Rainbow-electronics ATmega161L Manual de usuario descargar pdf (Pagina 39)

ATmega161(L)

1228C–AVR–08/02

Figure 30. Timer/Counter2 Prescaler

The clock source for Timer/Counter2 prescaler is named PCK2. PCK2 is by default con-

nected to the main system clock (CK). By setting the AS2 bit in ASSR, Timer/Counter2

is asynchronously clocked from the PD4(TOSC1) pin. This enables use of

Timer/Counter2 as a Real-time Clock (RTC). When AS2 is set, pins PD4(TOSC1) and

PD5(TOSC2) are disconnected from Port D. A crystal can then be connected between

the PD4(TOSC1) and PD5(TOSC2) pins to serve as an independent clock source for

Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal. Alterna-

tively, an external clock signal can be applied to PD4(TOSC1). The frequency of this

clock must be lower than one fourth of the CPU clock and not higher than 256 kHz. Set-

ting the PSR2 bit in SFIOR Resets the prescaler. This allows the user to operate with a

predictable prescaler.

Special Function IO Register –

SFIOR

• Bits 7..2 – Res: Reserved Bits

These bits are reserved bits in the ATmega161 and always read as zero.

• Bit 1

– PSR2: Prescaler Reset Timer/Counter2

When this bit is set (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, however, the bit will remain as one until the prescaler has been reset. See “Asyn-

chronous Operation of Timer/Counter2” on page 48 for a detailed description of

asynchronous operation.

10-BIT T/C PRESCALER

TIMER/COUNTER2 CLOCK SOURCE

PCK2

TOSC1

AS2

CS20

CS21

CS22

PCK2/8

PCK2/64

PCK2/128

PCK2/1024

PCK2/256

PCK2/32

PSR2

Clear

TCK2

Bit 765432 1 0

$30 ($50) ––––––PSR2 PSR10 SFIOR

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

InitialValue000000 0 0

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 158 159

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Rainbow-electronics ATmega161L Manual de usuario Pagina 39

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