7.1.4
Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter and the LCD controller
to be clocked directly from an external clock or an external 32 kHz clock crystal. The dedicated
clock domain allows using this Timer/Counter as a real-time counter even when the device is in
sleep mode. It also allows the LCD controller output to continue while the rest of the device is in
sleep mode.
7.1.5
ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
7.2
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Table 7-1.
Device Clocking Options Select(1)
Device Clocking Option
External Crystal/Ceramic Resonator
External Low-frequency Crystal
Calibrated Internal RC Oscillator
External Clock
CKSEL3:0
1111 - 1000
0111 - 0110
0010
0000
Reserved
0011, 0001, 0101, 0100
Note:
1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the start-
up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts
from reset, there is an additional delay allowing the power to reach a stable level before com-
mencing normal operation. The Watchdog Oscillator is used for timing this real-time part of the
start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table 7-
2. The frequency of the Watchdog Oscillator is voltage dependent as shown in ”Typical Charac-
teristics” on page 335.
Table 7-2.
Typ Time-out (VCC = 5.0V)
4.1 ms
65 ms
Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 3.0V)
4.3 ms
69 ms
Number of Cycles
4K (4,096)
64K (65,536)
30
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
