Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 104
maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the
Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The
TOV0 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 TOV0 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.
15.7.2 Clear Timer on Compare Match (CTC) mode
In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the
counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the
OCR0A. The OCR0A 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 15-5. The counter value (TCNT0) increases until a
Compare Match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 15-5. CTC mode, timing diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A 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
OCR0A is lower than the current value of TCNT0, 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 OC0A output can be set to toggle its logical level on each
Compare Match by setting the Compare Output mode bits to toggle mode (COM0A1:0 = 1). The OC0A 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
OC0
= f
clk_I/O
/2 when OCR0A is set to zero (0x00). The waveform frequency is
defined by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x00.
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
1 4
Period
2 3
(COMnx1:0 = 1)
f
OCnx
f
clk_I/O
2 N 1 OCRnx+
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