Tutorial 03: Flipping Bits

Now that we have access to the registers, how do we actually flip the switches (change the bits)?  We'll examine the ways to do this using the following example: we'll use a G2001 with an LED connected from P1.4 to ground.  If P1.4 is at 0 V, nothing happens, but if P1.4 is at Vcc, the LED will turn on.  Since we've connected to P1.4, we should be able to control it by fiddling with bit 4 in the P1 registers.  (Keep in mind that we're not changing the 4th bit, at least not in the way we normally think of it.  The 1st bit is bit 0, so bit 4 is technically the 5th bit, but who's counting?)

Before we look at setting bit values, we should take a look at the ways we can represent the values in 8-bit registers.  We could always use regular decimal notation, but unfortunately it has little advantage to counter the confusion caused by having to use 8-bit numbers in a non-multiple base (the decimal base is, of course, 10).  

The clearest way to represent the value is in binary notation, or base 2.  The register has 8 digits, each is a 0 or 1, corresponding to the value in each bit.  To make a binary 2 clear from a decimal 10, we can preface the number with the notation 0b (first character is a zero), such as 0b10.

Another common notation used is hexadecimal (base 16).  This notation is convenient because each 8-bit value is represented by exactly two digits, each a value from 0-9 or a-f.  The preface for hexadecimal is 0x, so the number 2 in hex is 0x02.  The number 12 would be 0x0c, since the digits a-f correspond to the numbers 10-15.  16-bit numbers are represented with 4 digits, such as 0x14da.  Many values in the MSP430 are 16-bit, so hexadecimal is very convenient here.  (I apologize for the change in notation from the previous tutorial; in the TI documentation they use the notation 14DAh for hex values, but here we're using 0x14da instead.  The 0x notation is used in the programming, and I will stick to that notation unless I'm citing from a TI document directly that uses the h notation.)

We could also use an octal notation (base 8), with the preface 0o.  I've not seen this in practice, however.

We'll start using some actual code in this post, so I want to establish a convention.  Anything written in the Courier font is a code snippet or example.  The coloring in the code matches the color scheme I usually see when I look at a .c file in a text editor, and for now doesn't make a difference.  You may have other color schemes that you see.  I'll try to use the colors that are used in CCS as best I can.

Binary notation is obviously the one that makes it clear what bits we're using and what they're set to.  Unfortunately, it also takes lots of characters to write them out.  When we write C programs for the MSP430, we can include a header file with information and shortcuts specific to the device we're using (such as #include <msp430g2001.h>).  These header files include some shortcut names for the binary values where each digit is 0 except for a specific bit; the header file allows us to use the name BIT2 to represent 0b00000100.  There is a BIT0 through BIT7 included in each header file, which makes for a nice shortcut and less typing.  In the discussion below, however, I'll try to keep it to the full 8 digit binary values to make it clear what is happening in the operations described.  Now, on to the fiddling!

For our LED example, we'll need to first make P1.4 an output so that we can control the voltage above the LED.  We do this by changing bit 4 in P1DIR to 1, for output mode.

1. Direct Assignment

If we know exactly what the binary (or hex, or even decimal) value corresponding to the configuration we need is, we can just set it directly:  P1DIR = 0b00010000; would set all of the bits in P1DIR to 0 except bit 4.  This method works pretty well for initializing the processor, but later in the program can be problematic when we might not know for certain what state bits we don't want to change are in.  Turning off the bit is easy, just set it to the value with bit 4 set at 0: P1DIR = 0b00000000, or just as easily for this particular case, P1DIR = 0.  (Using the BITx notation, we would write P1DIR = BIT4.)

2. Addition and Subtraction

If we don't know exactly the value needed for the configuration, or if we're just trying to turn on one bit, we could just add the bit's value to P1DIR (I'm using the colon character : to represent unknown digit values):  0b:::0:::: + 0b:::1:::: = 0b:::1::::, so we could turn on bit 4 by doing P1DIR += 0b00010000.  *(Confused by the +=?  See the footnote below!)  The problem with this method is if we accidentally turn it on twice: 0b:::1:::: + 0b00010000 is going to cause a carry over into the next bits, changing I/O directions where we don't want to!  This method should be used with caution.  Note that the equivalent to turn the bit off would be to subtract the value, of course.  (BITx notation: P1DIR += BIT4 or P1DIR -= BIT4.)

