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STARTING A PWM SIGNAL DIRECTLY AT HIGH LEVEL USING THE ST7 16-BIT TIMER

2 FIRST CASE: OLVL2=1 AND OLVL1=0

In this case, to force a high state on the OCMP1 pin when the timer is started:

– Initialize the timer in PWM mode and set the PWM frequency with a 0% duty cycle

– Reset the timer (at FFFC)

– Load the OC1R register with a value close to FFFC (FFFD for example) and configure

OLVL1=1 and OLVL2=0.

– Start the timer. It immediately reaches the value of the OC1R register and OCMP1 pin goes

into the state defined by the OLVL1 bit, which is high.

– Then, write the correct duty cycle value in the OC1R register and the correct state in the

OLVL2 and OLVL1 bits (OLVL2=1 and OLVL1=0).

The following code gives an example of how to restart (or start) the 16-bit timer this way. This

is for Timer A already initialized in PWM mode at 10KHz with the 16-bit timer clocked at 1MHz

(TACR2=10011000, TAOC2HR=$00 and TAOC2LR=$5F for 10KHz at 1MHz clock.

This is how to calculate the values to be put in the registers:

is 8MHz and the Timer prescaler is 8 because the Timer is at 1MHz.

OC2R=((t*Fcpu)/presc)-5=((100.10-6*8.10+6)/8)-5=95 (005F in hexadecimal).

– OC1R represents the pulse length, the duty cycle is 50% so the pulse length is

100µs*50%=50µs. So the value to be put in OC1R is:

OC1R=((50.10-6*8.10+6)/8)-5=45 (002D in hexadecimal).

Note: This method can be applied when the counter is first started as the starting value is

0000.

Initialization: In this example the duty cycle is first set to 0% with OLVL1=0 and OLVL2=0

ld A,#%00000000; set OLVL1=0 and OLVL2=0

ld TACR1,A

ld A,#%10011000; clock in/8=1MHz with 16 MHz quartz

ld TACR2,A

ld A,#$00

ld TAOC2HR,A

;10KHz frequency for the PWM signal (see formulas)

ld A,#$5F

ld TAOC2LR,A

Synchronization

ld A,#%00000001;Set OLVL1=1 and OLVL2=0

ld TACR1,A

;load corresponding control register 1

ld A,#$FF

ld TAOC1HR,A

;fix compare 1 to FFFD to force PWM high as soon as possible

ld A,#$FD