clear
clkTn
top
Clear TCNT2 (set all bits to zero).
Timer/Counter clock, referred to as clkT2 in the following.
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
bottom
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clkT2). clkT2 can be generated from an external or internal
clock source, selected by the Clock Select bits (CS22:0). When no clock source is
selected (CS22:0 = 0) the timer is stopped. However, the TCNT2 value can be accessed
by the CPU, regardless of whether clkT2 is present or not. A CPU write overrides (has
priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM21 and WGM20 bits
located in the Timer/Counter Control Register (TCCR2A) and the WGM22 located in the
Timer/Counter Control Register B (TCCR2B). There are close connections between
how the counter behaves (counts) and how waveforms are generated on the Output
Compare outputs OC2A and OC2B. For more details about advanced counting
sequences and waveform generation, see “Modes of Operation” on page 137.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation
selected by the WGM22:0 bits. TOV2 can be used for generating a CPU interrupt.
Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A and OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator
signals a match. A match will set the Output Compare Flag (OCF2A or OCF2B) at the
next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare
Flag generates an Output Compare interrupt. The Output Compare Flag is automatically
cleared when the interrupt is executed. Alternatively, the Output Compare Flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Gener-
ator uses the match signal to generate an output according to operating mode set by the
WGM22:0 bits and Compare Output mode (COM2x1:0) bits. The max and bottom sig-
nals are used by the Waveform Generator for handling the special cases of the extreme
values in some modes of operation (“Modes of Operation” on page 137).
Figure 56 shows a block diagram of the Output Compare unit.
134
ATmega48/88/168
2545D–AVR–07/04
ATMEL [ ATMEL ]
