SAM C21 SysTick Example Project


This page provides a simple SAM C21 System Timer (SysTick) GCC code example for the ATSAMC21J18A MCU. The project configures the SysTick module to produce interrupts every millisecond, using the default CPU clock frequency (4 MHz). The SysTick handler routine is used to count these interrupt events and toggle the onboard LED once every second.

The application is designed to work using the SAM C21 Xplained PRO Evaluation Kit (ATSAMC21-XPRO) which contains the ATSAMC21J18A Arm® Cortex®-M0+ MCU.


This application uses:

  • CPU Core SysTick Timer Module, clocked by the default CPU clock (4 MHz)
  • PA15 (User LED, LED0)

This code example uses a direct/indirect MCU register C-coding style (i.e., no software framework) and is built using the GCC compiler toolchain within the Atmel Studio 7 IDE.

Visit the following page to learn how to configure this periph eral:

SysTick Configuration
Learn more >


Hardware Tools

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SAM C21 Xplained Pro
Evaluation Kit

Software Tools

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Atmel® Studio
Integrated Development Environment

Exercise Files

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Example Project

We recommend extracting the ZIP file to your C:\ folder.

You should see the folder C:\MTT\32arm\samc21\code-examples-gcc\systick\systick-initialization containing the solution systick-initialization.atsln.

 Connection Diagram

SAM C21 Xplained Pro contains an Embedded Debugger (EDBG) that can be used to program and debug the ATSAMC21J18A using Serial Wire Debug (SWD) interface. The EDBG also includes a Virtual Com port interface over UART, a Data Gateway Interface (DGI) over SPI and TWI, and it monitors four of the SAM C21 GPIOs. Atmel Studio 7 is used as a front-end for the EDBG.

The LED0 is driven by this application and is connected to port PA15 as shown:


The virtual COM port in the EDBG requires the terminal software to set the Data Terminal Ready (DTR) signal to enable the UART pins connected to the ATSAMC21J18A. If the DTR signal is not enabled, the UART pins on the EDBG are kept in high-z (tristate) rendering the COM port unusable. The DTR signal is set automatically by some terminal software, but it may have to be manually enabled in your terminal.

VCC Selection Header

The SAM C21 Xplained Pro has a 3-pin header labeled VCC-SEL on the board. This header can be used to select between 3.3 V and 5.0 V as the supply voltage for the ATSAMC21J18A, peripherals, and extension headers by placing a jumper on the left (3.3 V) or right (5.0 V). Selecting 5.0 V supplies the kit directly from the USB or an external 5.0 V source. Selecting 3.3 V supplies the kit from an onboard regulator.

Select 3.3V as VCC by jumpering the left 2 pins on this header as shown here in the red box:



Attach the SAM C21 Xplained Pro board to your computer using a USB A-to-MicroB cable. Start Atmel Studio 7. If the board has been successfully enumerated, you should see the board image come up in Studio as shown:


The board is identified by the last four digits of its serial number (see the sticker on the bottom of the board). In this example, the last four digits are "3514".


Open the Solution

Select File > Open > Project/Solution…


Navigate to the solution folder and select the systick-initialization.atsln solution file:



Configure the Debugger

Next, you need to configure the debugger in Atmel Studio to discover and connect to the target EDBG IC on your Xplained Pro board.

First, select (click) on the project in the Solution Explorer window, then select Project > Properties as shown:


Next, under the project's 'Tool' settings, select your EDBG target from the pull-down. Select 'SWD' as the interface:


Save the tool setting by clicking the Save All button:



Rebuild/Program the Target

Finally, click on the Start Without Debugging icon in Atmel Studio which (i) re-builds the HEX file from the project source code, (ii) downloads/programs the HEX file onto the target MCU, (iii) and releases the target MCU Reset pin, allowing the program to execute.


If prompted, upgrade the EDBG firmware on the board:


You need to click on Start Without Debugging again after a EDBG firmware upgrade in order to rebuild/program the target.



LED0 toggles every second.


This project has provided a simple example of how to set up and use the SysTick timer on the SAM C21 MCU.

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