Page 350 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

I! .5 Gadfly Synchronization 327

LDAB #$80 ; Put one in leftmost bit of accumulator B
STAB TSCR ; Put it into the control register TSCR
LDD #$201 ; Generate 2 in Accumulator A, 1 in Accumulator B
STAA $8B ; Write 2 in rightmost bit of control register TCTLE
STAB $8E ; Write one in rightmost bit of control register TFLG1

In place of a delay loop, the gadfly loop itself is used whenever input or output is
done: do ; while{(TFLG1 & 1) == 0); TFLG1 = 1;. The loop waits until an edge
sets the flag bit, and the next statement clears the flag bit. This is optimally compiled to
assembly language as

L: BRCLR $8E,#1,L ; Wait for one in TFLG1 bit 0
LDAB #1 ; Put one in rightmost bit of accumulator B
STAB $8E ; Write it into control register TFLG1

We illustrate the use of gadfly synchronization for counting pulses from a Geiger
counter. Each time a pulse occurs, PORTT bit 0 sees a rising edge. To count the pulses,
execute the C procedure:
void main() { int i = 0;
TSCR = 0x80; TFLG1 = 1;TCTLE = 1;
do {while({TFLG1 & 1) == 0); TFLG1 = 1; i++; }while(1);
}

An optimized assembly language program segment for the body of the C procedure is
LDAB #$80 ; Put one in leftmost bit of accumulator B
STAB $86 ; Write it into control register TSCR
LDD #$102 ; Generate one in Accumulator A, 2 in Accumulator B
STAA $ 8E ; Write one in rightmost bit of control register TFLG1
STAB $ 8B ; Write 2 in rightmost bit of control register TCTLE
CLRA ; Clear msbyte of local variable i
CLRB ; Clear next byte of i
LOOP: BRCLR $ 8E, #1, LOOP ; Wait for one in TFLG1 bit 0
MOVE #1, $ 8E ; Write 1 into control register TFLG1 to clear bit 0
ADDD #1 ; Increment count
BRA LOOP ; Loop forever
You can stop this program after it has counted the pulses and examine accumulator D.