ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 102

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

15.8
102
Timer/Counter Timing Diagrams
Atmel ATmega48PA/88PA/168PA [Preliminary]
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the
OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted
PWM output can be generated by setting the COM0x1:0 to three: Setting the COM0A0 bits to
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.
• OCRnx changes its value from MAX, like in
• The timer starts counting from a value higher than the one in OCRnx, 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 15-8. Timer/Counter Timing Diagram, no Prescaling
f
TCNTn
OCnxPCPWM
(clk
TOVn
OCn pin value is the same as the result of a down-counting Compare Match. To ensure
symmetry around BOTTOM the OCnx value at MAX must correspond to the result of an
up-counting Compare Match.
misses the Compare Match and hence the OCnx change that would have happened on the
way up.
clk
clk
I/O
I/O
Tn
/1)
=
Figure 15-8
-------------------- -
N
f
clk_I/O
510
MAX - 1
contains timing data for basic Timer/Counter operation. The figure
Figure 15-7
Table 15-7 on page
OCnx has a transition from high to low even though
MAX
Figure
15-7. When the OCR0A value is MAX the
106). The actual OC0x value will only
BOTTOM
T0
) is therefore shown as a
BOTTOM + 1
9223B–AVR–09/11