PLL Generation using ARTimer

3/5

highest number: 255 (for best resolution). In this case 66.7

μs / 255 = 0.26μs is the smallest TC

clock period that we can use. With a prescaler ratio of 1, the TC clock period is 1/8MHz = 0.125

μs. So we need a prescaler ratio of 3, giving a TC clock period of 3/8 MHz = 0.375μs. This is the

elementary incrementing time of the TC counter, which gives the resolution of the phase shift

μs=

2667kHz).

The desired delay is 19

μs: CP - RC = 19μs /0.375μs = 50.67.

The TIMIN period is:

TOT = 66.7

μs / 0.375 μs = 177.78.

The duty cycle is: 0.25 = (255 - CP)/TOT

Rounding the decimals, this gives:

RC = 160; CP = 210; TOT = 178

The condition is met: Tvar is positive and smaller than CP: (255 - 160) < 178 < (255 - 160 + 210)

The TIMOUT signal will remain correct and stable as long as this condition is met, even if the in-

put frequency varies:

The input frequency limits for output correctness are:

(255 - 160) x 0.375

The output signal will remain locked in phase and frequency as long as the input signal TIMIN

is in the range:

8.7kHz < TIMIN frequency < 28kHz

Of course, it is also possible to modify by software the 19

μs delay and the 75% duty cycle, for

example by measuring repetitively an error voltage with the A/D converter and calculating the

modified delay or duty cycle.

;**************** A-R Timer Register Set *******************

RC

.def 0D9h,0FFh,0FFh

;reload/capture register

CP

.def 0DAh,0FFh,0FFh

;compare register

MC

.def 0D5h,0FFh,0FFh

;mode control register

SC0

.def 0D6h,0FFh,0FFh

;status/control register 0

SC1

.def 0D7h,0FFh,0FFh

;status/control register 1

LR

.def 0DBh,0FFh,0FFh

;load register

;============================================================

ldi CP, 210

;compare register = 210d

ldi RC, 160

;reload register = 160d

ldi SC1,00000101b

;clock source= CPU clock divided by 3

;rising TIMIN edge active

;pull-up disabled, prescaler ratio = 1

ldi MC, 11100011b

;load on TIMIN mode,interrupts disabled

;PWMOUT enabled, start timer

Program example