Datasheet

123
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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 in Figure 14-6. The counter value (TCNTn) increases until a
compare match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared.
Figure 14-6. CTC Mode, Timing Diagram
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
OC
n
A
= f
clk_I/O
/2 when OCRnA is set to zero (0x0000).
The waveform frequency is defined by the following equation:
The
N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x0000.
14.8.3 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 set on the compare match between TCNTn and OCRnx, and
cleared at TOP. In inverting Compare Output mode output is cleared on compare match and set at TOP. Due to
the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the phase
correct and phase and frequency correct PWM modes that use dual-slope operation. This high frequency
makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency
allows physically small sized external components (coils, capacitors), hence reduces total system cost.
TCNTn
OCnA
(Toggle)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 4
Period
2 3
(COMnA1:0 = 1)
f
OCnA
f
clk_I/O
2 N 1 OCRnA+
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