4.2.4.2 UPDI and /RESET

The UPDI one-wire interface can be a dedicated pin or a shared pin, depending on the target AVR device. Consult the device datasheet 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 datasheet. The practical implications of each choice are given here.

Table 4-2. RSTPINCFG[1:0] Fuse Configuration
RSTPINCFG[1:0]	Configuration	Usage
00	GPIO	General purpose I/O pin. In order 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. In order 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 12V 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 12V pulse is required on that pin in order 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 in order 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 12V 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 0Ω 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 JTAGICE3 does not support 12V on the UPDI line. In other words, if the UPDI pin has been configured as GPIO or RESET the JTAGICE3 will not be able to enable the UPDI interface.