The 8 pin J40 header block exposes some of the power management related signals.
The interesting pins here are pins 7 and 8 (the LATCH_SET_BTN pins), which are connected by a yellow jumper cable in the pictures below, and pins 1 and 2 (the BUTTON_PWR_ON pins), which are connected by a white jumper cable.
Normally when you connect the USB power cable the board boots-up straight away. Connecting pins 7 and 8 disables this behavior and instead lets us use pins 1 and 2 to initiate booting.
Pins 1 and 2 will be left unconnected most of the time - so before connecting the USB power cable, unplug one of the two as shown:
Then connect the USB power cable - nothing happens - unlike before the system does not automatically boot up.
Now connect pins 1 and 2 momentarily, i.e. plug in the jumper cable as shown and immediately disconnect it again. In a more permanent setup, you'd connect these two pins with a momentary button switch (as shown later).
Even though dusty_nv makes it clear in this post that you should only momentarily connect pins 1 and 2, it was this that confused me initially - leaving 1 and 2 connected, rather than connecting and immediately disconnecting them, resulted in confusing behavior.
So momentarily connecting pins 1 and 2 causes the system to boot up. What happens if you momentarily connect pins 1 and 2 once the system is up and running?
It causes the system to shutdown. This is safe, it does not cause a hard shutdown, it's essentially a hardware initiated shutdown now. The system does not power off immediately, instead it goes thru a normal orderly shutdown sequence.
Once the system is shutdown you can restart it by again momentarily connecting pins 1 and 2. This is exactly the behavior I was looking for when I first asked about this on the Nvidia forums. I.e. I wanted to restart the system without having to disconnect and reconnect the USB power cable.
So do pins 1 and 2 do anything if you don't have pins 7 and 8 connected together all the time as above?
Yes - when pins 7 and 8 are not connected pins 1 and 2 will do nothing when the system is in a powered off state but when the system is up and running momentarily connecting them will still trigger an orderly shutdown of the system.
To avoid accidentally connecting the wrong pins together (with potentially very serious consequences) a longer-term solution is to wire pins 1 and 2 together via a push button like so:
- My original thread on the Nvidia forums - where jonnymovo pointed me to the right answer.
- The thread pointed to by jonnymovo that explains the J40 header.
- The Jetson Nano developer kit user guide that covers all the headers on the development board (including J40).
- The datasheet for Jetson Nano system-on-module includes more details on the actual underlying module pins that the J40 header exposes (the final datasheet is not yet available but you can find preliminary versions via Google).