Embedded System Tutorial GPIO

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Revision as of 18:37, 20 October 2013 by Preet (talk | contribs) (Hardware)

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Interface your LPC17xx to a switch and an LED.

Although the interface may seem simple, you do need to consider hardware design and know some of the fundamental of electricity. There are a couple of goals for us :

  • No hardware damage if faulty software is written.
  • Circuit should prevent excess amount of current to avoid processor damage.

Required Background

  • You should know how to bit-mask variables
  • You should know how to wire-wrap
  • You should know fundamentals of electricity and the famous V = IR equation.


Hardware Registers

The hardware registers map to physical pins. If we want to attach our switch and the LED to our microcontroller's PORT0, then here are the relevant registers and their functionality :

LPC17xx Port0 Registers
LPC_GPIO0->FIODIR Direction of the port pins, 1 = output
Read : Sensed inputs of the port pins, 1 = HIGH
Write : Control voltage level of the pin, 1 = 3.3v
LPC_GPIO0->FIOSET Write only : Any bits written 1 are OR'd with FIOPIN
LPC_GPIO0->FIOCLR Write only : Any bits written 1 are AND'd with FIOPIN


We will interface our switch to PORT0.2, or port zero's 3rd pin (counting from 0).


Switch Circuit


/* Make direction of PORT0.2 as input */
LPC_GPIO0->FIODIR &= ~(1 << 2);

/* Now, simply read the 32-bit FIOPIN registers, which corresponds to
 * 32 physical pins of PORT0.  We use AND logic to test if JUST the
 * pin number 2 is set
if (LPC_GPIO0->FIOPIN & (1 << 2)) {
    // Switch is logical HIGH
else {
    // Switch is logical LOW


We will interface our LED to PORT0.3, or port zero's 4th pin (counting from 0).


Active-low LED circuit
Active-high LED circuit


/* Make direction of PORT0.3 as OUTPUT */
LPC_GPIO0->FIODIR |= (1 << 3);

/* Setting bit 3 to 1 of IOPIN will turn ON LED
 * and resetting to 0 will turn OFF LED.
LPC_GPIO0->FIOPIN |= (1 << 3);

/* Faster, better way to set bit 3 (no OR logic needed) */
LPC_GPIO0->FIOSET = (1 << 3);

/* Likewise, reset to 0 */
LPC_GPIO0->FIOCLR = (1 << 3);


At the end of this lab, you should be familiar with how a microcontroller's memory can access physical pins.