OCD and clocking
When the MCU enters stopped mode, the OCD clock is used as MCU clock. The OCD clock is either the JTAG TCK if the JTAG interface is being used, or the PDI_CLK if the PDI interface is being used.
SDRAM refresh in stopped mode
When the OCD is in stopped mode, the MCU is clocked by the PDI or JTAG clock, as described in the paragraph above. Since nothing is known of this frequency by the debugger or OCD, a low refresh period (0x20) is automatically used. This value can't be changed by the user, and it might not fit with all combinations of OCD clock frequency and SDRAM devices. If SDRAM problems are observed in stopped mode, try to adjust the OCD clock frequency.
I/O modules in stopped mode
Different from earlier Atmel megaAVR devices, in XMEGA the I/O modules are stopped in stop mode. This means that USART transmissions will be interrupted, timers (and PWM) will be stopped.
Hardware breakpoints
There are four hardware breakpoint comparators - two address comparators and two value comparators. They have certain restrictions:
All breakpoints must be of the same type (program or data)
All data breakpoints must be in the same memory area (I/O, SRAM, or XRAM)
There can only be one breakpoint if address range is used
Here are the different combinations that can be set:
Two single data or program address breakpoints
One data or program address range breakpoint
Two single data address breakpoints with single value compare
One data breakpoint with address range, value range or both
Atmel Studio will tell you if the breakpoint can't be set, and why. Data breakpoints have priority over program breakpoints, if software breakpoints are available.
External reset and PDI physical
The PDI physical interface uses the reset line as clock. While debugging, the reset pullup should be 10k or more or be removed. Any reset capacitors should be removed. Other external reset sources should be disconnected.
Debugging with sleep for ATxmegaA1 rev H and earlier
There was a bug in the early versions of the ATxmegaA1 family that prevented the OCD to be enabled while the device was in certain sleep modes. There are two methods to use to get back on the debugging:
Enter the Atmel AVR ONE! options dialog and enable "Always activate external reset when reprogramming device"
Perform a chip erase
The sleep modes that trigger this bug are:
Power-down
Power-save
Standby
Extended standby
The consequence of this bug is that attaching to a target using LiveDebug will not work when the target is in these sleep modes. The programming dialog is not affected by this bug.