Datasheet

155
2490R–AVR–02/2013
ATmega64(L)
The N variable represents the prescale factor (1, 8, 64, 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 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 67 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 a Compare Match.
• OCR2 changes its value from MAX, like in Figure 67. When the OCR2 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 OCR2, 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
The Timer/Counter is a synchronous design and the timer clock (clk
T2
) is therefore shown as a
clock enable signal in the following figures. The figures include information on when interrupt
flags are set. Figure 68 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 68. Timer/Counter Timing Diagram, no Prescaling
Figure 69 shows the same timing data, but with the prescaler enabled.
clk
Tn
(clk
I/O
/1)
TOVn
clk
I/O
TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1