Embedded System Tutorial GPIO

Objective

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.

Lecture

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
LPC_GPIO0->FIOPIN	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

Switch

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

/* 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 }

Note that the "inline" resistor is used such that if your GPIO is mis-configured as an OUTPUT pin, hardware damage will not occur from badly written software. Switch Circuit

LED

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

/* 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);

Given below are two configurations of an LED. Usually, the "sink" current is higher than "source", hence the active-low configuration is used more often. Active-low LED circuit Active-high LED circuit

Conclusion

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