This applies to Video iPods (5th or 5.5 generation). The symptoms are constant detection of USB power, inability to charge via USB, and inability to turn off from Rockbox.
USB power management and battery charging is done via an LTC4066 IC. The USB power part of the IC may fail so that USB power cannot be used and power is output via the IN pin (#9, 3rd from the left on the bottom). This causes the USB power input to stay at a few volts when nothing is connected, which makes other circuitry detect that USB power is present. This voltage can even be measured from the end of a USB iPod cable. This is an easy and non-invasive check which could be done first.
The iPod cannot stay off because false detection of USB power turns it on. The original firmware's "off" is actually sleep, so that still works. When Rockbox tries to turn off the iPod, it restarts.
There are several things which can be done about this:
- Use the original firmware. Its off is sleep, and sleep is still possible.
- Cut the trace leading to the IN pin of the LTC4066. This is what I did. The power output from the IN pin doesn't get to the USB power detection circuitry, so it's possible to turn off from Rockbox.
- For power while connected via USB, use a USB+FireWire cable and a FireWire charger on the FireWire end. (For example, here are some cables at Amazon.)
- Replace the LTC4066. I thought this was too hard because there is a large solder tab under the chip for heat sinking and there are very tiny parts very close to the chip.
- Cut the trace and connect a diode from the USB +5V input to OUT of the LTC4066 to restore USB charging ability. I wanted to do this but there wasn't enough space for the diode. I also had some concerns about voltage drop and the diode's heat dissipation.
- Short the USB +5V input to ground when not using USB, eg. via a modified dock connector. It should be easy, but would waste power. I didn't try this.
- Replace the motherboard.
- A purely software workaround which allows Rockbox to shut down the iPod. In firmware/drivers/pcf50605.c, pcf50605_standby_mode() sets conditions which turn on the iPod. Removing the condition relating to USB charging, probably CHGWAK, would allow the iPod to stay off, though FireWire charging might not turn it on either. But be careful, because according to the comment, it is possible to turn off the iPod in a way that requires disconnecting the battery or waiting for it to discharge (a month or more I guess).
Here is a closeup of the LTC4066 area. Remember that this is a small part of the iPod, and what you see here is actually very tiny.
(Both images are from another iPod which doesn't have this problem, which is okay because the LTC4066 doesn't have to look any different when it fails.)