Atmel ATmega8535, Руководство пользователя

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80
ATmega8535(L)
2502K–AVR–10/06
OCR0 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 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 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 33 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 33. 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 value higher 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
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 34 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 34. Timer/Counter Timing Diagram, no Prescaling
Figure 35 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