Hello, i have the task to make the same LED on the MCU blink at different frequencies when pushing a push button. I know that this is possible with External Interrupts.

Forum|Forum|5 years ago
January 7, 2021
6 replies
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I can make LEDs blink at different frequencies but only 1 frequency...

So far i managed to get the LED config going with CubeMX.

I want to make this by using External Interrupts.

But how can i make the same LED change frequency by pushing a button on my STM32F407VG?

I managed to make other LEDs light with interrupts, but the same? oh no :(

I would appreciate your help a lot ty:)

STM32F4 series

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waclawek.jan

Super User

Homework?

> I want to make this by using External Interrupts.

EXTI is not suitable for pushbuttons. They produce many edges at push/release due to mechanical bouncing of contacts.

For homework, what's wrong with

while(1) {
 LedToggle();
 if (ButtonInput() == 0) {
 Delay(LONG);
 } else {
 Delay(SHORT);
 }
}

?

JW

DKunz.1Author

Associate II

Ye homework...

Thanks for the quick answer

Hm i guess that works.

But we are supposed to practice external inputs x(

waclawek.jan

Super User

You mean, external interrupts.

Yes, a typical school assignment. It's the wrong task, as I've said above. And, actually, should you genuinely need to handle a noisy signal in EXTI, it's actually quite hard to do correctly.

What you are expected to do is:

have a global variable, say Speed
I expect you know that each module needs to have clock enabled in RCC, and I expect you've already read RM0090 chapters RCC, GPIO, SYSTICK and EXTI (that's a subchapter of the interrupts chapter)
set up given pin as GPIO input in GPIO (maybe with pullup or pulldown, depending on the actual circuit)
map given pin to EXTI in SYSCFG
enable detection of first edge in EXTI, say falling (if the button shorts to GND and there's a pullup), and enable interrupt on it in the EXTI interrupt mask register
enable the EXTI interrupt in NVIC
write an EXTI interrupt handler, which clears the pending register, then - based on current Speed value - clears one edge enable and sets the other, and toggles Speed,
in main()'s loop, perform the same blink as above, just not based on ButtonInput() but based on Speed

This will work, somehow.

Write the simple version I outlined above, without EXTI, first, and get it running.

Then try to write the interrupt version and debug it until it works somehow. If in trouble, try harder; if in serious trouble, ask.

If you want to go further and understand the problem which is in this approach, draw yourself waveform of a bouncing button http://www.ganssle.com/debouncing.htm then imagine that several of the edges occur in a rapid succession in between consecutive steps taken within the ISR, or in between invocations of the ISR.

JW