34.3.2.2 UPDI Enable with 12V Override of RESET Pin

GPIO or Reset functionality on the RESET pin can be overridden by the UPDI by using 12V programming. By applying a 12V pulse to the RESET pin, the pin functionality is switched to UPDI, independent of RSTPINCFG in FUSE.SYSCFG0. It is recommended to always reset the device before starting the 12V enable sequence.

During power-up, the Power-on Reset (POR) must be released before the 12V pulse can be applied. The duration of the pulse is recommended in the range from 100 μs to 1 ms, before tri-stating. When applying the rising edge of the 12V pulse, the UPDI will be reset. After tri-stating, the UPDI will remain in Reset until the RESET pin is driven low by the debugger. This will release the UPDI Reset and initiate the same enable sequence as explained in 34.3.2.1 UPDI Enable with Fuse Override of RESET Pin.

The following figure shows the 12V enable sequence.

Figure 34-6. UPDI Enable Sequence by 12V Programming

When enabled by 12V, only a POR will disable the UPDI configuration on the RESET pin, and restore the default setting. If issuing a UPDI Disable command through the UPDIDIS bit in UPDI.CTRLB, the UPDI will be reset and the clock request will be canceled, but the RESET pin will remain in UPDI configuration.

Note: If insufficient external protection is added to the UPDI Pin, an ESD pulse can be interpreted as a 12V override by the microcontroller and enables the UPDI.