Low Power Application on SAM D21 Using Harmony v3 Peripheral Libraries: Step 4

Step 4.1: Configure Power Manager (PM) Peripheral Library


Under the Available Components tab, expand Peripherals > PM.
Double click or drag and drop PM to add the PM peripheral library to the project graph.

Figure 1


Select the PM peripheral library in the Project Graph window and verify the default configuration as shown in Figure 2.

Figure 2

This completes the configuration of the PM peripheral library. In Standby mode, the CPU as well as the peripherals (except those which are configured to run in Standby mode) are stopped. The Standby Sleep mode provides very low power consumption with little overhead on wake-up time. CPU enters Standby mode by executing the Wait for Interrupt (WFI) instruction and exits from the Standby mode when an interrupt is generated.

Step 4.2: Configure NVMCTRL Peripheral Library


The Non-volatile Memory Controller (NVMCTRL) is added by default to the project graph. Select the NVMCTRL peripheral library and configure it as shown in Figure 3.

  • Verify that Wait States is configured as 1 to read the non-volatile memory. When the device is put in performance level 0, the device clock frequency cannot exceed 12 MHz and one wait-state is required.
  • Set the Power Reduction Mode During Sleep to WAKEUP INSTANT. This bit configures the NVMCTRL to wake up the Flash memory when the CPU wakes up from Standby mode, which allows it to reduce device wake-up time.
  • Disable the Instruction Cache to reduce the device's wake-up time. When the cache is enabled, the device will look for an instruction to fetch upon interrupt in the cache at first. If the instruction is found, the device wakes up quicker, but if not, a cache miss occurs and wake-up time is longer. This option is good to enable when the instruction to fetch upon an interrupt is always stored in the cache memory, which is not the case here, as we have different interrupt sources.
Figure 3

This completes the configuration of the NVMCTRL peripheral library. The NVMCTRL is set to optimize the performance for waking the device up while keeping the power consumption as low as possible.

Step 4.3: Configure LED and Wake-up Test Pins


Open the Pin Configuration tab by clicking Tools > Pin Configuration.

Figure 4


Once the MHC Pin Settings window is opened, scroll-down to pin number PA27 and PB30, and then configure these pins as shown in Figure 5:

  • Set the Pin Number #51 as GPIO:
    • Pin ID = PA27
    • Custom Name = WAKEUP_TEST
    • Function = GPIO
    • Direction = Out
    • Latch = High
  • Set the Pin Number #59 as GPIO:
    • Pin ID = PB30
    • Custom Name = LED0
    • Function = GPIO
    • Direction = Out
    • Latch = High
Figure 5

In the above step, the user LED is configured as per the following USER_LED design schematic on the SAM D21 Xplained Pro Evaluation Kit.

Figure 6

Step 4.4: Rename the Default main File


Under the Project Graph tab, click on System, and configure as below to give a name of your liking to the default template main file generated by the MHC.

Figure 7

The name of the default main.c is changed to main_d21.c to indicate that the referred main.c is for the Low Power application on SAM D21.

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