Wednesday, September 17, 2008

Adding Low Power modes in PXA - P3

Wakeup Sources

When PXA is in low power modes such as Standby, Sleep or DeepSleep, the processor can be brought out using "Wakeups".

Wakeup is an "external or selected internal event". For example, assume that the power button is connected to GPIO0 of the processor, pressing of power button can be a wakeup source to the processor. Not all the GPIOs can act as wakeup for processor. For PXA the GPIO <116,> can act as wakeup sources. Also there are other wakeup sources such as USB host & client, RTC are wakeup sources.

Now we may not need to wakeup the processor for activity from any of these sources. Hence we can select the required wakeup source using the Power Manager Wake-up Enable (PWER) register. Also for GPIO wakeup sources, the edges needs to be configured using the Power Manager Rising Edge Detect Enable (PRER) and Falling Edge Detect Enable (PFER) register, so that wakeup will only happen for the selected edge of the transition on the GPIO.

NOTE: Take special care in configuring the direction of the GPIO which is chosen as a wakeup. If a wakeup source is configured for a GPIO which is configured as output, some undesirable results can happen.

For keypad and usim wakup there are seperate set of registers too for configuring specifics.

So in short, while configuring a gpio wake source in pxa, the following things should be taken into consideration:
  • PWER, selecting the wakeup source
  • PFER and PRER, fo which edge of GPIO transition , system needs to wakeup
  • GPDR, gpio should be configured as input

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