151
2467S–AVR–07/09
ATmega128
overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-
tom (0x00). In normal operation the Timer/Counter overflow flag (
TOV
2) will be set in the same
timer clock cycle as the TCNT2 becomes zero. The
TOV
2 flag in this case behaves like a ninth
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the
TOV
2 flag, the timer resolution can be increased by software. There
are no special cases to consider in the normal mode, a new counter value can be written
anytime.
The output compare unit can be used to generate interrupts at some given time. Using the out-
put compare to generate waveforms in normal mode is not recommended, since this will occupy
too much of the CPU time.
Clear Timer on
Compare Match (CTC)
Mode
In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to manip-
ulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNT2) matches the OCR2. The OCR2 defines the top value for the counter, hence also its
resolution. This mode allows greater control of the compare match output frequency. It also sim-
plifies the operation of counting external events.
The timing diagram for the CTC mode is shown in
. The counter value (TCNT2)
increases until a compare match occurs between TCNT2 and OCR2 and then counter (TCNT2)
is cleared.
Figure 65.
CTC Mode, Timing Diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF2 flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the
TOP value. However, changing the TOP to a value close to BOTTOM when the counter is run-
ning with none or a low prescaler value must be done with care since the CTC mode does not
have the double buffering feature. If the new value written to OCR2 is lower than the current
value of TCNT2, the counter will miss the compare match. The counter will then have to count to
its maximum value (0xFF) and wrap around starting at 0x00 before the compare match can
occur.
For generating a waveform output in CTC mode, the OC2 output can be set to toggle its logical
level on each compare match by setting the compare output mode bits to toggle mode
(COM21:0 = 1). The OC2 value will not be visible on the port pin unless the data direction for the
pin is set to output. The waveform generated will have a maximum frequency of f
OC2
= f
clk_I/O
/2
when OCR2 is set to zero (0x00). The waveform frequency is defined by the following equation:
TCNTn
OCn
(Toggle)
OCn Interrupt Flag Set
1
4
Period
2
3
(COMn1:0 = 1)
f
OCn
f
clk_I/O
2
N
1
OCRn
+
(
)
⋅ ⋅
-----------------------------------------------
=
