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Servos use a motor to move a disk or control arm +/- 45 degrees from a centered position in response to an electrical pulse. The servo's position is proportional to the duration of the pulse, with a 1ms pulse corresponding to the endpoint of travel in one direction, a 2ms pulse corresponding to the endpoint of travel in the opposite direction, and a 1.5ms pulse corresponding to the center position. This simple servo subroutine accepts an input position value of between 0 and 255 and creates a corresponding pulse from 1 to 2 ms in duration. This servo control code was designed to run in a 4 MHz PIC.
;The trick here is the padded delay loop. The Delay code is made to
;be 4 instruction cycles long. At 4 MHz, one instruction cycle is 1 us,
;and 4 instruction cycles take 4 us to execute, so we can execute 250
;4-cycle delay loops per ms.
;The first time round, we wait for just under 1 ms (by preloading Counter
;with 248) after activating the servo. Then we use the position value to
;set our delay length (in 4 us increments). When the delay finishes, we
;deactivate the servo. The result is a 1-2 ms pulse corresponding to the
;position variable.
;Hardware Equates
Counter EQU 0Ch ;Servo delay counter register
Position EQU 0Dh ;Servo Position register
Servo1 EQU RB.0 ;Servo I/O port pin
Main .
.
Servo MOVLW 248 ;Load Counter with # of cycles for
MOVWF Counter ;1 ms Servo Delay
BSF Servo1 ;Activate the Servo1 output
CALL Delay ;Wait for 1ms to pass
MOVF Position,0 ;Load W with servo position
;00 & FF=extremes, 128=centre
MOVWF Counter ;and store in Counter for next delay
CALL Delay ;Keep servo active for position delay
BCF Servo1 ;Deactivate the Servo1 output
.
. ;Do other processing here, but
;remember to update each servo
;approximately once every 20 ms
Delay ;Servo time delay
NOP ;Pad the loop with one cycle
DECFSZ Counter ;Counter=Counter-1
GOTO Delay ;If Counter is not 0, repeat
RETURN ;If 0, return
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