XO1.5 1.2VSUS power sequencing ECO
This describes a change to early prototypes of the XO-1.5 motherboard. It fixes a problem with the sequencing of power supply turn-on to the VX855 chip, which prevents power up. Boards having this problem will occasionally fail to turn on, indicated by the EC flashing the red battery charge LED.
These power issues are not exacerbated by suspend/resume testing.
 Problem Description
- Lift PU5, pin 3 from its pad.
- Run wire from PU5, pin 3 to 3VPCU (anode of D19, next to Q22)
- Cut trace at top of PR70, and tie to +5VSUS (at PU5, pin 4 or PC31).
- Lift Q22, pin 2 (base), removing it from the 3VPCU net. Tie Q22, pin 2 to +1.2VSUS_GD (PU5, pin 1 or PR70).
- Replace PR6 ("top" side) with a 56.2K resistor
- Add a 0.1 uF capacitor from PR6 (thick trace) to ground (one side of PR7)