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ATMEGA163 Datasheet(PDF) 40 Page - ATMEL Corporation |
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ATMEGA163 Datasheet(HTML) 40 Page - ATMEL Corporation |
40 / 187 page 40 ATmega163(L) 1142E–AVR–02/03 Figure 31. Prescaler for Timer/Counter2 The clock source for Timer/Counter2 is named PCK2. PCK2 is by default connected to the main system clock CK. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchro- nously clocked from the PC6(TOSC1) pin. This enables use of Timer/Counter2 as a Real Time Clock (RTC). When AS2 is set, pins PC6(TOSC1) and PC7(TOSC2) are dis- connected from Port C. A crystal can then be connected between the PC6(TOSC1) and PC7(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. Setting the PSR2 bit in SFIOR resets the pres- caler. This allows the user to operate with a predictable prescaler. 8-bit Timer/Counter0 Figure 32 shows the block diagram for Timer/Counter0. The 8-bit Timer/Counter0 can select clock source from CK, prescaled CK, or an external pin. In addition it can be stopped as described in “Timer/Counter0 Control Register – TCCR0” on page 41. The overflow Status Flag is found in “The Timer/Counter Interrupt Flag Register – TIFR” on page 32. Control signals are found in the Timer/Counter0 Con- trol Register – TCCR0. The interrupt enable/disable settings for Timer/Counter0 are found in “The Timer/Counter Interrupt Mask Register – TIMSK” on page 31. When Timer/Counter0 is externally clocked, the external signal is synchronized with the Oscillator frequency of the CPU. To assure proper sampling of the external clock, the minimum time between two external clock transitions must be at least one internal CPU clock period. The external clock signal is sampled on the rising edge of the internal CPU clock. The 8-bit Timer/Counter0 features both a high resolution and a high accuracy usage with the lower prescaling opportunities. Similarly, the high prescaling opportunities make the Timer/Counter0 useful for lower speed functions or exact timing functions with infre- quent actions. 10-BIT T/C PRESCALER TIMER/COUNTER2 CLOCK SOURCE CK PCK2 TOSC1 AS2 CS20 CS21 CS22 0 PSR2 Clear TCK2 |
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