What is an analog Watchdog?

Question

I Quote from Wikipedia:

A watchdog timer (WDT; sometimes called a computer operating properly or COP timer, or simply a watchdog) is an electronic timer that is used to detect and recover from computer malfunctions.

While using STM32F429I-Discovery, I came across a term [in "stm32f4xx.h"] which uses a register to disable Watchdog:

#define  ADC_CR1_AWDIE    ((uint32_t)0x00000040)    //Analog Watchdog interrupt enable

Here, I am unable to understand Analog Watchdog

And if possible,

#define  ADC_CR1_JAWDEN    ((uint32_t)0x00400000)    //Analog watchdog enable on injected channels

What is injected channel here?

Answer 1

A watchdog timer can be thought of as two separate circuits, a timer circuit and a watchdog circuit. The timer circuit merely counts the time that passes. The watchdog circuit actively monitors the timer to see if a certain amount of time has passed without being reset by software. If so, the software is no longer running and the watchdog typically performs an automated function such as resetting the processor. The watchdog needs only to be told at initialization how much time to monitor for and it handles the rest of its operation without additional software interaction.

An analog watchdog operates in a similar manner. Only instead of monitoring a timer, it monitors an analog input channel. At initialization, you tell the watchdog what analog thresholds to monitor for. If a converted value on an analog input passes one of these thresholds, it will fire an interrupt for you to process the signal sample. This means you don't have to write code to continuously poll the analog inputs and check their levels. It is all handled autonomously in the background by the analog watchdog circuitry.

An injected channel can just be thought of as a high priority conversion channel. If a regular analog input is in the middle of performing a conversion and a conversion is triggered (either by a timer or because it is programmed in continuous conversion mode) on an injected channel, the conversion on the regular channel will halt and wait while the injected channel is converted before completing its own conversion. This is useful if you have an analog input that must be responded to in a realtime manner.

Here is an application note (which, for some strange reason, doesn't seem to be available in ST's website) that give a few examples of the use of the various ADC features. And here is another explanation of the two features your question was about.

Answer 2

The term "watchdog" in this context refers to the fact that the ADC channel is continuously monitored.

In this context the term is not related to a processor operation watchdog - which monitors processor operation. Although you could use it for brown-out detection or power-supply failure detection if your power supply as a reservoir capacitor or battery back-up capable of keeping the processor up long enough after the supply side drops-out.

The analogue watchdog on STM32 is simply a means of generating an interrupt when some external voltage drops below or exceeds a programmable threshold level. This is done without software intervention when the ADC conversion s configured to free-run, so if the application only needs to respond to thresholds, this can be implemented with zero software overhead for ADC polling.

You might use the feature for example for carrier sense detection in an RF application by using it to monitor the RSSI signal from an FM demodulator. Or it might be used in a in a bang-bang controller, such as a boiler thermostat. The AWD has upper and lower thresholds so can be used to implement hysteresis, and you can modify the thresholds dynamically to generate multiple events on a curve for example.