ATmega162 datasheet(118/324 Pages) ATMEL

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2513K–AVR–07/09

ATmega162/V

Compare Output Mode

and Waveform

Generation

The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the

OCnx Register is to be performed on the next Compare Match. For Compare Output actions in

the non-PWM modes refer to Table 53 on page 128. For fast PWM mode refer to Table 54 on

page 129, and for phase correct and phase and frequency correct PWM refer to Table 55 on

page 129.

A change of the COMnx1:0 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

FOCnx strobe bits.

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 (WGMn3:0) and Compare Output

mode (COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COMnx1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COMnx1:0 bits control whether the output should be set, cleared or toggle at a Compare

Match (See “Compare Match Output Unit” on page 117.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 126.

Normal Mode

The simplest mode of operation is the Normal mode (WGMn3:0 = 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 (TOVn) will be set in

the same timer clock cycle as the TCNTn becomes zero. The TOVn 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 TOVn 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.

Clear Timer on

Compare Match (CTC)

Mode

In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register

are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when

the counter value (TCNTn) matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 =

12). The OCRnA or ICRn 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 Figure 51. The counter value (TCNTn)

increases until a Compare Match occurs with either OCRnA or ICRn, and then counter (TCNTn)

is cleared.

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