121
ATmega64A [DATASHEET]
8160D–AVR–02/2013
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 Compare Match
occurs.
Figure 16-8. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA or
ICRn is used for defining the TOP value, the OCnA or ICFn flag is set accordingly at the same timer clock cycle as
the OCRnx Registers are updated with the double buffer value (at TOP). The interrupt flags can be used to gener-
ate 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. Note that when using fixed TOP values, the unused bits are
masked to zero when any of the OCRnx Registers are written. As the third period shown in
Figure 16-8 illustrates,
changing the TOP actively while the Timer/Counter is running in the phase correct mode can result in an unsym-
metrical output. The reason for this can be found in the time of update of the OCRnx Register. Since the OCRnx
update occurs at TOP, the PWM period starts and ends at TOP. This implies that the length of the falling slope is
determined by the previous TOP value, while the length of the rising slope is determined by the new TOP value.
When these two values differ the two slopes of the period will differ in length. The difference in length gives the
unsymmetrical result on the output.
It is recommended to use the phase and frequency correct mode instead of the phase correct mode when chang-
ing the TOP value while the Timer/Counter is running. When using a static TOP value there are practically no
differences between the two modes of operation.
In phase 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 non-inverted PWM and an inverted PWM output can be generated by set-
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 when the counter increments,
and clearing (or setting) the OCnx Register at Compare Match between OCRnx and TCNTn when the counter dec-
OCRnx / TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1
2
3
4
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
Period
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)