General Information on Data Breakpoint

Data Breakpoint can be edited/added only in debug mode
Local variables must always be qualified with the function name, which is also the case if the user wants to add a variable from the function that the program has stopped in
Data breakpoints on local variables can result in false hits due to the reuse of stack memory.

Tip:
Declare local variables as static and provide the static variable's address in the location field. The address of the static variable is fixed during compilation/linking.

Global variables are initialized with default values during the start-up. The valid data breakpoint for global variables will hit in the disassembly or initialization code during the start-up.
There can be several instructions to perform read/write data for a variable. E.g., the 'int' data type can have two individual bytes read/write instructions, so the data breakpoint hits twice for the same variable.
Data breakpoint event can occur when the bus access happens for the specific address

Maximum number of data breakpoint supported (This may vary based on specific device/family. Refer to the device specific data sheet.):

Architecture	Maximum Data Breakpoint Supported
ATmega	Two without Data Mask (OR) One without Data Mask and one with Data Mask
XMEGA	Two without Data Mask (OR) One without Data Mask and one with Data Mask (OR) Two with Data Mask
UC3	Two without Data Mask (OR) One without Data Mask and one with Data Mask (OR) Two with Data Mask
SAM	Device dependent. Refer to the device specific data sheet.
Tiny	Does not support data breakpoint

Most of the devices conform to the above limits.

Note: ATmega and SAM devices use multiple hardware resources when a data breakpoint with a data mask is set. Hence, using a data mask can reduce the number of data breakpoints to be set.

Software

General Information on Data Breakpoint