Data Sheet

127
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be
set when a compare match occurs.
Figure 16-8. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOTTOM. When either OCR1A
or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accordingly at the same timer clock
cycle as the OCR1x Registers are updated with the double buffer value (at TOP). The Interrupt Flags can 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 TCNT1 and the OCR1x. Note that when using fixed TOP values, the unused bits
are masked to zero when any of the OCR1x Registers are written. As the third period shown in Figure 16-8 on
page 127 illustrates, changing the TOP actively while the Timer/Counter is running in the phase correct mode
can result in an unsymmetrical output. The reason for this can be found in the time of update of the OCR1x
Register. Since the OCR1x 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
changing 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 OC1x pins. Setting
the COM1x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COM1x1:0 to three (See Table 16-3 on page 133). The actual OC1x value will only be visible on the
port pin if the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1 when the counter
increments, and clearing (or setting) the OC1x Register at compare match between OCR1x and TCNT1 when
the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated
by the following equation:
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
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
f
OCnxPCPWM
f
clk_I/O
2 NTOP
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