Page 355 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

332 Chapter 1! Input/ Output

unsigned char i;
void main() {
TSCR = 0x80; TTMSKl = TFLG1 = TCTLE = 1; asm CLI
do ; while(l);
}
void interrupt 8 hndlr(void){ TFLG1=1; /*clrreqst*/ i++; /* inc count */}

An optimized assembly-language program segment for the body of the C procedure is

i: DS. B 1 ; Global storage for 8-bit count i
LDAB #$80 ; Put one in leftmost bit of accumulator B
STAB $86 ; Write it into control register TSCR
LDAA #$ 1 ; Generate one in Accumulator A
STAA $ 8E ; Write one in rightmost bit of control reg. TFLG1
STAA $ 8C ; Write one in rightmost bit of control reg. TMSK1
STAA $8B ; Write 1 in rightmost bit of control reg. TCTLE
CLI ; Enable interrupt
LOOP: BRA LOOP ; loop forever

An optimized assembly-language program segment for the handler is

HNDLR: MOVE #1, TFLG1 ; Write one in rightmost bit of control register TFLG1
INC i ; Increment count
RTI ; Return to interrupted routine

The main procedure above initializes the control registers and loops forever. The I
condition code bit is generally clear after the 6812 is reset and must be cleared to enable
interrupts to occur. A high-level language like C generally does not have a way to enable
interrupts, except by inserting embedded assembly language. The statement asm CLI
inserts CLI into the assembly-language program. Each time another vector element is to
be output, a switch is closed, and PORTT bit 0 sees a rising edge. This causes an
interrupt, and the handler is entered. This handler increments the count.
We further illustrate an interrupt-based traffic light controller that is essentially the
same as the gadfly-based traffic light controller example. The main procedure initializes
the control registers, waits for all elements to be output, and then disables the interrupt.
Each time another vector element is to be output, a switch is closed, and PORTT bit 0
sees a falling edge. See Figure 11.9. This causes an interrupt, and the handler is entered.
This handler outputs an element from the vector. To output the vector, execute

char buffer[0x80], i = 0x80;
void main{) {
DDRA = OxFF; TSCR = 0x80; TFLG1 = TMSK1 = 1; TCTLE = 2; asm CLI
do ; while ( i ); /* wait until all are output */ TMSK1 = 0; /* disable interrupt */
}
void interrupt 8 hndlr (void) (TFLGl=l;PORTA=buff erf—i ];/* output data */}