Recovering ATMEGA328p chip from external clock fuse set

A quick post of tragedy and triumph!

Whilst breadboarding a new project with an Atmega328p chip (the heart of some of the Arduino ecosystem), I wanted to configure the AVR "fuses" (a desperately misleading term!) in order to use an external 16 MHz crystal, rather than the default internal 8 MHz oscillator.  This is straight forward and something I've done before.  Using the excellent fuse calculator, I wanted to get cracking and rushed in without double checking my fuse selection.  Big mistake!  I saw the option "external clock" (CKSEL=0000 SUT=00), and thought "great, that's it", also unselecting the CKDIV8 option (bit 7), generates the LOW fuse = 0xC0 and the HIGH fuse = 0xD9.  Two right mistakes here!  First of all the small one:  Selecting a box corresponds to setting the bit to '0', rather than '1', i.e. *unselecting/unticking* the CKDIV8 boxes will SET this bit as true (and divide the clock signal by 8).  It can't be only me that finds it confusing, but this also is how it is specified in the datasheet!  Secondly, the biggy.  An external 16 MHz CRYSTAL is not an external CLOCK!

Fuse reprogrammed.  Chip not responding to reprogramming or crystal oscillator.

Uh-oh.

Chip bricked! :(

Fortunately it is possible to provide an external clock, reprogram the fuses and carry on with life!  I suppose it's possible to use a 555 timer to provide the pulse, though I suspect their would be a frequency limit.  Also it's much simpler to simply hook up an Arduino Nano to provide the external clock signal.  This post also takes the same approach.  The Arduino sketch is insultingly simple and looks like this:

#define output_toggle(port,pin) port ^= (1<<pin)
#define set_output(portdir,pin) portdir |= (1<<pin)

void setup() {
set_output(DDRD, 6);
}

void loop() {
  while(1)
  {
  output_toggle(PORTD,6);
  __asm__("nop\n\t");
  __asm__("nop\n\t");
  }
}

Reading The Freakin' Manual/datasheet tells us what we need to do.  Power the bricked chip by the 5V and GND from the Arduino Nano, and hook up the D6 pin to the PB6 (XTAL1) pin of the bricked Atmega328p.

Now, the chip springs into life (note: a RESET might be required).  The fuses are now able to be reprogrammed, firstly back to the internal 8 MHz oscillator:

avrdude -p m328p -c usbtiny -e -U lfuse:w:0xe2:m -U hfuse:w:0xd9:m

Chip unbricked! And finally setting the fuses to the external CRYSTAL (lesson learnt!):

avrdude -p m328p -c usbtiny -e -U lfuse:w:0xff:m -U hfuse:w:0xd9:m

Now the crystal oscillator can be placed across PB6 and PB7 (with suitable capacitors to ground) and the chip skips along to the beat of the crystal.