Datasheet

ATmega32A
 2018 Microchip Technology Inc. Data Sheet Complete DS40002072A-page 84
OC0 Register at compare match between 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 15-7, OCn has a transition from high to low even though there is no Compare
Match. The point of this transition is to ensure symmetry around BOTTOM. There are two cases that give a transi-
tion without Compare Match:
• OCR0A changes its value from MAX, like in Figure 15-7. 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.
15.8 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 15-8 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 15-8. Timer/Counter Timing Diagram, no Prescaling
Figure 15-9 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 BOTTOM + 1