B4 Suspend ECR
This page documents a hardware modification to repair a design flaw in the B3/B4/C1 laptop motherboards. It is primarily a fix for ticket #1835, but includes changes which should improve the reliability of all laptops, especially when running from battery.
This page is still in flux. Unless you are a developer working with OLPC please wait for a final fix !
Production Change
In production, the CPU supervisor used in the ECO of existing machines will be replaced with a cheaper circuit based on an RC circuit and a buffer with hysteresis. This provides a relatively precise delay of the reset signal to the Southbridge, at a reduced cost. If the voltage does temporarily drops below the design specifications, there is a good chance that the system will continue to operate normally.
This circuit is included here for reference only, as it is much more complicated to implement.
Final ECO
The final ECO includes ECO #42 and replacing Q1 with an AO3420 (or some other N-channel MOSFET w. an Rds of 25 mOhm or less at a Vgs of 5V). Since the AO3420 is not available in the retail market, I am looking for a suitable alternative.
ECO #42
This consists of three parts:
- Tweaking the compensation of the WLAN_3.3V power supply by changing R20 to a 33K resistor (SMD0402, 5%).
- Increasing the voltage of the WLAN_3.3V power supply. There are two common methods:
- Increasing R22 to 32.37K by adding a 768 ohm resistor (SMD0402, 1%) in series to the existing 31.6K resistor
- Decreasing R21 by adding a 330Kohm (SMD0402, 1%) resistor in parallel with the existing 10K resistor
- Installing a 3.10V (3.05 to 3.15V) CPU supervisor, with a delay of at least 16 mS, driving the PWG signal on the motherboard
The first two changes are the same for all builds. R20, R21, and R22 are all located right next to U15, the WLAN_3.3V regulator, on the top of the motherboard.
C56, on the bottom of the motherboard underneath the Southbridge, must be removed.
Older Versions
1835 ECO II
The original ECO had laptops refusing to boot when powered by battery. This eventually led to a more exhaustive study of the laptop power supplies, but initially resulted in a second ECO, using a CPU supervisor with a lower voltage threshold (2.85V - 3.0V), the Micrel MIC811S.
This generally stopped the rebooting problems, but indicated a larger problem --- a significant droop on +3.3V! On some laptops, the rebooting continued (Trac ticket #3537).
1835 ECO
The original ECO which established this as a valid fix for Trac ticket #1835 was installing a 3.10V (3.05 to 3.15V) CPU supervisor, with a delay of at least 16 mS between "power good" and deassertion of the Southbridge reset signal (PWG) on the motherboard.
The supervisor used was a MCP130T-315 from Microchip.
