Debugging 3: I/O View Memory View and Watch

I/O View

The I/O view provides a graphical view of the I/O memory map of the device associated with the active project. This debug tool will display the actual register content when debugging, allowing verification of peripheral configurations. It can also be used to modify the content of a register without having to recompile.

Todo: Use I/O view to:

Get an overview of the device memory map.
Check current peripheral configurations.
Modify peripheral configurations.
Validate configuration changes.

Remove all breakpoints and start a new debug session.
Break code execution by pressing the Break All button .
Open the I/O view from the top menu bar by going to Debug → Windows → I/O.
Scroll through the list of peripherals and select I/O Ports (PORTB). Find the OUT register and click on Bit 4 in the Bits column, so the corresponding square changes color, as depicted in Figure 1. Observe that clicking Bit 4 in the PORTB.OUT register toggles the output level on GPIO pin PB4, which controls the LED on the ATtiny817 Xplained Pro.
Figure 1. Manipulate Bit Value in Register Using I/O View

Info: The I/O view is refreshed after any register has been modified, and all detected changes are highlighted in red.

Tip: Multiple bits can be modified simultaneously by double-clicking the value field and typing in the desired value to be assigned to the register.
Expand the Clock controller (CLKCTRL) in the I/O view, and answer the following questions:
- What is the currently selected clock source (Clock select)?
- What is the configured prescaler value (Prescaler division)?
- Is the main clock prescaler enabled (MCLKCTRLB.PEN)?
Result: The Clock controller should be configured with the ATtiny817 default clock settings; the main clock is running from the internal RC oscillator with prescaler enabled and a division factor of six.

Info: The default clock configuration guarantees that the device will execute code reliably over the entire supported operating voltage range, 1.8V to 5.5V. The Xplained Pro kit powers the ATtiny817 at 3.3V. According to the “General Operating Ratings” section in the device data sheet, the device can be safely run at 10 MHz with a 3.3V supply.

The code will now be changed to run the ATtiny817 at 10 MHz. Modify the start of main() as below:

int main(void)
{
    /*
     * Set the Main clock division factor to 2X, 
     * and keep the Main clock prescaler enabled.                
     */
    CLKCTRL.MCLKCTRLB = CLKCTRL_PDIV_2X_gc | CLKCTRL_PEN_bm;

Start a new debug session in order to recompile the project and program the device.
Halt code execution by clicking . Examine the clock settings in I/O view, depicted in Figure 2.
Figure 2. Clock Settings in I/O View Remain Unchanged

Result: There is a problem! The prescaler remains unchanged.
Select the MCLKCTRLB register in I/O view, as indicated in Figure 3.
Figure 3. Select MCLKCTRLB in I/O View
Push F1 on the keyboard to bring up a web-based register description.
Info: Internet access is required to use the web-based register description. Refer to an offline version of the ATtiny817 data sheet if internet access is not available.
Find out if any access restrictions apply to the MCLKCTRLB register.
Result: The register is protected by the Configuration Change Protection (CCP) mechanism. Critical registers are configuration change protected to prevent unintended changes. These registers can only be modified if the correct unlock sequence is followed, as described in the data sheet.
Replace the line of code which was just added with the following:
```
_PROTECTED_WRITE(CLKCTRL.MCLKCTRLB, CLKCTRL_PDIV_2X_gc | CLKCTRL_PEN_bm);
```
Info: _PROTECTED_WRITE() is an assembly macro that guarantees timing requirements for unlocking protected registers are met. It is recommended to use this macro when modifying protected registers.

Tip: Right-click the macro name in the code and select Goto Implementation to navigate to the implementation of the macro. This is also possible by placing the cursor at the macro name in the code and pressing Alt+G on the keyboard. The same process can also be used for variable declarations and function implementations.
Stop the previous debug session and launch a new session to program the device with the changes.
Break code execution and use the I/O view to verify that the prescaler is now successfully set to 2X, as indicated in Figure 4.
Figure 4. Clock Settings in I/O View Changed Successfully

Tip: The Processor Status window is the register view tool for the AVR Core. This tool can be opened from the top menu bar by going to Debug → Windows → Processor Status. This window will provide a detailed view of the status of the internal AVR Core registers. This view can be used to check if global interrupts are enabled; look for the I-bit in the status register.

Result: The capabilities of the I/O view have been used to find and fix a bug in the project.

Memory View

Todo: Write two strings to the beginning of the ATtiny817 EEPROM and use Memory view to verify the EEPROM contents.

Add #include <avr/eeprom.h> after the #include <avr/io.h> line.

