Digital I/O, such as the LED, is configured by modifying several registers pertaining to the port that they are attached to. Check the datasheet to find the respective port number of the peripheral you which to control. For more detailed information about Digital I/O on the MSP430 check Chapter 6: Digital I/O of the ez430's user guide. The basic logic behind seemingly trivial process of turning the LED on and off is the same behind the operation of every peripherial on the tool.

First, we must assign the direction of the corresponding I/O pins. This is done by setting the direction register, PxDIR, with the appropriate bit. By default, all I/O pins are assigned to be inputs.

On the ez430 the LED is located on port 1. The port number will correspond the to x value in registers such as PxIN, PxOUT, or PxDIR. Therefore, if we wanted to define the direction of a pin on port 1 we would write to P1DIR.

In order to modify values of registers you must first understand the following commonly used C logic operations:

~A
A | B
A & B
A ^ B
A |= B
A &= ~B
A ^=B

In C, the NOT: ~, OR: |, AND: & and XOR: ^ operators are all bitwise. This means that the operations are done bit by bit to the binary values of the two operands.

int A = 0b1101;
int B = 0b0101;
int C; 

/*
The results of these operations are very straightforward.  If we are dealing with the "|"
operator then we OR together each bit to make the result. This means 0b1101 | 0b0101 is 
computed as [1 OR 0, 1 OR 1, 0 OR 0, 1 OR 1] which is [1,1,1,1] thus the result is 0b1101;
*/

C = ~A;    // The value of C is 0b0010
C = A | B; // The value of C is 0b1101
C = A & B; // The value of C is 0b0101
C = A ^ B; // The value of C is 0b1000

Operators such as += or -=, implemented as A += B, are short forms for A = A + B. This is translated to "take the current value of A, add it to B, and finally store that result back into A." For -= it similarly translates to "subtract B from A and store the result back to A." If "@" is some operator, than A @= B would be "@ together A and B and store the result back into variable A."

If we combine the use of these two aspects of C then the aformentioned logic operations that are listed at the top of this section begin to make sense. The following examples will help you figure out why they are useful to us when we attempt to configure any register on our microcontroller. It is imperative that you understand how these function. You will use them in every program you write because these operations allow you to modify single bits of a register to modify a specific register and change a peripheral's settings.

P1DIR |= 0x01;

Output pins may be toggled using the PxOUT register. The LED is turned on by setting its corresponding register bit high.

P1OUT |= 0x01;     //Turns LED On
P1OUT &= ~0x01;    //Turns LED Off

P1OUT ^= 0x01;

#include "msp430x20x3.h"

void main(void)
{ 
   P1DIR |= 0x01;	// Set P1.0 as output
   P1OUT |= 0x01;	// Set P1.0's output value as high (turns the LED on)
}