How to trigger reset at long press of button

Question

I am trying to make a button control on/off and reset all in one, but I can't quite get my head around how to design it.

It should basically function like a PC power button. If short press it's either on or off, depending on state, and a long press is the reset that works when something completely locks up the computer. The on/off is easy as that's software controlled, but the reset has to be in hardware somehow as it has to ALWAYS work, no matter the state of the software or MCU.

The reset functionality should kick in after a long press of about 5~10 s. Once the reset signal goes low it should stay low for >10ms. How to do this?

I can find dedicated IC's that can set a signal low for x ms, which would work, but how do I trigger them in a good way? RC and then a comperator to trigger at a set voltage or...? And how do I avoid triggering the reset when starting up if using the RC approach?

Any help would be appriciated.

Answer 1

You could use a RC delay circuit.

A simple one is made out of a resistor and a capacitor. For example, charge the capacitor, attached to ground, through a pull-up resistor R1. Then discharge through another resistor R2 and the "long-press" reset button to the ground. The trick is to calculate the values of C, R1 and R2 to make the timings right, so that the capacitor slowly discharges to the digital zero threshold. Then the signal at the junction could be used for (inverted) reset signal.

You could add an inverter to make the reset signal more robust.

Here is an online calculator to estimate the delays: http://ladyada.net/library/rccalc.html

Answer 2

You can use a push-button/reset driver IC. These can automatically detect long presses, simultaneous presses of multiple buttons, etc. There also exist versions with a built-in watchdog timer, or brown-out detection, which will automatically reset your microcontroller if it hasn't responded within some time or if the voltage drops below an acceptable level, respectively.

An example is the TPS3422: https://www.ti.com/lit/ds/symlink/tps3422.pdf

Answer 3

Use a microcontroller that receives the button signal and drives the reset and on/off lines accordingly. Even the tiny PIC 10F200 can do this job easily. It comes in a SOT-23 6-pin package, with one input line and 3 I/O lines. You'll even have a extra I/O line left over.

The first thing the micro needs to do is debounce the button. This creates a debounced button signal internally. The rest is counting time to see how long the button is pressed. Depending on the length of time, the code either toggles the on/off line or asserts the reset line. You can also do the logic so that any press when off turns the unit on, since reset doesn't make sense when off.

The dedicated input pin of the 10F200 can have a internal pullup, so the only hardware you need is the button, the 10F200, and its bypass cap. Yes, it really is that easy.