Blink when input is constant?

Question

I would like to have a status LED to make sure my microcontroller software hasn't faulted. From software, I will alternate an output so if it dies, the output will be constant. I want the status LED to blink when there's a fault instead of blinking to show there is not one. So, I need a separate circuit that will blink the LED when its input is stuck high or low.

I can come up with a circuit myself, such as an edge detector that resets a 555. However, I would like to know first, is there a name for such a circuit? Or even better, if it comes in/can be made from a single IC?

EDIT: A completely external circuit is preferred because it'd be isolated and unlikely to be affected by software changes.

Answer 1

XY problem. This isn't a job for a 555. This needs to be dealt with by software, with a little help from RPi hardware. (Obi-wan voice: “Use the source, Luke…”)

What hardware? The ARM architecture in RPi has a watchdog timer resource. The point of a watchdog timer is for the system get unstuck and recover. I encourage you to use it (see the link below.) Further, RPi is powerful enough to run a robust RTOS, be it an open-source or commercial package. Any of these OS releases know how to deal with a watchdog and respond in an intelligent way (and if they don't, choose a different one.)

Related: https://diode.io/raspberry%20pi/running-forever-with-the-raspberry-pi-hardware-watchdog-20202/

What does that mean, 'respond in an intelligent way'? If you don’t want the system to recover but simply stop and inform the user that it has done so (why?), the watchdog timeout can instead cause a branch to a routine that flashes an LED and just sits there until the user intervenes. This routine can also, more importantly, log information for diagnostics later and also place the system in a 'safe' state. Again, this ability to respond to the watchdog and log errors would be something your OS would have built in.

Relevant: https://stackoverflow.com/questions/13217959/how-to-use-the-watchdog-timer-in-a-rtos

For some inspiration, this is how the Apollo Guidance Computer team did it: https://youtu.be/B1J2RMorJXM

Answer 2

You could program another microcontroller, maybe an SOT23-6 or SOIC-8 package, to do that.

It would be a simple program, running off an internal RC oscillator, and using an internal (reliable) BOR/POR peripheral. Just the chip and a bypass cap on the power supply.

Alternately, you could use a resettable monostable multivibrator (not necessarily a 555) and a blinking LED. The component count and board area would be higher.

Answer 3

I want the status LED to blink when there's a fault instead of blinking to show there is not one.

The following circuit may work for you, although I have not tested it.

^{simulate this circuit – Schematic created using CircuitLab}

The 555 is configured as an astable relaxation oscillator. However, the timing capacitor can be shorted by Q1. If this happens before the voltage on the timing capacitor rises to the trigger voltage, the 555 will remain in the same state and the LED will not blink.

Capacitor C2 limits the duration of the heartbeat pulse in case the source of the heartbeat gets stuck in the high state. C2 needs to pass current in each direction. In one direction it can flow through the base of Q1. D2 serves to provide a path for current in the opposite direction.

The resistors R1, R2 and Ctimer control the frequency of the blink. The heartbeat should be sent at least twice the circuit's blinking frequency to ensure blinking does not start. The heartbeat pulse needs to be sufficiently long to drain Ctimer through Q1.

If you want to change the default state of the LED, reverse it's polarity and connect the anode to Vcc.

Answer 4

is there a name for such a circuit

When you have your software pulsing something periodically to indicate that the software is running normally (not "locked up"),

• Watchdog: hardware that distinguishes between normal and locked-up software
• Feeding the watchdog: the bit of software that tells the watchdog that things are running normally. (Usually in only one place in the main loop). Sometimes called "petting the watchdog" or "kicking the watchdog".
• Watchdog's timer: the timer inside the watchdog that normally gets reset by the timer periodically. External watchdog ICs often use a resistor-capacitor network that slowly charges or discharges. Integrated watchdogs (on-chip watchdogs), on the same chip as the microcontroller, often use a a digital counter driven by some crystal oscillator or a fast RC oscillator.
• watchdog timeout: when the watchdog hasn't been fed in long time, the watchdog's timer runs out of time and the watchdog takes action, typically pulling a reset line that watchdog resets the CPU and all peripherals.
• consistency check : Rather than unconditionally feed the watchdog every time through the main loop, many systems do some quick checks (similar to assert() checks), then if all the checks look good, only then feed the watchdog. For example, this can catch problems where the main loop (responsible for feeding the watchdog) is still running, but some other thread has unexpectedly stopped.
• heartbeat signal: an external signal, typically connected to an LED, that indicates to outsiders that things are running normally. A few hard real-time systems have a main loop that executes extremely regularly, so wire running to an external watchdog chip is used directly as a heartbeat signal. Other systems, every time they feed the watchdog, then check if a second has gone by, and only then flip a separate wire connected to the heartbeat signal -- so humans see a regular, consistent, slow LED blink even when the normal main loop cycles too fast to see or cycles with irregular timing.
• supervisor chip: (aka voltage supervisor): a chip that drives the reset line of a CPU, holding it in reset as the power rails slowly ramp up to the normal voltage, until the voltage has stabilized. Many of these chips also include a watchdog timer.

A completely external circuit is preferred

Many people agree with you:

"no WDT internal to the CPU is really safe." -- Jack Ganssle

"If it's on the same die, it won't fly." -- George Farmer

Answer 5

Configure one of the spare hardware timers to toggle an output based upon the counter value. For example, the timer counts up and rolls over at 0xFFFF and the output toggles at 0x7FFF and 0x0000 for a 50% duty cycle. After the software configures the timer at startup, then the toggling output will be under control of the hardware timer and doesn't require software intervention. Connect the output to the LED so that the LED blinks.

Now create a software watchdog mechanism that periodically resets the timer's counter value to zero if the software is working properly. The software watchdog should reset the counter value before the timer output toggles. This will prevent the LED from flashing while the software is working. If the software ever fails, then the software watchdog mechanism will not reset the timer counter value, which will allow the output to toggle and flash the LED.

Answer 6

^{simulate this circuit – Schematic created using CircuitLab}

I haven't tested this, but it looks similar to what you want. R1 and C1 set the output blink frequency when the input is constant (either hi or lo). R2 and C2 set the delay time from when the input stops pulsing to when the output starts blinking. When the input is constant, R2 eventually drains C2 and enables the oscillator to blink. When the input oscillates, C2 and the diodes charge up C2, so that NAND1 input goes close to GND, and then the output of NAND1 stays hi, and then D1 stays dark.