Datasheet

19.9.2. Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value
(TCNTn) matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn
define the top value for the counter, hence also its resolution. This mode allows greater control of the
Compare Match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNTn) increases until a
Compare Match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared.
Figure 19-6. CTC Mode, Timing Diagram
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period
2 3
(COMnA[1:0] = 0x1)
An interrupt can be generated at each time the counter value reaches the TOP value by either using the
OCFnA or ICFn Flag according to the register used to define the TOP value. 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 running 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
OCRnA or ICRn is lower than the current value of TCNTn, the counter will miss the Compare Match. The
counter will then have to count to its maximum value (0xFFFF) and wrap around starting at 0x0000 before
the Compare Match can occur. In many cases this feature is not desirable. An alternative will then be to
use the fast PWM mode using OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be
double buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on
each Compare Match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The
OCnA value will not be visible on the port pin unless the data direction for the pin is set to output
(DDR_OCnA = 1). The waveform generated will have a maximum frequency of f
OCnA
= f
clk_I/O
/2 when
OCRnA is set to zero (0x0000). The waveform frequency is defined by the following equation:

OCnA
=

clk_I/O
2   
1 + OCRnA
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer Counter TOVn Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
19.9.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM options by its
single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-
inverting Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match between
TCNTn and OCRnx, and set at BOTTOM. In inverting Compare Output mode output is set on Compare
Match and cleared at BOTTOM. Due to the singleslope operation, the operating frequency of the fast
Atmel ATmega32A [DATASHEET]
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