Atmel ATmega128, Manual

Page: 102 / 386

102
2467S–AVR–07/09
ATmega128
PWM frequency for the output when using phase correct PWM can be calculated by the follow-
ing equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR0 Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR0 is set equal to BOTTOM, the out-
put 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 40 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:
• OCR0 changes its value from MAX, like in Figure 40 . When the OCR0 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 higher value than the one in OCR0, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the
way up.
Timer/Counter
Timing Diagrams
Figure 41 and Figure 42 contain timing data for the Timer/Counter operation. The Timer/Counter
is a synchronous design and the timer clock (clk
T0
) is therefore shown as a clock enable signal.
The figure shows the count sequence close to the MAX value. Figure 43 and Figure 44 show the
same timing data, but with the prescaler enabled. The figures illustrate when interrupt flags are
set.
The following figures show the Timer/Counter in Synchronous mode, and the timer clock (clk
T0
)
is therefore shown as a clock enable signal. In asynchronous mode, clk
I/O
should be replaced by
the Timer/Counter Oscillator clock. The figures include information on when interrupt flags are
set.
Figure 41 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 41. Timer/Counter Timing Diagram, No Prescaling
Figure 42 shows the same timing data, but with the prescaler enabled.
f
OCnPCPWM
f
clk_I/O
N
510
⋅
------------------ =
clk
Tn
(clk
I/O
/1)
TOVn
clk
I/O
TCNTn
MAX - 1 MAX BOTTOM 1