Microchip Technology Atmel-ICE User Manual page 41

selecting the 'Disable debugWIRE and Close' menu option from the 'Debug' menu. DebugWIRE will be temporarily
disabled, and the software front-end will use SPI programming to unprogram the DWEN fuse.
Having the DWEN fuse programmed enables some parts of the clock system to be running in all sleep modes. This
will increase the power consumption of the AVR while in sleep modes. The DWEN Fuse should, therefore, always be
disabled when debugWIRE is not used.
When designing a target application PCB where debugWIRE will be used, the following considerations must be made
for correct operation:
• Pull-up resistors on the dW/(RESET) line must not be smaller (stronger) than 10 kΩ. The pull-up resistor is not
required for debugWIRE functionality since the debugger tool provides this.
• Any stabilizing capacitor connected to the RESET pin must be disconnected when using debugWIRE since they
will interfere with correct operation of the interface
• All external Reset sources or other active drivers on the RESET line must be disconnected, since they may
interfere with the correct operation of the interface
Never program the lock-bits on the target device. The debugWIRE interface requires that lock-bits are cleared to
function correctly.
4.4.15 debugWIRE Software Breakpoints
The debugWIRE OCD is drastically downscaled when compared to the Microchip megaAVR (JTAG) OCD. This
means that it does not have any Program Counter breakpoint comparators available to the user for debugging
purposes. One such comparator does exist for purposes of run-to-cursor and single-stepping operations, but
additional user breakpoints are not supported in hardware.
Instead, the debugger must make use of the AVR BREAK instruction. This instruction can be placed in FLASH, and
when loaded for execution, it will cause the AVR CPU to enter Stopped mode. To support breakpoints during
debugging, the debugger must insert a BREAK instruction into FLASH at the point at which the users request a
breakpoint. The original instruction must be cached for later replacement. When single-stepping over a BREAK
instruction, the debugger has to execute the original cached instruction to preserve program behavior. In extreme
cases, the BREAK has to be removed from FLASH and replaced later. All these scenarios can cause apparent
delays when single-stepping from breakpoints, which will be exacerbated when the target clock frequency is very low.
It is thus recommended to observe the following guidelines, where possible:
• Always run the target at as high a frequency as possible during debugging. The debugWIRE physical interface
is clocked from the target clock.
• Try to minimize the number of breakpoint additions and removals, as each one requires a FLASH page to be
replaced on the target
• Try to add or remove a small number of breakpoints at a time, to minimize the number of FLASH page write
operations
• If possible, avoid placing breakpoints on double-word instructions
4.4.16 Understanding debugWIRE and the DWEN Fuse
When enabled, the debugWIRE interface takes control of the device's /RESET pin, which makes it mutually exclusive
to the SPI interface, which also needs this pin. When enabling and disabling the debugWIRE module, follow one of
these two approaches:
• Let the software front-end take care of things (recommended)
• Set and clear DWEN manually (exercise caution, advanced users only!)
Important: When manipulating DWEN manually, the SPIEN fuse must remain set to avoid having to use
High-Voltage programming.
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2020 Microchip Technology Inc.
Programmers and Debuggers
User Guide
On-Chip Debugging
DS50002999A-page 41