Arnab Chakraborty's assembly language tutorial: page 6 of 11

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Last updated on Wed Dec 22 10:24:57 IST 2010.

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Working with pins

The only way a microcontroller can interface with the external world is via its pins. Though the pins are grouped into ports for programming convenience, yet for many applications one needs to access the pins individually. So

during output one needs to
1. write 1 to some pins of a port without changing the other pins,
2. write 0 to some pins of a port without changing the other pins,
3. write a binary pattern to some pins of a port without changing the remaining pins,
4. toggle some pins of a port without changing the other pins,
during input one needs to
1. check if some pins of a port are 1
2. check if some pins of a port are 0
3. check if some pins of a port have some specific binary pattern

We discuss these one by one.

Output

Write 1 to some pins without changing the rest

Suppose that we want to write 1's to pins 2,3 and 7.


unsigned char mask = (1<<2) | (1<<3) | (1<<7); 
PORTB |= mask;

Write 0 to some pins without changing the rest

Suppose that we want to write 0's to pins 0,3 and 7.


unsigned char mask = (1<<0) | (1<<3) | (1<<7); 
PORTB &= ~mask;

Write a binary pattern to some pins without changing the rest

Suppose that we want to write 1's to pins 2,3, and 0's to pins 4 and 7.


unsigned char mask1, mask0;
mask1 = (1<<2) | (1<<3);
mask0 = (1<<4) | (1<<7);
PORTB = (PORTB | mask1) & ~mask0;

Toggle some pins without changing the rest

This is a rare requirement. But it is easy to do, anyway. Suppose that we want to toggle pins 0,3 and 7.


unsigned char mask = (1<<0) | (1<<3) | (1<<7); 
PORTB ⁼ mask;

Input

Check if some pins of a port show 1

Suppose that we want to check if the pins 0, 2 and 7 show 1. We can use


unsigned char mask = (1<<0) | (1<<2) | (1<<7);

if((PINB & mask) == mask) {
  ...
}

Check if some pins of a port show 0

Suppose that we want to check if the pins 0, 2 and 7 show 1. We can use


unsigned char mask = (1<<0) | (1<<2) | (1<<7);

if((PINB & mask) == 0) {
  ...
}

Check if some pins of a port show a given binary pattern

Suppose that we want to check if the pins 0, 2 and 7 show 101. We can use


unsigned char mask, posMask;
posMask   = (1 << 0) | (1 << 2) | (1 << 7);
valueMask = (1 << 0)          |   (1 << 7);
if((PINB & posMask) == valueMask) {
  ...
}