Figure 51. Phase Correct PWM Mode, Timing Diagram
OCn Interrupt Flag Set
OCRn Update
TOVn Interrupt Flag Set
TCNTn
(COMn1:0 = 2)
(COMn1:0 = 3)
OCn
OCn
1
2
3
Period
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOT-
TOM. 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 45 on
page 116). 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 Register 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:
f
clk_I/O
f
= -----------------
OCnPCPWM
N 510
The N variable represents the prescale 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.
At the very start of period 2 in Figure 51 OCn has a transition from high to low even
though there is no Compare Match. The point of this transition is to guarantee symmetry
around BOTTOM. There are two cases that give a transition without Compare Match:
•
OCR2A changes its value from MAX, like in Figure 51. When the OCR2A 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.
112
ATmega8(L)
2486M–AVR–12/03
ATMEL [ ATMEL ]
