Rainbow Electronics Atmega169L - Manual (Page 125)

125
ATmega169V/L
2514A–AVR–08/02
top Signalizes that TCNT2 has reached maximum value.
bottom Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
T2
). clk
T2
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 clk
T2
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). 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 128.
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.
Output Compare Unit 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 operat-
ing 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 special
cases of the extreme values in some modes of operation (“Modes of Operation” on page
128).
Figure 54 shows a block diagram of the Output Compare unit.
Figure 54. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
=
(8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMnX1:0
bottom