ATmega640/1280/1281/2560/2561
In phase and frequency correct PWM mode the counter is incremented until the counter
value matches either the value in ICRn (WGMn3:0 = 8), or the value in OCRnA
(WGMn3:0 = 9). The counter has then reached the TOP and changes the count direc-
tion. The TCNTn value will be equal to TOP for one timer clock cycle. The timing
diagram for the phase correct and frequency correct PWM mode is shown on Figure 57.
The figure shows phase and frequency 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 operation. The diagram includes non-inverted and inverted
PWM outputs. The small horizontal line marks on the TCNTn slopes represent compare
matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a com-
pare match occurs.
Figure 57. Phase and Frequency Correct PWM Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
OCnx
1
2
3
4
Period
The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the
OCRnx Registers are updated with the double buffer value (at BOTTOM). When either
OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag set when
TCNTn has reached TOP. The Interrupt Flags can then be used to generate an interrupt
each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is
higher or equal to the value of all of the Compare Registers. If the TOP value is lower
than any of the Compare Registers, a compare match will never occur between the
TCNTn and the OCRnx.
As Figure 57 shows the output generated is, in contrast to the phase correct mode, sym-
metrical in all periods. Since the OCRnx Registers are updated at BOTTOM, the length
of the rising and the falling slopes will always be equal. This gives symmetrical output
pulses and is therefore frequency correct.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By
using ICRn, the OCRnA Register is free to be used for generating a PWM output on
OCnA. However, if the base PWM frequency is actively changed by changing the TOP
value, using the OCRnA as TOP is clearly a better choice due to its double buffer
feature.
In phase and frequency correct PWM mode, the compare units allow generation of
PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a
153
2549A–AVR–03/05
ATMEL [ ATMEL ]
