Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
124
15.6.1 Normal Mode
The simplest mode of operation is the normal mode (WGM22:0 = 0). In this mode the counting direction is always up
(incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 8-bit value
(TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter overflow flag (TOV2) will be
set in the same timer clock cycle as the TCNT2 becomes zero. The TOV2 flag in this case behaves like a ninth bit, except
that it is only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the TOV2 flag,
the timer resolution can be increased by software. There are no special cases to consider in the normal mode, a new counter
value can be written anytime.
The output compare unit can be used to generate interrupts at some given time. Using the output compare to generate
waveforms in normal mode is not recommended, since this will occupy too much of the CPU time.
15.6.2 Clear Timer on Compare Match (CTC) Mode
In clear timer on compare or CTC mode (WGM22:0 = 2), the OCR2A register is used to manipulate the counter resolution. In
CTC mode the counter is cleared to zero when the counter value (TCNT2) matches the OCR2A. The OCR2A defines the top
value for the counter, hence also its resolution. This mode allows greater control of the compare match output frequency. It
also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 15-5. The counter value (TCNT2) increases until a compare match
occurs between TCNT2 and OCR2A, and then counter (TCNT2) is cleared.
Figure 15-5. CTC Mode, Timing Diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF2A flag. If the interrupt
is enabled, the interrupt handler routine can be used for updating the TOP value. However, changing TOP to a value close to
BOTTOM when the counter is running with none or a low prescaler value must be done with care since the CTC mode does
not have the double buffering feature. If the new value written to OCR2A is lower than the current value of TCNT2, the
counter will miss the compare match. The counter will then have to count to its maximum value (0xFF) and wrap around
starting at 0x00 before the compare match can occur.
For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical level on each compare
match by setting the compare output mode bits to toggle mode (COM2A1:0 = 1). The OC2A value will not be visible on the
port pin unless the data direction for the pin is set to output. The waveform generated will have a maximum frequency of
f
OC2A
= f
clk_I/O
/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the normal mode of operation, the TOV2 flag is set in the same timer clock cycle that the counter counts from MAX to
0x00.
12
TCNTn
(COMnx1:0 = 1)
OCnx
(Toggle)
Period
3
OCnx Interrupt
Flag Set
4
f
OCnx
f
clk_I/O
2 N 1 OCRnx+()⋅⋅
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