ATMEGA8-16AI SL383 Atmel, ATMEGA8-16AI SL383 Datasheet - Page 88

• Manufacturer Part Number: ATMEGA8-16AI SL383
• Description: Semiconductors and Actives, Microprocessors, Microcontrollers
• Manufacturer: Atmel

Compare Output Mode
and Waveform
Generation
Modes of
Operation
Normal Mode
Clear Timer on
Compare Match (CTC)
Mode
88
ATmega8(L)
The waveform generator uses the COM1x1:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COM1x1:0 = 0 tells the waveform generator that no action on the
OC1x Register is to be performed on the next Compare Match. For compare output actions in
the non-PWM modes refer to
page
98.
A change of the COM1x1:0 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
FOC1x strobe bits.
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 (WGM13:0) and Compare Output
mode (COM1x1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM1x1:0 bits control whether the PWM out-
put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM1x1:0 bits control whether the output should be set, cleared or toggle at a Compare
Match.
For detailed timing information refer to
The simplest mode of operation is the Normal mode (WGM13:0 = 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 16-bit value (MAX = 0xFFFF) and then restarts from the
BOTTOM (0x0000). In normal operation the Timer/Counter Overflow Flag (TOV1) will be set in
the same timer clock cycle as the TCNT1 becomes zero. The TOV1 Flag in this case behaves
like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow
interrupt that automatically clears the TOV1 Flag, the timer resolution can be increased by soft-
ware. There are no special cases to consider in the Normal mode, a new counter value can be
written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum
interval between the external events must not exceed the resolution of the counter. If the interval
between events are too long, the timer overflow interrupt or the prescaler must be used to
extend the resolution for the capture unit.
The Output Compare units 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.
In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 Register
are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when
the counter value (TCNT1) matches either the OCR1A (WGM13:0 = 4) or the ICR1 (WGM13:0 =
12). The OCR1A or ICR1 define 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 oper-
ation of counting external events.
The timing diagram for the CTC mode is shown in
increases until a Compare Match occurs with either OCR1A or ICR1, and then counter (TCNT1)
is cleared.
97, and for phase correct and phase and frequency correct PWM refer to
See “Compare Match Output Unit” on page 87.
Table 36 on page
“Timer/Counter Timing Diagrams” on page
97. For fast PWM mode refer to
Figure
37. The counter value (TCNT1)
Table 38 on page
2486W–AVR–02/10
95.
Table 37 on