Low Power Application on SAM L21 Using MPLAB® Harmony v3 Peripheral Libraries: Step 2

Last modified by Microchip on 2023/11/09 09:07

Configure I²C, USART, and RTC Peripheral Libraries

Configure RTC Peripheral Library

Under the bottom left tab, in Available Components, expand Peripherals > RTC.

Double click or drag and drop RTC to add the RTC Peripheral Library (PLIB) to the project graph.

Available Components and Project Graph pane​​

Verify that the RTC clock is set to run at 1 kHz using the 1 kHz Ultra Low Power Internal Oscillator (OSCULP1K).

RTC clock set to run at 1 kHz​​

Note:

  • When a module is added to the project graph, MPLAB Harmony Configurator (MHC) automatically enables the clock to the module. The default RTC clock source is OSCULP1K.
  • On the SAM L21 device, RTC can be clocked through several low power clock sources of 1 kHz and 32 kHz as shown above. The 1 kHz clock source retained is enough to generate periods of 500 milliseconds.

Go back to the project graph and configure the RTC PLIB to generate a compare interrupt every 500 milliseconds.

Configuration Options window​​

Note: The Compare Value is set as 0x200. This compare value generates an RTC compare interrupt every 500 milliseconds

  • RTCprescaler=1
  • FGCLK−RTC=1024Hz
  • FRTC−CNT=FGCLK−RTC=1024Hz
  • Tinterrupt−period=500∗10−3s

CompareValue=Tinterrupt−period∗FRTC−CNT=500∗10−3∗1024=512(0x200)

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Configure I²C Peripheral Library, I²C Pins, and Verify I²C Clock

Configure I²C Peripheral

Under the Available Components tab, expand Peripherals > SERCOM.

Double-click on SERCOM2 or drag and drop to add the SERCOM instance 2 to the project.

Available Components and Project Graph pane​​

Select the SERCOM 2 peripheral library and configure it for I²C protocol:

Configuration Options pane​​

Configure I²C Pins

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

Tools > Pin Configuration menu​​

Once the MHC Pin Settings window is opened, find the Order drop-down list, and select Pins to sort the entries by port names.

Pin Settings window​​

Now, select the MHC Pin Table tab and then scroll down to the SERCOM2 module as shown:

  • Enable I²C data (I2C_SDA) on PA08 (pin #17)
  • Enable I²C clock (I2C_SCL) on PA09 (pin #18)

Pin Table​​

Go back to the Pin Settings table and customize the PA08 and PA09 pins names as listed:

  • Pin ID: PA08
    • Custom Name: I2C_SDA
  • Pin ID: PA09
    • Custom Name: I2C_SCL

Pin Settings pane​​

Verify I²C Clock

Open the Clock Configuration tab by clicking on Tools > Clock Configuration.

Tools > Clock Configuration menu​​

Open the Peripheral Clock Configuration tab by clicking on the button in the Peripheral Clock Selection.

Peripheral Clock Configuration tab​​

Once the window is opened, scroll down to the SERCOM2_CORE peripheral and select GCLK0 (12 MHz) as the source clock to generate the peripheral clock frequency.

Peripheral Clock Configuration pane​​

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When a peripheral is added to the project, the peripheral clock is automatically fed by the GCLK0. However you must configure the peripheral clocks according to your needs (power consumption, performance, …).

This completes the configuration of the I²C peripheral library. The application code will use the I²C PLIB APIs to read temperature from the temperature sensor.

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Configure USART Peripheral Library, USART Pins, and Verify USART Clock

Configure USART Peripheral

Under the tab Available Components tab, expand Peripheral > SERCOM. Double-click on SERCOM3 or drag and drop to add the SERCOM instance 3 to the project.

Available Components and Project Graph pane​​

Under the tab Available Components tab, expand Tools and double-click on STDIO or drag and drop to add the STDIO tool to the project.

Available Components and Project Graph pane​​

Connect the SERCOM3 USART output to the STDIO USART input.

SERCOM3 USART and STDIO USART​​

Select the SERCOM 3 peripheral library in the Project Graph and configure it for USART protocol.

Configuration Options window​​

Note: In this lab, the SERCOM3 (as USART) interrupt is disabled as the application does not need a callback on USART transfer complete. A USART transmit buffer empty event triggers the DMA to transfer one byte of data from source (user buffer) to destination (USART Tx register). When all the requested bytes are transmitted, the DMA PLIB notifies the application by calling the registered DMA callback event handler.

Configure USART Pins

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

Tools > Pin Configuration menu​​

Now, select the MHC Pin Table tab and then scroll down to the SERCOM3 module as listed below:

  • Enable USART Transmit (SERCOM3_TX) on PA22 (pin #43)
  • Enable USART Receive (SERCOM3_RX) on PA23 (pin #44)

Pin Table window​​

Go back to the Pin Settings table and customize the PA22 and PA23 pins names as indicated below:

  • Pin ID: PA22
    • Custom Name: SERCOM3_TX
  • Pin ID: PA23
    • Custom Name; SERCOM3_RX

Pin Settings window​​

Verify USART Clock

Open the Clock Configuration tab by clicking on Tools > Clock Configuration.

Tools > Clock Configuration menu​​

Open the Peripheral Clock Configuration tab by clicking on the button in the Peripheral Clock Selection.

Peripheral Clock Configuration tab​​

Once the window is opened, scroll down to the SERCOM3_CORE peripheral and select GCLK0 (12 MHz) as the source clock to generate the peripheral clock frequency.

Peripheral Clock Configuration pane​​

This completes the configuration of the USART peripheral library. The application code will use the USART PLIB APIs to print messages on the serial terminal.

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Configure DMA Peripheral Library

Launch the DMA Configurator by going to the MHC tab in MPLAB X IDE and then selecting Tools > DMA Configuration.

Tools > DMA Configuration menu​​

Click on the DMA Settings tab.

Configure DMA Channel 0 to transfer the application buffer to the USART TX register. The DMA transfers one byte from the user buffer to the USART transmit buffer on each trigger.

Based on the trigger source, the DMA channel configuration is automatically set by MHC.

  • Trigger Action: Action taken by DMA on receiving a trigger.
    • One beat transfer: Generally used during a memory-to-peripheral or peripheral-to-memory transfer.
    • One block transfer: Generally used during the memory-to-memory transfer on a software trigger.
  • Source Address Mode, Destination Address Mode: Select whether to increment Source/Destination Address after every transfer. Automatically set by MHC based on the trigger type. For example:
    • If the trigger source is USART transmit, then the Source Address is incremented, and the Destination Address is fixed.
    • If the trigger source is USART receive, then the Source Address is fixed, and the Destination Address is incremented.
  • Beat Size: Size of one beat. The default value is 8 bits. For example:
    • If the SPI peripheral is configured for 16-bit/32-bit mode, then the beat size must be set to 16/32 bits respectively.

DMA settings window

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