WinCE

So far,we have discussed about how to put pxa into low power mode, how to wake it up, and briefly on how operating system restore things to the prior state after a transition to low power mode. Now lets put the bits and pieces together and see how windows ce does the low power mode transition in case of PXA.


As I have mentioned in the previous posts, we need to add the wakeup configuration first and then put the processor in to the low power mode.

For purpoes of explaining the concept, I will how to add a wakeup for GPIO 11 which is connected to the activity line of a communication module.

Adding the wakeup source
1) In OAL interrupt code(intr.c and bsp_cfg.h), add the code for creating a new interrupt corresponding to the wakeup source.

bsp_cfg.h - Add the new sysintr for the wakeup line

....
#define SYSINTR_COMMMOD_WAKEPUP (SYSINTR_FIRMWARE+22)
....

intr.c - map the sysintr to gpio 11 and add code for managing that
BOOL BSPIntrInit()
{
....
OALIntrStaticTranslate (SYSINTR_COMMMOD_WAKEPUP,IRQ_GPIOXX_2_GPIO11);
....
}

2) In OAL power code (power.c), add code for setting the pwer register. In mainstone bsp, the wakeups are register while OS starts (BSPPowerOffInit function)and the pwer value is written only while going into low power mode (BSPPowerOff).

BSPSetWakeSrc function - add a case for gpio 11 and set the gpio11 bit in the pPwer global variable. This variable will be written to the actual PWER register while going into suspend.

......
case IRQ_GPIOXX_2_GPIO11:
pPwer->gpio11 = 1;
OALMSG(1, ( L"BSPSetWakeSrc: IRQ Communication module source\r\n", irq));
break;
......

BSPGetWakeSrc function - add case for gpio 11 here also.This function to return the IRQ value if the PEDR (Power manager edge detect register) bit for gpio 11 is set.

....
else if (pPedr->gpio11)
{
*pSrcIrq = IRQ_GPIOXX_2_GPIO11;
}
....

BSPPowerOffInit function - use the OALIoCtlHalEnableWake function to set the global variable (g_oalWakeMask, an array) with the sysintr value corresponding to gpio11.
UINT32 commSysIntr = SYSINTR_COMMMOD_WAKEPUP;
...

OALIoCtlHalEnableWake(IOCTL_HAL_ENABLE_WAKE, &commSysIntr, sizeof(commSysIntr), NULL, 0, NULL);
...

While the BSPPowerOff function finds out which sysintr is mapped as a wakeup using OALPowerWakeSource and OALIntrTranslateSysIntr functions and sets the pwer bits with BSPSetWakeSrc function call.

When system resumes, BSPGetWakeSrc is used to get the wake cause and is stored in g_oalWakeSource variable which can be retrieved using kernel ioctl IOCTL_HAL_GET_WAKE_SOURCE

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

Power Modes in PXA

PXA has the following power modes

• Normal mode
• Idle mode
• Deep idle mode
• Standby mode
• Sleep mode
• Deep Sleep mode

Each mode is characterized by the power domains that will be kept active and the clocks that will be running. For example in Sleep mode all the external low voltage power domains (i.e. domains controlled by PWR_EN pin of PXA) is disabled. While on the other hand, in Deep Sleep mode both the external low voltage and high voltage power domains are disabled as both PWR_EN and SYS_EN pins are de-asserted while entering the low power mode. The choice of which power mode to use for each depends solely on the power consumption requirements of the product.

In the beginning of the project, we mapped the Standby mode of PXA to POWER_STATE_SUSPEND state of Windows CE. But as we had a tight power consumption requirement for the Suspend mode, we had to re map the state to Sleep mode of PXA instead of standby.

Transition of Power Modes

For initiating a power mode transition in PXA, we need write in to the coprocessor 14 , register C7 (PWRMODE). Writing the bit pattern for each pattern corresponding to the power mode in to the PWRMODE[M] bits initiates the transition. For eg:

XllpPmEnterDeepSleep FUNCTION

ldr r3, =0x00000007
mcr p14, 0, r3, c7, c0, 0
nop
.......

A call to this assembly routine will initiate a transition to deep sleep mode.

Context Restoration

While going into low power modes such as Sleep or Deep Sleep, the processor contents can get corrupted.

Lets take the example of Sleep mode. In Sleep mode, the PC (Program Counter) value is corrupted along with some other set of registers. So in order to tackle this situation, operating systems are doing a context storage and restoration while going into low power mode. It stores the status of all the required registers such as GPIO configurations, level, stack address, core configuration registers in to a memory area. The address of this memory area is kept in a register which will be retained during the low power mode. In the case of PXA, it is the Power Manager Scratch Pad Register (PSPR). This register value will be maintained during a sleep reset, but will be cleared during GPIO, power on , hardware or watch dog resets.

So while going in to sleep mode, the required information is stored in a data area in RAM and the address to this area is stored in PSPR register. While resuming from sleep mode, the boot loader will check the PSPR register and will restore the configuration that is stored in the data area.

NOTE: bear in mind that the data area is in RAM, so the RAM needs to be properly initialized after a sleep reset, otherwise the boot loader can hand while trying to access the restoration area.