ATMEGA640 datasheet(172/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

20.4.1

Normal Mode

The simplest mode of operation is the Normal mode (WGM22:0 = 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 (TOV2) will be set in the same timer clock cycle as the TCNT2 becomes zero. The TOV2 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 TOV2 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.

20.4.2

Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM22:0 = 2), the OCR2A Register is used to manipulate the counter

resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT2) matches the OCR2A. The

OCR2A 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 20-3. The counter value (TCNT2) increases until a com-

pare match occurs between TCNT2 and OCR2A, and then counter (TCNT2) is cleared.

Figure 20-3. CTC Mode, Timing Diagram

An interrupt can be generated each time the counter value reaches the TOP value by using the OCF2A 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 OCR2A is lower

than the current value of TCNT2, 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.

For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical level on each

compare match by setting the Compare Output mode bits to toggle mode (COM2A1:0 = 1). The OC2A value will

not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated will have

a maximum frequency of fOC2A = fclk_I/O/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by

the following equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the counter counts

from MAX to 0x00.

TCNTn

OCnx

(Toggle)

OCnx Interrupt Flag Set

Period

(COMnx1:0 = 1)

fOCnx

fclk_I/O

2 N

1 OCRnx





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