Step 2.1: Configure RTC Peripheral Library
From the Clock Easy View MCC plugin, verify that the RTC clock is set to run at 1 kHz internal ultra-low-power clock.
When a module is added to the project graph, MCC automatically enables the clock to the module. The default RTC clock source is an external 1 kHz clock (OSCULP1K).
Note: RTC can be clocked through several low power clocks sources of 1 kHz and 32 kHz as shown above on the SAM E51 device. The 1 kHz clock source retained (OSCULP1K) is enough to generate time periods of 500 milliseconds, 1 second, 2 seconds, and 4 seconds.
Go back to the project graph and configure the RTC PLIB to generate a compare interrupt every 500 milliseconds.
Note: 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 (0x200).
Step 2.2: Configure USART Peripheral Library and USART Pins
Under the left tab Resource Management (MCC), go to Device Resources and expand Libraries > Harmony > Peripherals > SERCOM.Click on the symbol beside SERCOM 5 to add the SERCOM instance 5 (SERCOM 5 PLIB) to the project.
Select the SERCOM 5 Peripheral Library in the Project Graph and configure it for USART protocol.
Verify the default baud rate is set to 115,200 Hz.
Note: The application will use the SERCOM 5 (as USART) PLIB for printing messages on the serial terminal. Hence, only the transmit functionality is enabled and the receive functionality is disabled.
Step 2.4: Configure DMA Peripheral Library
Click on the DMA Settings tab. Configure DMA Channel 0 to transfer application buffer to the USART TX register. The DMA transfers 1 byte from the user buffer to USART transmit buffer on each trigger.
Based on the trigger source, the DMA channel configuration is automatically set by MCC.
- 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 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 MCC 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.
Disable USART Interrupt
Under the Project Graph window, click on the SERCOM 5 block.
In the Configuration Options pane, change Operating Mode to Blocking mode from Non-blocking mode.
Note: In this lab, SERCOM 5 (as USART) interrupt is disabled as the application does not need a callback on USART transfer complete. A USART transmit buffer empty event triggers DMA to transfer 1 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 event handler.