ATmega128 datasheet(151/386 Pages) ATMEL

151 / 386 page

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2467S–AVR–07/09

ATmega128

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 (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 out-

put 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 (WGM21:0 = 2), the OCR2 Register is used to manip-

ulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value

(TCNT2) matches the OCR2. The OCR2 defines the top value for the counter, hence also its

resolution. This mode allows greater control of the compare match output frequency. It also sim-

plifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 65. The counter value (TCNT2)

increases until a compare match occurs between TCNT2 and OCR2 and then counter (TCNT2)

is cleared.

Figure 65. CTC Mode, Timing Diagram

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

OCF2 flag. 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 run-

ning 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 OCR2 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 OC2 output can be set to toggle its logical

level on each compare match by setting the compare output mode bits to toggle mode

(COM21:0 = 1). The OC2 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 f

OC2 = fclk_I/O/2

when OCR2 is set to zero (0x00). The waveform frequency is defined by the following equation:

TCNTn

OCn

(Toggle)

OCn Interrupt Flag Set

Period

(COMn1:0 = 1)

fOCn

clk_I/O

OCRn

()

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