75
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
Figure 11-9.
Phase Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV0) 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 OC0x pins. Setting
the COM0x[1:0] bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by setting
the COM0x[1:0] to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare Matches if
the WGM02 bit is set. This option is not available for the OC0B pin (See
Table 11-4 on page 78). The actual OC0x
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 OC0x Register at the Compare Match between OCR0x and TCNT0 when the
counter increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0
when the counter decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output in
the phase correct PWM mode. If the OCR0A 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.
At the very start of period 2 in
Figure 11-9 OCn has a transition from high to low even though there is no Compare
Match. The point of this transition is to guaratee symmetry around BOTTOM. There are two cases that give a tran-
sition without Compare Match, as follows:
OCR0A changes its value from MAX, like in
Figure 11-9. When the OCR0A value is MAX the OCn pin value is
the same as the result of a down-counting Compare Match. To ensure symmetry around BOTTOM the OCn
value at MAX must correspond to the result of an up-counting Compare Match.
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1
2
3
TCNTn
Period
OCn
(COMnx[1:0] = 2)
(COMnx[1:0] = 3)
OCRnx Update
f
OCnxPCPWM
f
clk_I/O
N510
------------------
=