Low Power Application on SAM E54 Using Harmony v3 Peripheral Libraries: Step 2

Step 2.1: Configure Real-Time Clock (RTC) Peripheral Library

1

Under the bottom left Available Components tab, expand Peripherals > RTC.
Double click or drag and drop RTC to add the RTC Peripheral Library (PLIB) to the project graph.

rtc_adding.png

2

In Clock Easy View, verify the RTC clock is set to run at 1 kHz internal ultra-low-power clock.

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 an internal 1 kHz ultra-low-power clock (OSCULP1K).

rtc_clock_setup.png

On the SAM E54 device, the RTC can be clocked through several low power clock sources of 1 kHz and 32 kHz. The 1 kHz clock source retained (OSCULP1K) is enough to generate time periods of 500 milliseconds.

3

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

rtc_configuration_setup.png

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

  • RTC clock = 1024 Hz
  • RTC Prescaler = 1
  • Required Interrupt rate = 500 ms

Hence, Compare Value = (500/1000) x 1024 = 512 (i.e., 0x200).

Step 2.2: Configure I²C Peripheral Library, I²C Pins, and Verify I²C Clock

1

Under the Available Components tab, expand Peripherals > SERCOM.

Double click on SERCOM3 to add the SERCOM instance 3 to the project.

sercom_selection.png

Select the SERCOM 3 Peripheral Library and configure it for the I²C protocol.

sercom_setup.png
  • The SERCOM3 (as I²C) retains the default 100 kHz speed because the temperature sensor chip on I/O1 Xplained Pro Extension Kit can operate at 100 kHz I²C speed.
  • The SERCOM3 (as I²C) retains the default 50-100 nanoseconds hold time for Serial Data (SDA) Hold Time because it aligns with the minimum (50 nanoseconds) start hold time in the specification of the temperature sensor chip (AT30TSE758).
  • The SERCOM3 (as I²C) retains the default 100 nanoseconds for I2C Trise time because it aligns with the maximum (300 nanoseconds) input rise time in the specification of the temperature sensor chip (AT30TSE758).

2

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

open_pin_configuration.png

3

Select the MHC Pin Settings tab and sort the entries by Ports names.

sercom_pins_setup_1.png

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

  • Enable I²C Clock (TWI_SCL)(SERCOM3_PAD1) on PA23 (Pin #93)
  • Enable I²C Data (TWI_SDA)(SERCOM3_PAD0) on PA22 (Pin #92)
sercom_pins_setup_2.png

4

In Clock Easy View, open the Peripheral Clock Configuration by clicking on the button Peripheral Clock Configuration.

peripheral_clock_config.png

Once the window is opened, scroll down to the SERCOM3_CORE peripheral and select GCLK2 (1 MHz) as the source clock to generate the peripheral clock frequency.

sercom_3_clock_configuration.png

When a peripheral is added to the project, the peripheral clock is automatically fed by the GCLK1. 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 Application Programming Interfaces (APIs) to read the temperature from the temperature sensor.

Step 2.3: Configure Universal Synchronous Asynchronous Receiver Transmitter (USART) Peripheral Library, USART Pins, and Verify USART Clock

1

Under the tab Available Components tab, expand Peripherals > SERCOM.

Double click on SERCOM2 to add the SERCOM instance 2 to the project.

sercom_selection_for_uart.png

Associate STDIO tool with SERCOM instance 2 (USART) peripheral by right clicking on the yellow diamond.

stdio_selection.png
stdio_linked.png

Select the SERCOM2 Peripheral Library in the Project Graph window and verify default SERCOM Operation Mode configuration is set as USART and configure it.

uart_sercom_setup.png

Verify the default baud rate is set to 115200 Hz.

Note: In this lab, the SERCOM2 (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.

2

Select the Pin Table tab and then scroll down to the SERCOM2 module as follows:

  • Enable USART_TX (SERCOM2_PAD0) on PB25 (Pin #101)
  • Enable USART_RX (SERCOM2_PAD1) on PB24 (Pin #100)
uart_sercom_pins_setup.png

3

In Clock Easy View, open the Peripheral Clock Configuration by clicking on the button Peripheral Clock Configuration.

peripheral_clock_config.png

Once the window is opened, scroll down to the SERCOM2_CORE peripheral and select GCLK2 (1 MHz) as the source clock to generate the peripheral clock frequency.

sercom_2_clock_configuration.png

Step 2.4: Configure DMA Peripheral Library

1

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

open_dma_configuration.png

2

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 the 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-bits/32-bits respectively.
  • Burst Length: this is a very fast data transfer mode. It can perform up to 16 transfers (beats) before releasing the control of the system bus back to the CPU. The default value of burst length is a single beat (one transfer).
    • In the default configuration, USART is configured for one-byte data at a time. So, the burst length is configured as a single beat.
  • FIFO Threshold: FIFO threshold level determines the size when the data in the internal DMA FIFO should be transferred to/from memory.
    • When the size of the data filled (in the internal DMA FIFO) becomes equal to the FIFO threshold size, the actual burst transfer (from the internal DMA buffer to USART peripheral) takes place, thereby improving the overall performance.
    • In the default configuration, the DMA transfers 1 beat of data as burst transfer.

Click on Add Channel to add the DMA channel and configure the DMA channel.

sercom_dma_setup.png

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, DMA PLIB notifies the application by calling the registered DMA callback event handler.



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