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9-21
TIMER/COUNTER UNIT
Example 9-2. Configuring a Square-Wave Generator
$mod186
name example_timer1_square_wave_code
;FUNCTION: This function generates a square wave of given
; frequency and duty cycle on Timer 1 output pin.
;
; SYNTAX: extern void far clock(int mark, int space)
;
; INPUTS: mark - This is the mark (1) time.
; space - This is the space (0) time.
;
; The register compare value for a given time can be
; easily calculated from the formula below.
;
; CompareValue = (req_pulse_width*f)/4
;
; OUTPUTS: None
;
; NOTE: Parameters are passed on the stack as required by
; high-level Languages
T1CMPA equ xxxxH ;substitute register offsets
T1CMPB equ xxxxH
T1CNT equ xxxxH
T1CON equ xxxxH
lib_80186 segment public ’code’
assume cs:lib_80186
public _clock
_clock proc far
push bp ;save caller’s bp
mov bp, sp ;get current top of stack
_space equ word ptr[bp+6] ;get parameters off the stack
_mark equ word ptr[bp+8]
push ax ;save registers that will be
push bx ;modified
push dx
mov dx, T1CMPA ;set mark time
mov ax, _mark
out dx, al
mov dx, T1CMPB ;set space time
mov ax, _space
out dx, al
mov dx, T1CNT ;Clear Timer 1 Counter
xor ax, ax
out dx, al
mov dx, T1CON ;start Timer 1
mov ax, C003H
out dx, al