L9803 datasheet(52/126 Pages) STMICROELECTRONICS

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On-Chip Peripherals

L9803

52/126

The Pulse Width Modulation mode uses the complete Output Compare 1 function plus the

OC2R register, and so these functions cannot be used when the PWM mode is activated.

Procedure

To use Pulse Width Modulation mode:

Load the OC2R register with the value corresponding to the period of the signal using

the formula in the section.

Load the OC1R register with the value corresponding to the period of the pulse if

OLVL1=0 and OLVL2=1, using the formula in the section.

Select the following in the CR1 register:

–

Using the OLVL1 bit, select the level to be applied to the OCMP1 pin after a

successful comparison with OC1R register.

–

Using the OLVL2 bit, select the level to be applied to the OCMP1 pin after a

successful comparison with OC2R register.

Select the following in the CR2 register:

–

Set OC1E bit: the OCMP1 pin is then dedicated to the output compare 1 function.

–

Set the PWM bit.

–

Select the timer clock (CC[1:0]) (see Table 9: Clock Control Bits).

If OLVL1=1 and OLVL2=0, the length of the positive pulse is the difference between the

OC2R and OC1R registers.

If OLVL1=OLVL2 a continuous signal will be seen on the OCMP1 pin.

The OCRi register value required for a specific timing application can be calculated using

the following formula:

Where:

= Signal or pulse period (in seconds)

fCPU

= CPU clock frequency (in hertz)

PRESC = Timer prescaler factor (2, 4 or 8 depending on CC[1:0] bits,

see Table 9: Clock Control Bits

If the timer clock is an external clock the formula is:

OCiR = t · fEXT -5

Where:

Counter

Counter is reset

to FFFCh

OCMP1 = OLVL2

Counter

= OCR2

OCMP1 = OLVL1

When

= OCR1

Pulse Width Modulation cycle

ICF1 bit is set

OCiR Value

CPU

⋅

PRESC

----------------------

–

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