ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 126

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Output Compare Unit
126
ATmega325/3250/645/6450
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
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 clk
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). There are close connections
between how the counter behaves (counts) and how waveforms are generated on the
Output Compare output OC2A. For more details about advanced counting sequences
and waveform generation, see “Modes of Operation” on page 129.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation
selected by the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A). Whenever TCNT2 equals OCR2A, the comparator signals a match. A match
will set the Output Compare Flag (OCF2A) at the next timer clock cycle. If enabled
(OCIE2A = 1), the Output Compare Flag generates an Output Compare interrupt. The
OCF2A Flag is automatically cleared when the interrupt is executed. Alternatively, the
OCF2A Flag can be cleared by software by writing a logical one to its I/O bit location.
The Waveform Generator uses the match signal to generate an output according to
operating mode set by the WGM21:0 bits and Compare Output mode (COM2A1:0) bits.
The max and bottom signals are used by the Waveform Generator for handling the spe-
cial cases of the extreme values in some modes of operation (“Modes of Operation” on
page 129).
Figure 55 shows a block diagram of the Output Compare unit.
Figure 55. Output Compare Unit, Block Diagram
top
bottom
bottom
FOCn
top
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
OCRnx
T2
Waveform Generator
WGMn1:0
). clk
=
(8-bit Comparator )
DATA BUS
T2
T2
is present or not. A CPU write overrides (has
can be generated from an external or internal
COMnX1:0
TCNTn
OCFnx (Int.Req.)
OCnx
2570A–AVR–09/04