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ATMEGA161 Scheda tecnica(PDF) 39 Page - ATMEL Corporation |
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ATMEGA161 Scheda tecnica(HTML) 39 Page - ATMEL Corporation |
39 / 159 page 39 ATmega161(L) 1228D–AVR–02/07 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 CK PCK2 TOSC1 AS2 CS20 CS21 CS22 0 PSR2 Clear TCK2 Bit 765 43 2 1 0 $30 ($50) ––– –– – PSR2 PSR10 SFIOR Read/Write RR RR RR R/W R/W Initial Value 0 0 0 0 0 0 0 0 |
Codice articolo simile - ATMEGA161_07 |
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Descrizione simile - ATMEGA161_07 |
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