ATMEGA640 datasheet(150/449 Pages) ATMEL

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ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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 Over-

flow 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 software. 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 maxi-

mum 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

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 operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 53. The counter value

(TCNTn) increases until a compare match occurs with either OCRnA or ICRn, and then

counter (TCNTn) is cleared.

Figure 53. CTC Mode, Timing Diagram

An interrupt can be generated at each time the counter value reaches the TOP value by

either using the OCFnA or ICFn Flag according to the register used to define the TOP

value. If the interrupt is enabled, the interrupt handler routine can be used for updating

the TOP value. However, changing the 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

OCRnA or ICRn is lower than the current value of TCNTn, the counter will miss the com-

pare match. The counter will then have to count to its maximum value (0xFFFF) and

wrap around starting at 0x0000 before the compare match can occur. In many cases

this feature is not desirable. An alternative will then be to use the fast PWM mode using

TCNTn

OCnA

(Toggle)

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

Period

(COMnA1:0 = 1)

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