ATmega8U2/16U2/32U2
15.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0x[1:0] bits differently in Normal, CTC, and PWM
modes. For all modes, setting the COM0x[1:0] = 0 tells the Waveform Generator that no action
on the OC0x Register is to be performed on the next Compare Match. For compare output
actions in the non-PWM modes refer to Table 15-2 on page 102. For fast PWM mode, refer to
Table 15-3 on page 102, and for phase correct PWM refer to Table 15-4 on page 103.
A change of the COM0x1:0 bits state will have effect at the first Compare Match after the bits are
written. For non-PWM modes, the action can be forced to have immediate effect by using the
FOC0x strobe bits.
15.7 Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is
defined by the combination of the Waveform Generation mode (WGM0[2:0]) and Compare Out-
put mode (COM0x[1:0]) bits. The Compare Output mode bits do not affect the counting
sequence, while the Waveform Generation mode bits do. The COM0x[1:0] bits control whether
the PWM output generated should be inverted or not (inverted or non-inverted PWM). For non-
PWM modes the COM0x[1:0] bits control whether the output should be set, cleared, or toggled
at a Compare Match (See “Compare Match Output Unit” on page 95.).
For detailed timing information see “Timer/Counter Timing Diagrams” on page 100.
15.7.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM0[2:0] = 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 bot-
tom (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 Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
15.7.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM0[2:0] = 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.
The timing diagram for the CTC mode is shown in Figure 15-5. The counter value (TCNT0)
increases until a Compare Match occurs between TCNT0 and OCR0A, and then counter
(TCNT0) is cleared.
96
7799D–AVR–11/10
ATMEL [ ATMEL ]
