Datasheet

127
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
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 Com-
pare Match between OCR0x and TCNT0 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 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 16-7 on page 126 OCnx 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.
• OCR0A changes its value from MAX, like in Figure 16-7 on page 126. 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.
• The timer starts counting from a value higher than the one in OCR0A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the
way up.
16.8 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T0
) is therefore shown as a
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set. Figure 16-8 contains timing data for basic Timer/Counter operation. The figure
shows the count sequence close to the MAX value in all modes other than phase correct PWM
mode.
Figure 16-8. Timer/Counter Timing Diagram, no Prescaling
Figure 16-9 on page 128 shows the same timing data, but with the prescaler enabled.
f
OCnxPCPWM
f
clk_I/O
N 510
------------------=
clk
Tn
(clk
I/O
/1)
TOVn
clk
I/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1