107
ATmega8A [DATASHEET]
8159E–AVR–02/2013
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.
The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct PWM mode the coun-
ter is incremented until the counter value matches MAX. When the counter reaches MAX, it changes the count
direction. The TCNT2 value will be equal to MAX for one timer clock cycle. The timing diagram for the phase cor-
rect PWM mode is shown on
Figure 18-7. The TCNT2 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 hor-
izontal line marks on the TCNT2 slopes represent compare matches between OCR2 and TCNT2.
Figure 18-7. Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The Interrupt Flag can
be used to generate an interrupt each time the counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC2 pin. Setting the
COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM output can be generated by setting the
COM21:0 to 3 (see
Table 18-5 on page 113). The actual OC2 value will only be visible on the port pin if the data
direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the OC2 Regis-
ter at the Compare Match between OCR2 and TCNT2 when the counter increments, and setting (or clearing) the
OC2 Register at Compare Match between OCR2 and TCNT2 when the counter decrements. The PWM frequency
for the output when using phase correct PWM can be calculated by the following equation:
The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2 Register represent special cases when generating a PWM waveform output in
the phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the output will be continuously low and if set
equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will
have the opposite logic values.
TOVn Interrupt Flag Set
OCn Interrupt Flag Set
1
2
3
TCNTn
Period
OCn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCRn Update
fOCnPCPWM
f
clk_I/O
N 510
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