XO1.5 WLAN SR ECO

Scope

This describes a change to B3 prototypes of the XO-1.5 motherboard. It fixes a silicon error which will not be fixed. Boards without this ECO will not reliably maintain their WLAN upon resuming, and may crash hard instead.

This only applies to XO-1.5 B3 phase motherboards.

Problem Description

Glitch in Power Control

There appear to be three problems: a glitch on the power control line for the WLAN card, a drooping WLAN_RESET# line, and finally, too slow a rise time on the +3.3V rail when resuming.

Power Control Glitch

Driving Force

The glitch is a surprise, and is present in both OFW and Linux. It happens early enough in the resume process (before PCIRESET# has been deasserted) that it is caused by hardware. We take a look at it by disconnecting the motherboard pullup resistor on WLAN_EN#, and instead adding a 0.1 uF cap (see Driving Force thumbnail). This transforms the glitch in the WLAN_EN# signal to one which looks like it is driven by a very high impedance driver, such as the built-in pullup resistor.

If we instead add a resistor between the VX855 output and the inverter, along with a filtering capacitor to ground, we can slow down the turn-off process enough that legitimate software usage is allowed, but this glitch is eliminated.

After Modification

WLAN_RESET

We also need to populate a resistor to pull-up WLAN_RESET# to a sane value. This was predictable, and a resistor position (R120) is pre-wired.

+3.3V Rise Time

Finally, if the rise time of the +3.3V rail isn't fast enough, the card detect debouncer in the SDIO interfaces takes very long (> 450 mS) to settle on resume. This rise time is determined by a capacitor in the power switch (C253) which needs to be decreased in size.

Procedure

1. Remove R325. R325 and Q38 are located right below the WLAN connector, on the "bottom" side of the PCB (farthest from the LCD).
2. Cut the trace between Q38, pin 2 and R325 (disconnecting Q38, pin 2 from the WLAN_EN# signal).
3. Add a 47K resistor between WLAN_EN# (available at R325) and Q38, pin 2.
4. Add a 0.1uF, SMD-0402 resistor between pins 1 and 2 of Q38.
5. Populate R120 with a 4.7K resistor. R120 is located roughly in the middle of the board, on the "top" side.
6. Replace C253 with a 0.01 uF SMD0402 capacitor. C253 is located above the battery charger and near the EC.