STR720 datasheet(316/401 Pages) STMICROELECTRONICS

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STR720 - EXTENDED FUNCTION TIMER (EFT)

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21.3.4.1 Procedure

To use the output compare function, select the following in the CR1/CR2 registers:

– Set the OCiE bit if an output is needed then the OCMPi pin is dedicated to the output com-

pare i function.

– Select the timer clock (ECKGEN) and the prescaler division factor (CC7

÷CC0).

Select the following in the CR1/CR2 registers:

– Select the OLVLi bit to applied to the OCMPi pins after the match occurs.

– Set OCAIE (OCBIE) if only compare A (compare B) needs to generate an interrupt.

When match is found:

– OCFi bit is set.

– The OCMPi pin takes OLVLi bit value (OCMPi pin latch is forced low during reset and stays

low until valid compares change it to OLVLi level).

– A timer interrupt is generated if the OCAIE (or OCBIE) bit in CR2 register is set, the OCAR

(or OCBR) matches the timer counter (i.e. OCFA or OCFB is set).

Clearing the output compare interrupt request is done by a write access to the SR register

while the OCFi bit is cleared, 14-bit at ‘0’ for OCAR and 12-bit at ‘0’ for OCBR.

If the OCiE bit is not set, the OCMPi pin is at ‘0’ and the OLVLi bit will not appear when match

is found.

The value in the 16-bit OCiR register and the OLVLi bit should be changed after each

successful comparison in order to control an output waveform or establish a new elapsed

timeout.

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

following formula:

Where:

∆t

= Desired output compare period (in seconds)

fAPB

= APB clock frequency

CC7

÷CC0

= Timer clock prescaler

∆ OCiR = ∆t * fAPB

(

CC7

÷CC0+1)

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