Low Power Application on SAM L21 (Cortex® M0+) MCUs Using MPLAB® Harmony v3 Peripheral Libraries

Objectives

This tutorial shows you how to create a low-power application on a SAM L21 using the MPLAB^® Harmony v3 software framework.

The SAM L21 is a 32-bit Arm^® Cortex^® M0+ based microcontroller (MCU) that provides features to reduce power consumption through different Sleep modes such as Idle, Standby, Backup, and Off.

• In Idle mode, the CPU and the synchronous clocks are stopped, except when requested. This mode allows power optimization with a fast wake-up time
• In Standby mode, the CPU and the peripherals are stopped, except those that are running using the Run in Standby mode feature. This mode allows the device to consume the lowest power with little overhead on wake-up time
• In Backup mode, the device is entirely powered off, except for the peripherals in the backup domain. This mode allows you to achieve the lowest power consumption aside from Off mode but increases the wake-up time compared to Standby mode
• In Off mode, the device is entirely powered off. It allows you to achieve the lowest power consumption. Since the device must be reset to recover from this mode, the wake-up time is the highest

The Power Manager (PM) module is responsible for controlling the Low Power modes. Additionally, the SAM L21 provides SleepWalking, an advanced low-power operation mode, which is based on event propagation managed by the Event System (EVSYS).

This application uses the SAM L21 Xplained Pro B evaluation kit and the I/O1 Xplained Pro extension kit (sold separately).

The application is developed on the MPLAB Harmony v3 software framework. MPLAB Harmony is a modular framework that provides interoperable firmware libraries for application development on 32-bit microcontrollers and microprocessors. It includes an easy-to-use Graphical User Interface (GUI) (MPLAB Code Configurator (MCC)) for selecting, configuring, and generating starter codes, peripheral libraries, and middlewares (USB, TCP/IP, graphics, and so on).

On power-up, the device is set to Standby Sleep mode. The device wakes up from Standby mode and enters Active mode when you cover the light sensor (by placing a hand over it, for example) on the I/O1 Xplained Pro extension kit. The application periodically prints room temperature on a serial terminal while the light sensor is covered. The device goes back to Standby mode when you uncover the light sensor. LED0 in the SAM L21 evaluation kit is toggled whenever the temperature is displayed on the serial console. When you press the switch button, SW0, the application switches Sleep mode from Standby mode to Idle mode.

The application you create will utilize the following peripherals:

• SERCOM2 (as I²C) peripheral library to read the temperature from the temperature sensor
• Real-time clock (RTC) peripheral library to periodically generate an event for the ADC to sample the light sensor
• SERCOM3 (as Universal Synchronous Asynchronous Receiver Transmitter (USART)) peripheral library to print messages on a serial console running on a computer
• Direct Memory Access Controller (DMAC) peripheral library to send the converted temperature value to the SERCOM3
• PORT peripheral library to toggle the LED0
• Analog-to-Digital Converter (ADC) peripheral library to sample the light sensor analog input and detect whether the light sensor is covered or not
• PM and Supply Controller (SUPC) peripheral libraries to configure Low Power modes
• External Interrupt Controller (EIC) peripheral library to control the user button SW0
• EVSYS peripheral library to trigger the start of the ADC conversion on every RTC compare match event. The EVSYS allows for peripheral-to-peripheral communication without CPU intervention. This reduces the burden on the CPU and other resources when compared to the conventional interrupt-based systems

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There are two approaches for this tutorial:

1. Create the project from scratch:
   • Use the provided source files and step-by-step instructions below
2. Use the solution project as an example:
   • Build the solution project and download it to the SAM L21 Xplained Pro evaluation kit to observe the expected behavior

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Lab Objectives

1. Create an MPLAB X IDE Harmony v3 project for a SAM L21 microcontroller from scratch.
2. Use MCC to configure and generate Harmony v3 Peripheral Libraries code for RTC, I²C, USART, Direct Memory Access (DMA), ADC, PM, SUPC, EIC, EVSYS, and PORT peripherals.
3. Use the Harmony v3 Peripheral Library Application Programming Interfaces (APIs) to implement and demonstrate a low-power application.

