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[iMX28] Using a GPIO as an input counter

Question asked by olistudent on Sep 19, 2012
Latest reply on Nov 13, 2012 by Yuji Sasaki

Hello all.

 

I want to use a GPIO as an input counter on an iMX28 board. I'm using kernel 2.6.35 from the Freescale BSP.

 

For testing I connected a digital output of my board to a digital input. The good news first. When reading the value of the GPIO everything seems okay. It changes whenever I switch my digital output.

 

The bad news: I'm observing three problems.

 

1. Despite configuring for rising or falling edges I'm seeing the interrupt counter increasing everytime I do switch the digital output. I expected that there isn't any rising edge when switching from 1 to 0. Or did I misunderstood the meaning of rising edge?

 

2. I'm not able to configure the trigger to detect "both" edges. Why?

root@iMX28:/# echo both > /sys/class/gpio/gpio116/edge

[  642.890000] setting trigger mode 3 for irq 244 failed (mxs_gpio_set_irq_type+0x0/0x58)

 

3. For one event the interrupt counter increases by a higher value depending on the utilization of the system (up to 9). I would expect it to increase by one for one event. I even looked at the signal with an oscillator but couldn't observe any strange things with the ramps.

 

Log:

# detect rising edges

root@iMX28:/# echo rising > /sys/class/gpio/gpio116/edge

# print gpio configuration for gpio 116

root@iMX28:/# cat /sys/kernel/debug/gpio | grep 116

gpio-116 (Digital Input 1     ) in  hi irq-244 edge-falling

# show interrupt counter

root@iMX28:/# cat /proc/interrupts |grep gpio

244:          1           -  gpiolib

# switch to 1

root@iMX28:/# echo 1 > /sys/class/gpio/gpio74/value

# show interrupt counter again

root@iMX28:/# cat /proc/interrupts |grep gpio

244:          5           -  gpiolib

# switch to 0

root@iMX28:/# echo 0 > /sys/class/gpio/gpio74/value

# show interrupt counter

root@iMX28:/# cat /proc/interrupts |grep gpio

244:         14           -  gpiolib

 

I also tried with an external signal generator but the problem persists. Now the question:

Hardware problem? Kernel problem? pebcak?

 

Any help is appreciated.

 

Regards

Oliver

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