The UPDI one-wire interface can be a dedicated pin or a shared pin, depending on the target AVR device. Consult the device data sheet for further information.

When the UPDI interface is on a shared pin, the pin can be configured to be either UPDI, /RESET, or GPIO by setting the RSTPINCFG[1:0] fuses.

The RSTPINCFG[1:0] fuses have the following configurations, as described in the data sheet. The practical implications of each choice are given here.

Table 1. RSTPINCFG[1:0] Fuse Configuration
RSTPINCFG[1:0] Configuration Usage
00 GPIO General purpose I/O pin. To access UPDI, a 12V pulse must be applied to this pin. No external Reset source is available.
01 UPDI Dedicated programming and debugging pin. No external Reset source is available.
10 Reset Reset signal input. To access UPDI, a 12V pulse must be applied to this pin.
11 Reserved NA
Note: Older AVR devices have a programming interface, known as “High-Voltage Programming” (both serial and parallel variants exist.) In general, this interface requires 12 V to be applied to the /RESET pin for the duration of the programming session. The UPDI interface is an entirely different interface. The UPDI pin is primarily a programming and debugging pin, which can be fused to have an alternative function (/RESET or GPIO). If the alternative function is selected then a 12 V pulse is required on that pin to re-activate the UPDI functionality.
Note: If a design requires the sharing of the UPDI signal due to pin constraints, steps must be taken to ensure that the device can be programmed. To ensure that the UPDI signal can function correctly, as well as to avoid damage to external components from the 12 V pulse, it is recommended to disconnect any components on this pin when attempting to debug or program the device. This can be done using a 0 Ω resistor, which is mounted by default and removed or replaced by a pin header while debugging. This configuration effectively means that programming should be done before mounting the device.
Important: The Atmel-ICE does not support 12 V on the UPDI line. In other words, if the UPDI pin has been configured as GPIO or RESET, the Atmel-ICE will not be able to enable the UPDI interface.