ATMEGA644PV ATMEL [ATMEL Corporation], ATMEGA644PV Datasheet - Page 101

• Manufacturer Part Number: ATMEGA644PV
• Description: 8-bit Microcontroller with 16/32/64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

13.8
8011D–AVR–02/07
Timer/Counter Timing Diagrams
one allows the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is
not available for the OC0B pin (See
be visible on the port pin if the data direction for the port pin is set as output. The PWM wave-
form is 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
Compare Match between OCR0x and TCNT0 when the counter decrements. The PWM fre-
quency 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
there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-
TOM. There are two cases that give a transition without Compare Match.
• OCR0A changes its value from MAX, like in
• The timer starts counting from a value higher than the one in OCR0A, and for that reason
The Timer/Counter is a synchronous design and the timer clock (clk
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set.
shows the count sequence close to the MAX value in all modes other than phase correct PWM
mode.
Figure 13-8. Timer/Counter Timing Diagram, no Prescaling
Figure 13-9
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.
misses the Compare Match and hence the OCn change that would have happened on the way
up.
TCNTn
(clk
TOVn
clk
clk
I/O
I/O
Tn
/1)
shows the same timing data, but with the prescaler enabled.
Figure 13-8
MAX - 1
contains timing data for basic Timer/Counter operation. The figure
Figure 13-7
f
Table 13-4 on page
OCnxPCPWM
OCnx has a transition from high to low even though
MAX
Figure
ATmega164P/324P/644P
=
----------------- -
N 510
f
13-7. When the OCR0A value is MAX the
clk_I/O
⋅
104). The actual OC0x value will only
BOTTOM
T0
) is therefore shown as a
BOTTOM + 1
101