iMX233-OLinuXino GPIOs FASTER and FASTER :)

In my previous post https://olimex.wordpress.com/2012/09/07/imx233-olinuxino-working-with-gpios/ I experimented with GPIOs trying to see how fast they could be changed and had in my TODO list to access them via /dev/mem directly writing to the registers, David Whittaker made this before me 🙂 : http://tech.groups.yahoo.com/group/olinuxino/message/2022

He made this wonderful tool gpio-mmap.h where he define several useful functions:

gpio_map(); // this function should be called to initialize and map the GPIOs to the /dev/mem

gpio_output( bank, pin); // initialize GPIO as output you can see which GPIO correspond to bank/pin in IMX233 user manual, I prepare cross reference for all GPIOs which will post soon here on the blog

GPIO_WRITE_PIN(pin,value); //this writes to the GPIO pin#

the quick code I did you try gpio-mmap.h tool was:

#include “gpio-mmap.h”

int main() {
gpio_map();
gpio_output(2,1); //bank 2 bit 1 = GPIO65 the LED on board
while (1) {
GPIO_WRITE_PIN(65,1);
GPIO_WRITE_PIN(65,0);
}
}

the oscolloscope connected to the GREEN LED show nice square with 175 nS period, i.e. the direct memory register write can achieve as high as 5.7 MHz wow 🙂

note that this code was running without other codes to interrupt it, in Linux which is multitasking OS you can run many processes, for instance if I add in background the led_blink script from the previous blog post, I can see with the oscilloscope LED glitch on ever second when the scrip toggle the LED, i.e. now I have 5.7Mhz frequency distrubed from time to time with the 1Hz led_blink script, so the more processed you run on iMX233 the less determinant will be this frequency and will slow down, this is like on your desktop when you run video for instance and start several other tasks like opening open office or copy some big file on the disk the video may freeze for a second when the video buffer goes empty and other tasks keep the CPU busy as this moment.

Nevermind gpio-mmap.h is one very useful tool, now we can implement software SPI with bit-banging as SPI is broken in imx233 kernel 2.6.35 and 5Mhz is pretty decent clock speed for most of the tasks, so we can now make some good code examples for our SPI UEXT modules.

You can download gpio-mmap.h source from GitHub: https://github.com/OLIMEX/OLINUXINO/tree/master/SOFTWARE/iMX233/gpio-mmap.h