ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 101

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

15.7.4
9223B–AVR–09/11
Phase Correct PWM Mode
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 wave-
form generated will have a maximum frequency of f
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.
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 BOT-
TOM. 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
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.
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
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 BOT-
TOM value.
TCNTn
OCnx
OCnx
Period
Atmel ATmega48PA/88PA/168PA [Preliminary]
Figure
15-7. The TCNT0 value is in the timing diagram shown as a histogram for
1
2
OC0
= f
clk_I/O
3
/2 when OCR0A is set to zero.
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
(COMnx1:0 = 2)
(COMnx1:0 = 3)
101