Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 106
The extreme values for the OCR0A Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each
MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output
(depending on the polarity of the output set by the COM0A1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to
toggle its logical level on each Compare Match (COM0x1:0 = 1). The waveform generated will have a maximum
frequency of f
OC0
= f
clk_I/O
/2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
15.7.4 Phase Correct PWM mode
The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct PWM waveform
generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts
repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1,
and OCR0A when WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared
on the Compare Match between TCNT0 and OCR0x while upcounting, and set on the Compare Match while
down-counting. 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.
In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the
counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock
cycle. The timing diagram for the phase correct PWM mode is shown on Figure 15-7. The TCNT0 value is in the
timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-
inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare
Matches between OCR0x and TCNT0.
Figure 15-7. Phase Correct PWM mode, timing diagram
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt Flag
can be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting
the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1 2 3
TCNTn
Period
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Update