6.1 Data Polling Example Code

A Mass Storage Class example is used as an example on how to use the data polling and control of data variables features. A SAM L21 Xplained Pro board is connected to a host computer trough both the Target USB and Debug USB connectors on the kit. The ATSAML21 target device is running the USB Device MSC Example from ASF for SAM L21 Xplained Pro.

To be able to work through this example, the following is required:

Host computer with Atmel Studio 7 (or later) installed (Data Visualizer is included)
ATSAML21 Xplained Pro kit

Todo:

Connect both the Target USB and Debug USB connectors on the SAM L21 Xplained Pro board

This example makes use of the USB Device MSC Example from ASF for SAM L21 Xplained Pro.

Todo:

In Atmel Studio, create a New Example Project
In the New Example dialog, select the SAM L21 device family (or other relevant device) and filter by the keyword “MSC”
Select the USB Device MSC Example
Build the project/solution (F7)

Todo:

Open the project properties (right click the project in the Solution Explorer and select Properties)
On the Tool tab, select the appropriate tool and interface

Now see how Data Visualizer can poll variables from the target and display their values in graphical form.

Important: Data polling is only available when Data Visualizer is run as an extension within Atmel Studio. This is because it needs to access the debug system on the device through the Atmel Studio debugger backend.

First, add a few lines of code containing variables to poll.

Todo: Open ui.c and add two global variables to the top of the file.

volatile uint32_t write_count = 0;
volatile uint32_t read_count = 0;

Important: Declaring variables you are interested in polling as volatile will ensure that they are placed in SRAM and that their values will not be cached in registers by the compiler. Registers cannot be polled, only SRAM locations.

Tip: Data polling operates on absolute SRAM locations. It is thus advised to use global variables for this purpose so that they are always available at the same location in SRAM. Polling locations in the stack can yield unpredictable results based on the stack context at the time of polling.

Todo: Modify the two ̔startʼ functions in ui.c to increment read and write counters on each access started.

void ui_start_read(void)
{
	port_pin_set_output_level(EXT1_PIN_GPIO_0, true);
	read_count++;
}

void ui_start_write(void)
{
	port_pin_set_output_level(EXT1_PIN_GPIO_1, true);
	write_count++;
}

Todo:

Build the project/solution (F7)
Open Data Visualizer
Connect

For data polling functionality, enable the Code Profiling interface.

Todo:

Start the Data Visualizer session
Launch the debug session using Start Debugging and Break (Alt + F5)

Data polling operates on SRAM locations, so to find out where variables are located in SRAM we need to use the Atmel Studio Watch window.

Todo:

Locate the two global variables added to ui.c
Right-click each variable and select Add to Watch
Examine the type field of each variable in the Watch window to find its location

Switch back to the Data Visualizer to set up the Code Profiling interface and to connect the two variables to a graph.