Add the following code before the while(1) loop in main():

    uint8_t hello[] = "Hello World";
    eeprom_write_block(hello, (void *)0, sizeof(hello));
    uint8_t hi[] = "AVR says hi";
    eeprom_write_block(hi, (void *)0, sizeof(hi));

Place a breakpoint next to the first call to eeprom_write_block() as in Figure 5.
Figure 5. Breakpoint to Halt for Checking EEPROM
Start a new debug session in order to program the device with the updated code.
After the breakpoint has been hit, open the memory window from the top menu bar by going to Debug → Windows → Memory → Memory 1. Look at the current content of the EEPROM.
Push F10 on the keyboard to step over the eeprom_write_block() call and verify the EEPROM write.
Allow the ATtiny817 to execute the next EEPROM write before verifying the write using the Memory view. The view should appear as in Figure 6 at each interval respectively.
Figure 6. Memory View Updating After EEPROM Writes

Tip: The Memory view tool can also be used to investigate the contents of other AVR memory sections, including the program memory. This can be useful when debugging bootloaders.

Result: The content of the EEPROM is updated after each call to eeprom_write_block(). The updated content is highlighted in red, and the ASCII interpretation of the EEPROM content matches the written strings. Therefore, the contents of EEPROM after writing to it have been verified using Memory view.

Watch Window

This is covered in more detail in section Debugging 2: Conditional- and Action-Breakpoints, however, the note on how to cast pointers as an array in the Watch window is repeated here.

Info: A variable can also be added to the Watch window by clicking on an empty field name and typing the variable name. This way, it is even possible to cast a variable to a different data type for better readability in the Watch window. This is especially useful if it is required to look at an array that is passed to a function as a pointer.

For example, if an array is passed to a function, it will be passed to the function as a pointer. This makes it impossible for Atmel Studio to know the length of the array. If the length of the array is known, and it needs to be examined in the Watch window, the pointer can be cast to an array using the following cast:

*(uint8_t (*)[<n>])<name_of_array_pointer>

Where <n> is the number of elements in the array and <name_of_array_pointer> is the name of the array to be examined.

This can be tested on the SW0_edge_count variable by entering the following in an empty name field in the Watch window:

*(uint8_t (*)[5])&SW0_edge_count

Note that the '&' symbol must be used in this case to obtain a pointer to the variable.

Result: Atmel Studio has now been used to inspect and modify the contents of variables in the code.

Code used for Debugging 3

#include <avr/io.h>
#include <avr/eeprom.h>

void LED_on(void);
void LED_off(void);
void LED_set_state(uint8_t state);
uint8_t SW_get_state(void);
uint8_t SW_get_state_logic(void);

int main(void)
{
    PORTB.DIRSET   = PIN4_bm;          /* Configure LED Pin as output */
    PORTB.PIN5CTRL = PORT_PULLUPEN_bm; /* Enable pull-up for SW0 pin  */
    
	_PROTECTED_WRITE(CLKCTRL.MCLKCTRLB, CLKCTRL_PDIV_2X_gc | CLKCTRL_PEN_bm);
	
	uint8_t Hello[] = "Hello World!";
	save(Hello,sizeof(Hello));
	uint8_t Hi[] = "AVR says hi!";
	save(Hi,sizeof(Hi));
	
	while(1)
    {
        uint8_t SW0_state = SW_get_state_logic(); /* Read switch state */
        LED_set_state(SW0_state);           /* Set LED state     */
    }
}

void save(const uint8_t* to_save, uint8_t size)
{
	eeprom_write_block(to_save,(void*)0,size); 
}

uint8_t SW_get_state()
{
	return !(PORTB.IN & PIN5_bm);
}

uint8_t SW_get_state_logic(void)
{
    static uint8_t SW0_prv_state  = 0;
    static uint8_t SW0_edge_count = 0;

    uint8_t SW0_cur_state = !(PORTB.IN & PIN5_bm); /* Read the current SW0 state  */
    if (SW0_cur_state != SW0_prv_state)            /* Check for edges             */
    {
        SW0_edge_count++;
    }
    SW0_prv_state = SW0_cur_state;                /* Keep track of previous state */

    /*
     * Report the switch as pushed when it is pushed or the edge counter is a
     * multiple of 3
     */
    return SW0_cur_state || !(SW0_edge_count % 3);
}

void LED_off(void)
{
    PORTB.OUTSET = PIN4_bm; /* Turn LED off  */
}

void LED_on(void)
{
    PORTB.OUTCLR = PIN4_bm; /* Turn LED on */
}

void LED_set_state(uint8_t state)
{
    if (state)
    {
        LED_on();
    }
    else
    {
        LED_off();
    }
}