Hardware Projects » Misc Hardware Projects

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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. JP3

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|GLOBAL_EN|open collector watchdog output; xpected to be connected to the !RESET of the CPU we're guarding (or GLOBAL_EN in case of Raspi)|-|

|2|VCC|3.3V supply voltage powering the watchdog|VCC|

|3|TPICLK|-|TPICLK|

|4|PB0|-|TPIDATA|

|5|!RESET|-|!RESET|

|6|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_wdt_v2_top.jpg)}} {{thumbnail(attiny_wdt_v2_bot.jpg)}}

h2. Firmware source code + Design Files

See https://gitea.osmocom.org/electronics/attiny-wdt

h2. Flashing

You need

* an attiny-wdt

* a TPI-capable programmer for ATtiny9, such as the "guloprog":https://guloshop.de/shop/Mikrocontroller-Programmierung/guloprog-der-Programmer-von-guloshop-de::70.html

* a 2x3 to 1x5pin adapter cable (custo made) to connect the guloprog TPI Connector to attiny JP3

* a Linux machine with "avrdude":https://www.nongnu.org/avrdude/ (e.g. installed via @apt install avrdude@ on Debian/Ubuntu). Tested with avrdude 6.3

h3. Physical connection

h4. full setup with usb cable, guloprog, 2x3-1x5 cable and attiny-wdt:

{{thumbnail(attiny_wdt_with_gulprog_cable.jpg)}}

h4. detailed view of 2x3 cable in guloprog

{{thumbnail(guloprog_with_cable.jpg)}}

h4. detailed view of 1x5 cable in attiny-wdt

{{thumbnail(attiny_with_cable.jpg)}}

h3. avrdude command for flashing

Assuming your firmware file is called @attiny-wdt.hex@, you can use the fa command like:

<pre>

avrdude -p attiny9 -P usb -c usbasp -U flash:w:attiny-wdt.hex

</pre>

The successful output looks like this:

<pre>

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e9008 (probably t9)

avrdude: NOTE: "flash" memory has been specified, an erase cycle will be performed

         To disable this feature, specify the -D option.

avrdude: erasing chip

avrdude: reading input file "attiny-wdt.hex"

avrdude: input file attiny-wdt.hex auto detected as Intel Hex

avrdude: writing flash (116 bytes):

Writing | ################################################## | 100% 0.41s

avrdude: 116 bytes of flash written

avrdude: verifying flash memory against attiny-wdt.hex:

avrdude: load data flash data from input file attiny-wdt.hex:

avrdude: input file attiny-wdt.hex auto detected as Intel Hex

avrdude: input file attiny-wdt.hex contains 116 bytes

avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 0.04s

avrdude: verifying ...

avrdude: 116 bytes of flash verified

avrdude done.  Thank you.

</pre>

h2. LED test assembly.

This is a small helper that allows to test attiny-wdt without a Raspi target device.  It will illuminate a yellow LED whenever the GLOBAL_EN (reset) is asserted.

{{thumbnail(attiny_wdt_led_test.jpg)}}