88
ATmega8A [DATASHEET]
8159E–AVR–02/2013
compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a Compare Match
occurs.
Figure 17-8. 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 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 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 17-8 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 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 2 will produce a non-inverted PWM and an inverted PWM output can be generated by setting
the COM1x1:0 to 3. See
Table 17-4 on page 93. 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 clear-
ing) 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 decre-
ments. 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
(COMnx1:0 = 2)
(COMnx1:0 = 3)
fOCnxPCPWM
f
clk_I/O
2
NTOP
----------------------------
=