Attiny-wdt » History » Revision 6
Revision 5 (laforge, 08/23/2022 02:26 PM) → Revision 6/8 (laforge, 08/23/2022 02:28 PM)
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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)}}
{{thumbnail(attiny-wdt1.jpg)}} {{thumbnail(attiny-wdt2.jpg)}} {{thumbnail(attiny-wdt3.jpg)}}
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>
