85
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
14.7
Modes of operation
The mode of operation, that is, the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generating mode (WGM02:0) and Compare Output mode (COM0x1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generating mode bits do.
The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a
14.7.1
Normal mode
The simplest mode of operation is the Normal mode (WGM02:0 = 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 (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 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 TOV0 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.
14.7.2
Clear Timer on Compare Match (CTC) mode
In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the counter
resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the OCR0A. The
OCR0A 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.
until a compare match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 14-5. CTC mode, timing diagram.
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A 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 OCR0A is lower
than the current value of TCNT0, 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.
TCNTn
OCn
(toggle)
OCnx interrupt flag set
1
4
Period
2
3
(COMnx1:0 = 1)