AN2476 datasheet(17/49 Pages) STMICROELECTRONICS

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AN2476

Power supply and clocks

17/49

This procedure is performed by the function of the library described in the Section 2.5:

MRCC_EnterWFIMode on page 29.

LP mode control parameters

If the Flash is used in Burst mode, it must be kept enabled in WFI mode

(WFI_FLASH_EN bit = 1).

●

If the WFI_FLASH_EN bit is set, the Flash memory is kept enabled in WFI mode and

the system can perform DMA transfers.

●

The LP_PARAM bits in the MRCC_PWRCTRL register have no effect in this case.

If the Flash is used in Non-Burst mode, the Flash may be disabled in WFI mode

(WFI_FLASH_EN bit = 0) and the LP_PARAM bits in the MRCC_PWRCTRL register used

to further reduce power consumption in WFI mode as follows:

●

If WFI_FLASH_EN is reset, DMA must be disabled when entering WFI mode.

●

FLASH OFF (LP_PARAM14). The Flash can be put in low power mode

(LP_PARAM14=0) or disabled (LP_PARAM14=1). In the second case, the static

consumption of the Flash is removed, but latency is incurred when exiting WFI mode.

●

Other LP_PARAM bits have no effect in WFI mode.

Wake-up events

Wake-up from WFI mode can be performed either by an external interrupt from an I/O pin or

an internal interrupt generated by one of the internal peripherals (assuming that its clock

has not been disabled by software) or by NCKDF (no clock detected) interrupt.

DMA interrupts can also be used, but only if the WFI_FLASH_EN bit in the Power

Management Control register (MRCC_PWRCTRL) has been previously set.

You can also combine WFI mode with PCG mode. However in this case, the peripherals that

are disabled cannot generate the interrupt used to exit from WFI mode and another interrupt

source has to be available.

It is also possible to wake-up from WFI when triggered by an external interrupt line

(configured in the EXTIT registers) without having enabled the corresponding IRQ in the

EIC. In this case, the external interrupt event will cause a wake-up from WFI, but no IRQ

routine will be invoked. The program will simply resume execution.

The RTC can wake up the MCU from WFI in two ways:

●

either using the global interrupt of the RTC (Alarm, Second or Overflow)

●

or through external interrupt line #15 which is connected only to the RTC alarm. In this

case, this external interrupt must be properly configured.

However, it is recommended to use the global interrupt of the RTC to wake-up from WFI.

Wake-up latency

If the Flash memory is kept enabled (WFI_FLASH_EN) or in low power mode

(LP_PARAM14=0), there will be no latency when resuming after wake-up from WFI mode.

If Flash memory is disabled in WFI Mode, using the LP_PARAM14 bit in the Power

Management Control register (MRCC_PWRCTRL), latency occurs when restarting due to

the Flash start-up time. Wait states are inserted until the Flash is ready. This latency is not

deterministic and may vary, depending on temperature or process dispersion.

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