for your own purposes and want to avoid ending up in safe mode, add this to config.txt:
P1 Pin diagram: RPi Low-level peripherals
Wake from Halt
When using bootcode.bin dated 12/04/2012 or later, the same two pins (5 & 6) can be momentarily shorted to wake the Raspberry Pi from the halted state.
puts the cpu/gpu into a very low power state making the system safe to power off. Previously the only simple way to restart the Raspberry Pi from this state was to cycle the power. Recent changes to the bootcode.bin file will now allow the Raspberry Pi to be started by momentarily connecting pins 5 & 6.
Just about anything made of conductive metal can be used to short these two pins for the moment require to start the boot up process, including a button or switch being wired to the pins.
This method works on both Rev. 1 and Rev. 2 Model B boards.
Be careful as power pins are located right next to these pins and you could permanently damage your Raspberry Pi if you accidentally short a pin to the 5v power pin.