Chapter 10: C Structures

Unions

A union provides a way to have a type that may be of different sizes depending on the circumstances of its use.

We use a union in prgmspacehlp.h to store a float or an int in the same program memory:

union

{

int i[2]; // uint16_t float f;

} u;

Bit-fields

ANSI C defines bit-fields as a member of a structure or union that is defined to be a cluster of bits. This cluster can be a single bit, as would be used for a flag, or a 4-bit nibble, or any number of bits you might want to define. These fields can be very useful, but unfortunately, in many microcontrollers, these bit-fields slow things down (the compiler promotes bit to larger data types) so for efficiency sake, bits are best dealt with using bit masking, which compiles to faster and smaller assembly code. Bit masking simply uses a constant to define the position of a bit in a byte and allows you to read or write only that bit using the bitwise operators. We will look at the C-way, since we are learning C, then the mask-way since we want to be as efficient as possible.

Bit-Fields the C-way

In our examples above we have often declared an object as an unsigned char when that object could only have two values: TRUE or FALSE. Using bit-fields we can declare eight similar variables in a single unsigned char (note – not true for WinAVR, which promotes them to eight bytes).

We could define:

unsigned char calibrate = FALSE;

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to control a loop:

while(!calibrate) { // do something while calibrate == FALSE};

which runs as long as calibrate equals FALSE. We could have used:

struct {

unsigned int calibrate : 1; unsigned int this : 1; unsigned int that : 1; unsigned int the : 1; unsigned int other : 1; unsigned int tobe: 1; unsigned int or!tobe : 1; unsigned int hello : 1;

} flags;

Setting aside 8 flags in the space formerly used by calibrate. Now we control the loop:

while(!flags.calibrate)

That is, we could have done this in an ideal world. In the real world, our compiler would allow us to use the above syntax, but would assign 8 bytes to do the job. K&R notes: “Almost everything about fields is implementation dependent.”

Bit-fields the masking-way

This is mostly a review of stuff presented with the bitwise operator section, but reviews are good. Let’s look at a bit-masking example from OSCCAL.C:

We define an alias name for the Timer/Counter 2 Interrupt Flag Register:

#define TIFR2 _SFR_IO8(0x17)

Noting that the _SFR_IO8(0x17) is itself an alias defined elsewhere, but eventually is aliased to a specific register address on our ATMEGA169.

We next define two ‘bit-fields’ in the TIFR2 register;

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Next we write a function that causes our code to wait for the timer2 compare flag, OCF2A, which is bit one of the Timer/Counter2 Interrupt Flag Register:

while ( !(TIFR2 && (1<<OCF2A)) );// wait for timer2 compareflag

So this usage will do the same as a bit-field, but with greater efficiency.

Let’s look at another example where we assign Port B to a ‘bit-field structure’ without using bit-fields or structures. First we get the address of the Port B registers from io169.h which also has defines for each bit.

In the Butterfly software main.c file, the initialization function has:

And we ask, what’s with the 15? Well, 15 in hex is 0xE which in binary is 1110, so since PB0 == 0, what we are doing is setting PORTB equal to 00001110 << 0, remembering that ‘<<’ is the left-shift operator we know that we actually aren’t doing anything, or rather we are left shifting 15 zero times, which is doing nothing if I’ve ever seen nothing, which I haven’t but… back to our story, we are setting PORTB pins 1, 2, and 3 to 1 thus enabling the pull-ups on port B pins 2, 3, and 4. And that ends this discussion.

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This alternative bit-field technique is a compromise between the K&R way of doing things in C and the machine efficient way of doing things in C for microcontrollers.

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