L9805 datasheet(38/103 Pages) STMICROELECTRONICS

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L9805

16-BIT TIMER (Cont’d)

5.2.3.4 Output Compare

In this section, the index, i, may be 1 or 2.

This function can be used to control an output

waveform or indicating when a period of time has

elapsed.

When a match is found between the Output Com-

pare register and the free running counter, the out-

put compare function:

– Assigns pins with a programmable value if the

OCIE bit is set

– Sets a flag in the status register

– Generates an interrupt if enabled

Two 16-bit registers Output Compare Register 1

(OCR1) and Output Compare Register 2 (OCR2)

contain the value to be compared to the free run-

ning counter each timer clock cycle.

These registers are readable and writable and are

not affected by the timer hardware. A reset event

changes the OCRi value to 8000h.

Timing resolution is one count of the free running

counter: (

fCPU/(CC1.CC0)).

Procedure

To use the output compare function, select the fol-

lowing in the CR2 register:

– Set the OCiE bit if an output is needed then the

OCMPi pin is dedicated to the output compare i

function.

– Select the timer clock (CC1-CC0) (see Table 7

Clock Control Bits).

And select the following in the CR1 register:

– Select the OLVLi bit to applied to the OCMPi pins

after the match occurs.

– Set the OCIE bit to generate an interrupt if it is

needed.

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 a high level).

– A timer interrupt is generated if the OCIE bit is

set in the CR2 register and the I bit is cleared in

the CCR register (CCR).

Clearing the output compare interrupt request is

done by:

3. Reading the SR register while the OCFi bit is

set.

4. An access (read or write) to the OCLRi register.

Note: After a processor write cycle to the OCHRi

until the OCLRi register is also written.

If the OCiE bit is not set, the OCMPi pin is a gen-

eral I/O port and the OLVLi bit will not appear

when match is found but an interrupt could be gen-

erated if the OCIE bit is set.

The value in the 16-bit OCRi register and the OLVi

bit should be changed after each successful com-

parison in order to control an output waveform or

establish a new elapsed timeout.

The OCRi register value required for a specific tim-

ing application can be calculated using the follow-

ing formula:

Where:

= Desired output compare period (in

seconds calculated from the current

counter)

fCPU

= Internal clock frequency (see Miscel-

laneous register)

CC1-CC0 = Timer clock prescaler

The following procedure is recommended to pre-

vent the OCFi bit from being set between the time

it is read and the write to the OCRi register:

– Write to the OCHRi register (further compares

are inhibited).

– Read the SR register (first step of the clearance

of the OCFi bit, which may be already set).

– Write to the OCLRi register (enables the output

compare function and clears the OCFi bit).

MS Byte

LS Byte

OCRi

OCHRi

OCLRi

∆ OCRi register Value =

t * fCPU

(CC1.CC0)

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