Rainbow-electronics ATmega64L Instrukcja Użytkownika Pobierz pdf (Strona 127)

127

ATmega16(L)

2466B–09/01

Timer/Counter Prescaler Figure 64. Prescaler for Timer/Counter2

The clock source for Timer/Counter2 is named clk

T2S

. clk

T2S

is by default connected to

the main system I/O clock clk

. By setting the AS2 bit in ASSR, Timer/Counter2 is asyn-

chronously clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real

Time Counter (RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from

Port C. A crystal can then be connected between the TOSC1 and TOSC2 pins to serve

as an independent clock source for Timer/Counter2. The oscillator is optimized for use

with a 32.768 kHz crystal. Applying an external clock source to TOSC1 is not

recommended.

For Timer/Counter2, the possible prescaled selections are: clk

T2S

/8, clk

T2S

/32, clk

T2S

/64,

clk

T2S

/128, clk

T2S

/256, and clk

T2S

/1024. Additionally, clk

T2S

as well as 0 (stop) may be

selected. Setting the PSR2 bit in SFIOR resets the prescaler. This allows the user to

operate with a predictable prescaler.

Special Function IO Register –

SFIOR

• Bit 1 - PSR2: Prescaler Reset Timer/Counter2

When this bit is written to one, the Timer/Counter2 prescaler will be reset. The bit will be

cleared by hardware after the operation is performed. Writing a zero to this bit will have

no effect. This bit will always be read as zero if Timer/Counter2 is clocked by the internal

CPU clock. If this bit is written when Timer/Counter2 is operating in asynchronous

mode, the bit will remain one until the prescaler has been reset.

10-BIT T/C PRESCALER

TIMER/COUNTER2 CLOCK SOURCE

clk

I/O

clk

T2S

TOSC1

AS2

CS20

CS21

CS22

clk

T2S

clk

T2S

/64

clk

T2S

/128

clk

T2S

/1024

clk

T2S

/256

clk

T2S

/32

PSR2

Clear

clk

Bit 7 6 5 4 3 2 1 0

ADTS2 ADTS1 ADTS0 ADHSM ACME PUD PSR2 PSR10 SFIOR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

1 2 ... 122 123 124 125 126 127 128 129 130 131 132 ... 297 298

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