4.4.11 Advanced Debugging (AVR® JTAG/debugWIRE devices)
I/O Peripherals
Most I/O peripherals will continue to run even though the program execution is stopped by a breakpoint. Example: If a breakpoint is reached during a UART transmission, the transmission will be completed and corresponding bits set. The TXC (transmit complete) flag will be set and be available on the next single step of the code even though it normally would happen later in an actual device.
All I/O modules will continue to run in Stopped mode with the following two exceptions:
- Timer/Counters (configurable using the software front-end)
- Watchdog Timer (always stopped to prevent Resets during debugging)
Single Stepping I/O Access
Since the I/O continues to run in Stopped mode, care should be taken to avoid certain timing issues. For example, the code:
OUT PORTB, 0xAA IN TEMP, PINB
When running this code normally, the TEMP register would not read back
0xAA because the data would not yet have been latched physically to
the pin by the time it is sampled by the IN operation. A NOP
instruction must be placed between the OUT and the IN
instruction to ensure that the correct value is present in the PIN register.
However, when single-stepping this function through the OCD, this code will
always give 0xAA in the PIN register since the I/O is running at full
speed even when the core is stopped during the single-stepping.
Single Stepping and Timing
Certain registers need to be read or written within a given number of cycles after enabling a control signal. Since the I/O clock and peripherals continue to run at full speed in Stopped mode, single-stepping through such code will not meet the timing requirements. Between two single steps, the I/O clock may have run millions of cycles. To successfully read or write registers with such timing requirements, the whole read or write sequence should be performed as an atomic operation running the device at full speed. This can be done by using a macro or a function call to execute the code or use the run-to-cursor function in the debugging environment.
Accessing 16-Bit Registers
The Microchip AVR peripherals typically contain several 16-bit registers that can be accessed via the 8-bit data bus (e.g., TCNTn of a 16-bit timer). The 16-bit register must be byte accessed using two read or write operations. Breaking in the middle of 16-bit access or single-stepping through this situation may result in erroneous values.
Restricted I/O Register Access
Certain registers cannot be read without affecting their content. Such registers include those which contain flags which are cleared by reading, or buffered data registers (e.g., UDR). The software front-end will prevent reading these registers when in Stopped mode to preserve the intended non-intrusive nature of OCD debugging. Also, some registers cannot safely be written without side-effects occurring. These registers are read-only. For example:
- Flag registers, where a flag is
cleared by writing
1to any bit. These registers are read-only. - UDR and SPDR registers cannot be read without affecting the state of the module. These registers are not accessible.