ATmega64A
14.10 Timer/Counter Prescaler
Figure 14-12. Prescaler for Timer/Counter0
clkOSC
clkT0S
10-BIT T/C PRESCALER
Clear
TOSC1
AS0
PSR0
0
CS00
CS01
CS02
TIMER/COUNTER0 CLOCK SOURCE
clkT0
The clock source for Timer/Counter0 is named clkT0S. clkT0S is by default connected to the main
system clock clkOSC. By setting the AS0 bit in ASSR, Timer/Counter0 is asynchronously clocked
from the TOSC1 pin. This enables use of Timer/Counter0 as a Real Time Counter (RTC). When
AS0 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can then be con-
nected between the TOSC1 and TOSC2 pins to serve as an independent clock source for
Timer/Counter0. The Oscillator is optimized for use with a 32.768 kHz crystal. Applying an exter-
nal clock source to TOSC1 is not recommended.
For Timer/Counter0, the possible prescaled selections are: clkT0S/8, clkT0S/32, clkT0S/64,
clkT0S/128, clkT0S/256, and clkT0S/1024. Additionally, clkT0S as well as 0 (stop) may be selected.
Setting the PSR0 bit in SFIOR resets the prescaler. This allows the user to operate with a pre-
dictable prescaler.
105
8160C–AVR–07/09
ATMEL [ ATMEL ]
