SparkFun GPIO Block for Intel Edison - Digital Input slow

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VaultDweller
Posts: 2
Joined: Tue May 10, 2016 9:09 am

SparkFun GPIO Block for Intel Edison - Digital Input slow

Post by VaultDweller » Tue May 10, 2016 9:34 am

I was having problems reading some digital input using the Sparkfun GPIO block for the Intel Edison, so I decided to really simplify things. I created a very simple circuit with the 3.3 VCC connected to a 10k Ohm resistor (R1) and a jumper I could remove connecting it to ground, and a line between the resistor and the jumper going to GP44 on the GPIO board (See image below). In theory when the jumper is removed the pin should read high, and when the jumper is present it should read low. I created a very simple C++ program to just set the pin as input and read it 4 times per second, printing the response. What I saw happening was that when I started the program with no jumper it was reading high, and as soon as I added the jumper it would drop low, everything working as it should. However, once I took the jumper out it would stay low for about 5 seconds before going high again. When I hooked up my multimeter I could see as soon as I took the jumper out the voltage to the input pin was only around 1.2V for that 5 seconds (hence reading low) and would then go back to the 3.3. It is acting like there is a large capacitor it is charging. And sure enough, when I replaced the 10k Ohm resistor with a 1k Ohm resistor it now registers the change almost instantly (faster than I can notice at least). Worrying that it was something in my code I connected to the Edison using the serial port, ensured that GP44 was in input mode and read the values from "/sys/kernel/debug/gpio_debug/gpio44/current_value" and I was still seeing the same behavior. Is 0.33 mA not enough current to drive the input of this board? I'm very new to this so I don't fully understand how the internal pull up resistor may be effecting this, the pullmode is currently the default "pullup" and pullstrength is the default "50k", is this something I should change? I'm guessing this has something to do with the 3.3V level shifting on this board, but don't really understand what is happening. I'm mostly just curious as to why I apparently need to use a 1k resistor here.

Poorly drawn diagram:
Image

C++ code used for testing:

Code: Select all

#include "mraa.hpp"  
  
int main() {  
  mraa_gpio_context pin31 = mraa_gpio_init(31);  
  mraa_gpio_dir(pin31, MRAA_GPIO_IN);  
  
  int in;  
  for (;;) {  
    in=mraa_gpio_read(pin31);  
    printf("Read: %d \n",in);  
    usleep(250000);  
  }  
  
  return mraa::SUCCESS;  
}

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