ATA8742 ATMEL Corporation, ATA8742 Datasheet - Page 42

• Manufacturer Part Number: ATA8742
• Manufacturer: ATMEL Corporation

14.2
14.3
14.4
42
Clock Sources
Default Clock Source
Crystal Oscillator
ATA8742
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 14-1.
Note:
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
commencing 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
14-2 on page
Table 14-2.
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is therefore the Internal RC Oscillator running at 8.0 MHz with longest
start-up time and an initial system clock prescaling of 8, resulting in 1.0 MHz system clock. This
default setting ensures that all users can make their desired clock source setting using an
In-System or High-voltage Programmer.
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can
be configured for use as an On-chip Oscillator, as shown in Figure 14-2. Either a quartz
crystal or a ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in
ues given by the manufacturer should be used.
Device Clocking Option
External Clock
Calibrated Internal RC Oscillator 8.0 MHz
Watchdog Oscillator 128 kHz
External Low-frequency Oscillator
External Crystal/Ceramic Resonator
Reserved
1. For all fuses “1” means unprogrammed while “0” means programmed.
42.
Typ Time-out
Device Clocking Options Select
Number of Watchdog Oscillator Cycles
64 ms
4 ms
Table 14-3 on page
(1)
43. For ceramic resonators, the capacitor val-
Number of Cycles
8K (8,192)
512
0101, 0111, 0011,0001
1000-1111
CKSEL3.0
9151A–INDCO–07/09
0000
0010
0100
0110
Table