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Materials

Hardware Tools

• SAM L21 Xplained Pro Evaluation Kit
• I/O1 Xplained Pro Extension Kit

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Software Tools

• MPLAB X Integrated Development Environment (IDE)
• MPLAB XC32/32++ Compiler
• MPLAB Harmony v3 Software Framework
   

For this lab, download the following repositories from GitHub:

• CSP: The following table shows the summary of contents

• DEV_PACKS: The following table shows the summary of contents

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Overview

This lab shows you how to create an MPLAB Harmony v3 project from scratch. You will configure and generate Harmony v3 peripheral library code for the RTC, I²C, USART, ADC, DMA, PM, SUPC, EIC, EVSYS, and PORT peripherals. It also demonstrates the Low-Power mode features of the SAML21 MCU using light and temperature sensors on the I/O1 Xplained Pro extension kit.

The application flow is as follows:

• On power-up, the application runs in Standby Sleep mode
• The RTC is configured as an event generator to generate a periodic (every 500 ms) event to trigger the ADC to start conversion
• The ADC acknowledges the event from the RTC and starts the conversion of the light sensor signal
• If the ADC conversion result is greater than the defined threshold, the ADC produces an interrupt to wake-up the CPU
• In Active mode, the CPU submits an I²C read transfer request to read the temperature value from the temperature sensor on the I/O1 Xplained Pro evaluation kit
• The application submits a DMA transfer request to transfer the latest temperature value (in a formatted message) to the USART (serial console)
• After the DMA transfer is complete, the device re-enters Standby Sleep mode

The application also monitors the pressing of the switch SW0. If a switch press is detected, the application moves the device to Idle Sleep mode. As in Standby Sleep mode, covering the light sensor with your hand wakes up the device from Idle Sleep mode. Once awake, the application prints room temperature values on the serial console and re-enters Standby Sleep mode once the light sensor is uncovered. 

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Lab Source Files and Solutions

This ZIP file contains the completed solution project for this lab. It also contains the source files needed to perform the lab by following the step-by-step instructions (see the "Procedure" section on this page).

ZIP

• Windows®
• Linux®
• macOS® X

Extracting the ZIP file creates the following folders:

• saml21_low_power contains the lab solution (in the firmware folder) and source files (in the dev_files folder)
  • dev_files contains subfolder sam_l21_xpro containing application source files and other support files (if any) required to perform the lab (see "Procedure" section below)
  • firmware contains the completed lab solution project. It can be directly built and downloaded on the hardware to observe expected behavior

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Procedure

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Lab Index

Step 1: Create a Project and Configure the SAM L21

• Step 1.1 - Verify that MCC Plug-in is Installed in MPLAB X IDE
• Step 1.2 - Create MPLAB Harmony v3 Project Using MPLAB X IDE
• Step 1.3 - Configure Clock Settings

Step 2: Configure RTC, I²C, USART, and DMA Peripheral Libraries

• Step 2.1 - Configure RTC Peripheral Library
• Step 2.2 - Configure I²C Peripheral Library, I²C Pins, and Verify I²C Clock
• Step 2.3 - Configure USART Peripheral Library, USART Pins, and Verify USART Clock
• Step 2.4 - Configure DMA Peripheral Library

Step 3: Configure ADC, Event System, and EIC Peripheral Libraries

• Step 3.1 - Configure Analog-to-Digital Converter (ADC) Peripheral Library (PLIB)
• Step 3.2 - Configure External Interrupt Controller (EIC) PLIB for Switch Button
• Step 3.3 - Configure Event System (EVSYS) PLIB

Step 4: Configure PM, SUPC and NVMCTRL Peripheral libraries, LED, and Wake-up Test Pins

• Step 4.1 - Configure Power Manager (PM) Peripheral Library
• Step 4.2 - Configure the Supply Controller (SUPC) Peripheral Library
• Step 4.3 - Configure the Non-volatile Memory Controller (NVMCTRL) Peripheral Library
• Step 4.4 - Configure LED and Wake-up Test Pins

Step 5: Generate Code
Step 6: Add Application Code to the Project
Step 7: Build, Program, and Observe the Outputs

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