Datasheet

125
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better choice due to its
double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the
COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to three.
Refer to Table 14-1 on page 131, Table 14-2 on page 132, and Table 14-3 on page 132.
The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn, and clearing (or setting) the OCnx Register at the timer clock cycle the
counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow spike
for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or low output
(depending on the polarity of the output set by the COMnx1:0 bits).
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA to
toggle its logical level on each compare match (COMnA1:0 = 1). This applies only if OCR1A is used to define
the TOP value (WGM13:0 = 15). 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). This feature is similar to the OCnA toggle in CTC mode, except the double
buffer feature of the Output Compare unit is enabled in the fast PWM mode.
14.8.4 Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 1, 2, 3, 10, or 11)
provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is,
like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter counts
repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output
mode, the Output Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while
upcounting, and set on the compare match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope
operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for
motor control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn
or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum
resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the
following equation:
In phase correct PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or the value in
OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count direction. The TCNTn
value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is
shown on Figure 14-8 on page 126. The figure shows phase correct PWM mode when OCRnA or ICRn is used
to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope
f
OCnxPWM
f
clk_I/O
N 1 TOP+
-----------------------------------=
R
PCPWM
TOP 1+log
2
log
-----------------------------------=