• Table of contents
• attiny-wdt
  • Operating Principle
  • Pinout
    • JP1
    • JP2
  • Pictures

attiny-wdt¶

attiny-wdt is an OSHW + open source firmware project of turning an Attiny{4,5,9,10} microcontroller into a hardware watchdog that can be used to reset another CPU/SoC when it gets stuck.

The primary use case was to use it with Raspberry Pi devices. Those do have a built-in hardware watchdog themselves, but unfortuantelly it is not documented at the register-level and there is no option to enable it straight from the boot loader, so it is not really providing the kind of guarantees one would expect from a watchdog.

Operating Principle¶

The general idea is to use the Timer/Counter block of the ATTiny9 in the following way:

• Clear the output OC0B on BOTTOM (counter == 0)
  • this generates a rising edge on the !RESET signal of the CPU we're guarding
• Set the output OC0B when counter reaches OCR0B
  • this generates a falling edge on the !RESET signal of the CPU we're guarding
• Wrap the counter once it reaches OCR0A
  • this determines the amount of time until a reset
• Configure any falling edge on the INT0 pin to re-set the counter to zero
  • this is what the software on the CPU we're guarding nees to do to avoid the watchdog from expiring

Pinout¶

The board contains two connectors:

JP1¶

This header is used in two cases:

1. programming phase: the firmware into the ATtiny9 (using the TPI protocol)
2. normal use: To supply VCC to the watchdog and to connect its output with !RESET/GLOBAL_EN of the target

Number	Name	Description	Programming Function
1	VCC	3.3V supply voltage powering the watchdog	VCC
2	GLOBAL_EN	watchdog output; expected to be connected to the !RESET of the CPU we're guarding (or GLOBAL_EN in case of Raspi)	TPICLK
3	PB0	-	TPIDATA
4	!RESET	-	!RESET
5	GND	Ground	GND

JP2¶

Number	Name	Description
1	GND	Ground
2	PB2	Input; any falling edge re-sets the counter to zero. Connected to GPIO26 on Raspi
3	PB0/GPIO19	No function; RFU

Pictures¶

The pictures below show one of the first hand-soldered prototypes: