Page 344 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

11.2 Parallel Ports 321

Note that DDRA is declared an unsigned char variable. Then, any time after that, to
output a char variable i to PORTA, put
PORTA = i;
Note that PORTA is declared an unsigned char variable. To make PORTA an input
port, we can write
DDRA = 0;
Then, any time after, to input PORTA into an unsigned char variable i we write
i = PORTA;
Generally, the direction port is written into before the port is used the first time and need
not be written into again. However, one can change the direction port from time to time,
PORTA and PORTS together, and their direction ports DDRA and DDRB together, can
be treated as a 16-bit port because they occupy consecutive locations. Therefore, they can
be read from or written into using LDD and STD instructions. To make PORTAB an
output port, we can write in assembly language:
LDD #$FFFF ; generate all ones
STD $2 ; put them in direction bits for output
Then, any time after that, to output accumulator D to PORTAB we can write
STD $0 ; output accumulator D
To make PORTAB an input port, we can write
CLR $2 ; put zeros in high direction bits for input
CLR $ 3 ; put zeros in low direction bits for input
Then, any time after that, to input PORTAB into accumulator D we can write
LDD $ 0 ; read PORTA into accumulator D
Also, some of the 16 bits can be made input, and some can be output. In manner similar
to how 8-bit ports are accessed in C, 16-bit ports can be declared in a header file that is
#included in each program as follows:
int PORTAB@0, DDRAB@2;
To make PORTA and PORTS an output port, we can write: DDRAB = Oxf ff f;.
Note that DDRAB is declared an int variable. Then, any time after that, to output an
int variable i, high byte to PORTA and low byte to PORTS, we can write PORTAB =
i;. Note that PORTAB is declared an int variable. To make PORTA and PORTS an
input port, we write: DDRAB = 0;. Then, any time after that, to input PORTA (as high
byte) and PORTB (as low byte) into an int variable i we can write i = PORTAB;.
The ports A and B, or the combined port AB, can be made an input or output port and
can be easily accessed in assembly language or in C.
As a simple example of the use of an input port, consider a home security system.
See Figure 11.4a. Three switches, each attached to a different window, are normally
closed. When any window opens, its switch opens, and the pull-up resistor makes
PORTA's input high; otherwise the input is low. This signal is sensed in PORTA bit 0.
The C program statement if (PORTA & 1) alarm(); will execute procedure alarm if
any switch is opened. It is optimally programmed into assembly language as follows: