Datasheet

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16-BIT TIMER (Cont’d)
10.4.3.4 Output Compare
In this section, the index, i, may be 1 or 2 because
there are two output compare functions in the 16-
bit timer.
This function can be used to control an output
waveform or indicate 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
(OC1R) and Output Compare Register 2 (OC2R)
contain the value to be compared to the counter
register each timer clock cycle.
These registers are readable and writable and are
not affected by the timer hardware. A reset event
changes the OC
iR value to 8000h.
Timing resolution is one count of the free running
counter: (
f
CPU/
CC[1:0]
).
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
signal.
– Select the timer clock (CC[1:0]) (see Table 1).
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 a match is found between OCiR register
and CR register:
– OCFi bit is set.
– The OCMPi pin takes OLVLi bit value (OCMPi
pin latch is forced low during reset).
– A timer interrupt is generated if the OCIE bit is
set in the CR1 register and the I bit is cleared in
the CC register (CC).
The OC
iR register value required for a specific tim-
ing application can be calculated using the follow-
ing formula:
Where:
Δt = Output compare period (in seconds)
f
CPU
= CPU clock frequency (in hertz)
PRESC
= Timer prescaler factor (2, 4 or 8 de-
pending on CC[1:0] bits, see Table 1)
If the timer clock is an external clock, the formula
is:
Where:
Δt = Output compare period (in seconds)
f
EXT
= External timer clock frequency (in hertz)
Clearing the output compare interrupt request
(that is, clearing the OCFi bit) is done by:
1. Reading the SR register while the OCFi bit is
set.
2. An access (read or write) to the OCiLR register.
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 OC
iR register:
– Write to the OCiHR 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 OCiLR register (enables the output
compare function and clears the OCFi bit).
MS Byte LS Byte
OCiROCiHR OCiLR
Δ OCiR =
Δt
*
f
CPU
PRESC
Δ OCiR = Δt
*
f
EXT