Amruta (talk) 11:24, 17 March 2015 (IST)
In this tutorial we will be looking at accessing the GPIO of ARM. In this tutorial series we will be using LPC1768 cortex ARM M3. We will be simply reading a switch and making a LED act accordingly.
Contents
Schematic
Code
In this code, we will demonstrate LED and Switch interfacing with the LPC1768.
We will monitor the switch condition and will glow LED when switch is pressed.
#include "lpc17xx.h" // NXP Library containing all port abd register definations #include "stdutils.h" // Explore Embedded library #define LED 29 //P1.29 #define SWITCH 28 //P1.28 int main() { /* Setup and initialize the microcontroller system */ SystemInit(); LPC_PINCON->PINSEL3 = 0x00; // Configure Pin as GPIO util_BitSet(LPC_GPIO1->FIODIR,LED); // Set pin direction as output util_BitClear (LPC_GPIO1->FIODIR,SWITCH); // Set pin direction as input while(1) { if (util_IsBitCleared (LPC_GPIO1->FIOPIN,SWITCH)) // check if switch is pressed util_BitSet (LPC_GPIO1->FIOSET,LED); // turn on LED else util_BitSet (LPC_GPIO1->FIOCLR,LED); // turn off LED } }
Basics
lpc17xx header file
This is peripheral access header file for NXP LPC17xx device series. It contains all required register and peripheral definitions which one requires in order to access any peripheral.
If you take a look at this header file, you will notice that all registers associated with a peripheral are clubbed together in a structure. So to access a register we need to use the respective structure
Code Details
SystemInit()
It initializes the microcontroller system. Typically, this function configures the oscillator (PLL) that is part of the microcontroller device.
In-short, it configures and initializes microcontroller for our use.
Register Details
PINCONNECT Block
Before using any port pin, it is required to configure its function.
The pin connect block allows most pins of the microcontroller to have more than one potential function. It contains Pin Function Select registers and Pin Mode Select registers.
Usually we don't deal with Pin Mode registers unless we are very much familiar with the microcontroller and want to use its internal resistor features. For time being, we will leave Pin Mode register with its default value and let's see details of Pin Function Select register.
- PINSELx (Pin function select register)
There are four alternate functions for each pin. So to select a function for one pin, 2 bits are required. That means 2 PINSEL registers for each port, one for pins 0 to 15 and another for pins 16 to 31.
The PINSEL registers control the functions of device pins as shown below :
| Value | Function |
|---|---|
| 00 | Primary (default) function, typically GPIO port |
| 01 | First alternate function |
| 10 | Second alternate function |
| 11 | Third alternate function |
GPIO Block
The LPC1768 implements portions of five 32-bit General Purpose I/O ports. The registers discussed below represents the enhanced GPIO features available on all of the GPIO ports. Here we will discuss only commonly used GPIO registers.
- FIODIR(Fast GPIO Port Direction control register)
This register individually controls the direction of each port pin.
| Values | Direction |
|---|---|
| 0 | Input |
| 1 | Output |
- FIOPIN (Fast Port Pin value register )
The current state of digital port pins can be read from this register, regardless of pin direction or alternate function selection (as long as pins are not configured as an input to ADC).
Writing to this register places corresponding values on digital port pins
- FIOSET (Fast Port Output Set register ) and FIOCLR (Fast Port Output Clear register )
These registers controls the state of output pins
| Values | FIOSET | FIOCLR |
|---|---|---|
| 0 | No Effect | No Effect |
| 1 | Sets High on Pin | Sets Low on Pin |
- FIOMASK (Fast Mask register )
Writes, sets, clears, and reads to port (done via writes to FIOPIN, FIOSET, and FIOCLR, and reads of FIOPIN) alter or return only the bits enabled by zeros in this register.
As its default value is zero, no need to configure it here.