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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.
How do we switch the pin (P1.0) corresponding to the LED to be an output?
P1DIR |= 0x01; 0x01 in hex is equivalent to 0b00000001 in binary. You may refer to the module about
Binary and Hexadecimal Notation to learn how to do this conversion or you could use the Windows Calculator (
Start->Run...->'Calc' ) to do it quickly. We can now easily see that P1.0 is set to 1 which makes it an output.
|= instead of just
= because it won’t overwrite bits that are not set to 1 in the mask.Output pins may be toggled using the PxOUT register. The LED is turned on by setting its corresponding register bit high .
What code would turn on the LED without modifying any other bits in the register? What would turn it back off?
P1OUT |= 0x01; //Turns LED On
P1OUT &= ~0x01; //Turns LED Off This will turn on and turn off the LED assuming it had already been set to be an output.
What would be the proper syntax for toggling the value of P1.0 so that we can turn the light on if it is off or vice versa?
P1OUT ^= 0x01; The XOR operation will invert the bit in question so that we may toggle it regardless of its current state. If this doesn't make sense, do the computation by hand to see how that single bit is flipped.
Now, write the full C program to turn on the LED. Do the following:
return 0; , which is not necessary. All of your programs will use this as a skeleton#include "msp430x20x3.h"; #define led_on ~0x01 to the top of the file (after the
#include ) we may call
led_on every time we wanted the value ~0x01. Similarly we may add
#define led_off 0x01 to use when we wanted 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)
}
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