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h1. 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.

h2. 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

h2. Pinout

The board contains two connectors:

h3. JP1

This header is used in two cases:

# *programming phase:* the firmware into the ATtiny9 (using the TPI protocol)

# *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|

h3. 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|

h2. Pictures

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

{{thumbnail(attiny-wdt1.jpg)}} {{thumbnail(attiny-wdt2.jpg)}} {{thumbnail(attiny-wdt3.jpg)}}