3. Logic Operators & and |

To avoid the problems with addition and subtraction, we can use logic operations.  Let's say we have a bit with the value x.  (x could be 0 or 1, we'll just look at this generally.)  The operations and (&) and or (|) can be used to change the value of the bit to be specifically 1 or 0.  Note that x & 0 = 0 no matter what value x is.  Similarly, x | 1 = 1 regardless of the value of x.  Also, x & 1 = x and x | 0 = x, so the value of x is unchanged. for these operations.  So what would be the result of the operation P1DIR | 0b00010000?  Bits 0-3 and 5-7 would be unchanged (remember, | 0 doesn't change the value) while bit 4 is set to 1.  (It may have been 1 before, but is definitely 1 after the operation!)  On the other hand, P1DIR &= 0b11101111 would have the effect of setting bit 4 to zero and leaving the others unchanged.  Note that 0b11101111 is the not operation (~) applied to the original 0b00010000, so we could write this as P1DIR &= ~0b00010000.  (In BITx notation: P1DIR |= BIT4 turns bit 4 on, and P1DIR &= ~BIT4 turns it off.)

Reader Exercise:  Sometimes you'll see example code that uses the notation a + b; and sometimes a | b; to do the same thing.  Convince yourself that these are equivalent in particular circumstances; when would you not want to use one or the other?

4.  Toggling with ^

Ok, I said there were three choices, and there really are only three.  This fourth option is similar to the third choice, but is an effective way if we just want to toggle the value, not set it to be specifically 0 or 1.  The exclusive or operator (^) has this effect:  x^0 = 1 if x is 1, and 0 if x is 0.  x^1 = 1 if x is 0, and 0 if x is 1.  So x^0 leaves the bit unchanged, and x^1 changes the value of x to its opposite.  P1DIR ^= 0b00010000 flips bit 4 (to 1 if it was 0, to 0 if it was 1) just like toggling a switch.  (In BITx notation: P1DIR ^= BIT4.)

Now, in our program we need to initialize the pin to be an output:

P1DIR = BIT4;  
(An equivalent way to do this is to use a logic operator rather than an assignment:  P1DIR |= BIT4;)

The value in P1OUT could be either 0 or 1, so if we care what state the LED starts in we should set that:

P1OUT = 0;

(A better practice is to initialize the output state before we change the pin to an output.)

Now we have an output pin set at 0, keeping the LED off.  To turn it on, we give the command:

P1OUT |= BIT4;

To turn it off, we give:

P1OUT &= ~BIT4;

To toggle its state (off if on, on if off):

P1OUT ^= BIT4;

As a final example, demonstrating some power in the other assignment methods, let's put another LED on P1.6.

First, let's configure the two pins as outputs with an initial state off:

P1OUT = 0;

P1DIR = BIT4 + BIT6;

(adding the two values together is easily verified to be 0b01010000).

We can turn each one on and off individually:

P1OUT |= BIT4;    // P1.4 on

P1OUT &= ~BIT4;   // P1.4 off

P1OUT |= BIT6;    // P1.6 on

P1OUT &= ~BIT6;   // P1.6 off

Or we can turn them on and off together:

P1OUT |= BIT4 + BIT6;     // both on

P1OUT &= ~(BIT4 + BIT6);  // both off

And finally, toggle them simultaneously:

P1OUT ^= BIT4 + BIT6;     // toggle both

Reader Exercise:  Write down the necessary code to use LEDs on pins 3 and 7 of port 1, with pin 3 initially off and pin 7 initially on.  Then toggle the LEDs so that 3 is on and 7 is off.


*  += is a shorthand operator in C.  If I say x += 1;, the command adds one to x and reassigns the result to x.  In other words, it's the same as saying x = x + 1;.  While algebraically that statement makes no sense, in programming it's very common.  Don't think of = as meaning the two sides are equal, read it as "x is now equal to the value currently in x plus 1".