;*** Measure the width of an external pulse with an AVR Mega8
;; PWM sginal from R/C receiver. One 1-2ms pulse every 20ms.
;; R/C PWM signal connected to ICP (PortB, Pin0).
;; Mega8 running at 4Mhz.
;; Demo code writes lower 8 bits of Timer1/ICR1 to PortD.
;; Note that if upper 8 bits of ICR1 are not all zero,
;; then pulse width exceeds valid range.
;; -rkumar at hyperreal dot org
.org 0x0000 ; Reset vector
.org 0x0005 ; Timer1 Capture Event Interupt vector
ldi r16, low(RAMEND) ; Set up stack pointer on reset
out SPL, r16
ldi r16, high(RAMEND)
out SPH, r16
ldi r16, (1<<ICES1)|(1<<CS11)|(1<<CS10)
out TCCR1B, r16 ; Set clock prescaler to 32
; Also set the Input Capture Edge Select bit to 1
; (trigger on rising edge)
ldi r16, 1<<ICF1
out TIFR, r16 ; Clear ICF1, Clear pending interupts.
ldi r16, 1<<TICIE1 ; Set bit 4.
out TIMSK, r16 ; Enable Timer1 capture event interupt.
sei ; Enable global interupts.
ser r16 ; Set all bits in r16.
out DDRD, r16 ; Set PortD as output.
cbi DDRB, PB0 ; Set PB0 (ICP) as input.
loop: ; Loop forever! Wait for interupt!
; Timer1 Capture Handler_________________________________________________
; We don't need to perserve registers, since they
; are not used outside this ISR. But be careful
; when extending this code. Note that this ISR
; returns from two different places, depending on ICES1!
;;push r16 ; Preserve r16
;;in r16, SREG ; Save MCU status register
in r20, ICR1L ; Read Input Capture Register to r20/21
in r21, ICR1H
; Invert the edge select bit.. This is NOT the best way to do this..
; We have 32 registers, and we should use them.
; However, it is unclear right now which registers our main loop
; will need.
in r16, TCCR1B ; Read Timer1 Control Register B into r16.
bst r16, ICES1 ; Store edge select bit in T.
bld r17, ICES1 ; Load edge select bit from T into r17.
com r17 ; Invert bits in r17.
bst r17, ICES1 ; Store edge select bit in T.
bld r16, ICES1 ; Now we have the inverted the edge select bit in r16.
out TCCR1B, r16 ; Set Timer1 Control Register with inverted edge select bit.
out TCNT1H, r17 ; Clear Timer1
out TCNT1L, r17
sbrs r16, ICES1 ; In r16, if edge select bit==0 (which means we entered on rising edge)
reti ; then return from interupt and wait for falling edge.
; If we reach this point in the ISR, it means we entered on a negative edge.
; The pulse width is already stored in r20/21.
com r20 ; Invert bits (led's connected backwards)
out PORTD, r20 ; Write lower 8 bits of Timer1 ICR to PortD
;clr r16 ; clearing Timer1 here should be unnecessary
;out TCNT1H, r16 ; clear Timer1
;out TCNT1L, r16
;;pop r16 ; Remember this isn't the only place we return from the ISR.
;;out SREG, r16