Wanting to work with an AD9833 signal generator chip, which needs an external master clock source, I was wondering if I could somehow tap into the 16 Mhz clock already feeding the 328P. The answer was yes, it is possible on the 328P to feed this to pin PB0 (a.k.a., digital pin 8).
The only tricky part is you have to program a fuse bit on the chip, which can not be done through self-programming, the way you normally upload code through the arduino bootloader. You need to set bit 6 (CKOUT) in the lfuse byte to 0:
So, you have to have a separate programmer, and also you must write the whole byte, meaning you want to be careful to preserve the other bit settings. Fortunately, through the ArduinoISP sketch, you can use another Arduino to program it.
This is fairly straightforward to wire up. However, somebody on #arduino IRC had to explain to me that it is necessary to put a 60 Ohm resistor across VCC and rst pins on the programmer, if using an Uno or Nano for the programmer. I didn’t catch that reading the tutorial.
Once wired up, you can use avrdude to write the lfuse byte. In my case.
avrdude -c stk500v1 -p m328p -P /dev/ttyUSB0 -b 19200 -U lfuse:w:0xBF:m
The last part of the command instructs avrdude to write byte 0xBF to the lfuse byte. A few things that might not be obvious about that: (1) in 328P fuse bytes, a “0” is considered the “programmed” state, the opposite of what you would intuitively expect; (2) I had to look at the boards.txt file in the arduino ide could to figure out that the “standard” byte value (for an MC used on an Arduino Uno board) is 0xFF. So, if I’m just changing bit 6 to zero, 0xFF needs to become 0xBF.
The fuse change was successful, and I was able to measure 16 Mhz out of PB0 (normally digital pin 8).