Rainbow-electronics Atmega169L Bedienungsanleitung PDF herunterladen (Seite 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

). clk

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

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 B U S

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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