ATtiny's UPDI interface activates permanently upon noise input!?

UPDI - Unified Program and Debug Interface - is the new programming and debugging interface on new ATtiny MCUs from Microchip. The UPDI requires just a single pin on the MCU, so it is very suitable for low-pin count chips. Physically it is implemented as a bidirectional (half-duplex) UART protocol. UPDI pin is typically located with the RESET pin on the ATtiny MCUs. A programmable fuse in the MCU switches the RESET/UPDI between these three modes:

  1. UPDI programming interface - factory default. Pin always behaves as the UPDI interface. In this mode the MCU could be programmed using many cheap programmers, e.g. one possibility is mentioned here.
  2. /RESET - the pin behaves as a RESET pin (active low). Programming the MCU when the pin UPDI pin has been switched into the RESET mode requires first applying a 12V pulse followed by a special KEY word. As of this writing in August 2019 only the ATMEL POWER DEBUGGER (not cheap - 169 EUR) is capable of generating the 12V UPDI activation sequence. I don’t posses this debugger.
  3. GPIO mode - the pin could be used as another GPIO pin. A 12V activation pulse is required to start the UPDI communication as in the case of RESET mode.

UPDI Activation

In a normal MCU working mode, when there is no debugger connected, the internal UPDI interface is in a power-down state and it consumes basically no power. This is very important feature for any low-power applications of the MCU.

In the idle state the UPDI pin is pulled high by a built-in resistor. When a debugger/programmer is connected, it will pull the UPDI pin down for a moment, thus activating the UPDI interface. Then the UPDI protocol could start. This process is shown in datasheet picture below:

UPDI Enable Sequnce, (c) Microchip

After UPDI activation low-pulse, the SYNCH character 0x55 must be sent from the debugger within 14ms. If not, the interface is supposed to de-activate itself.

The Problem

It was discovered, by analysing a problem situation, that the UPDI interface in ATtiny will not deactivate itself when it gets activated by a random electrical noise on the UPDI pin:

Noise on UPDI caused by dettaching programming connector

This kind of electrical noise comes for example from the programming connector, when attaching or dettaching the connector while the target is under power.

Why this is a problem? Since active UPDI draws cca 2mA of current from VDD, it will quickly drain battery in any low-power application.

Mitigation

  1. In typical use-cases the ATtiny target is powered down after software is programmed, this clears UPDI, and after the next start there is no problem!
  2. In case you need to disconnect the programmer cable from the target while the target is running, do it while the programmer is still powered up.
  3. Perhaps the best mitigation would be to switch the UPDI pin into the reset mode, and use the 12V-capable programmer. At the moment I don’t the programmer capable of this mode, so I cannot test it.