ATA8743_14 Datasheet, PDF (38/215 Pages) ATMEL Corporation – Microcontroller with UHF ASK/FSK Transmitter

14.1.4 ADC Clock – clk_ADC
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.
14.2
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 14-1. Device Clocking Options Select⁽¹⁾
Note:
Device Clocking Option
External clock
Calibrated internal RC oscillator 8.0MHz
Watchdog oscillator 128kHz
External low-frequency oscillator
External crystal/ceramic resonator
Reserved
1. For all fuses “1” means unprogrammed while “0” means programmed.
CKSEL3..0
0000
0010
0100
0110
1000-1111
0101, 0111, 0011,0001
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 Table 14-2.
Table 14-2. Number of Watchdog Oscillator Cycles
Typ Time-out
4ms
64ms
Number of Cycles
512
8K (8,192)
14.3
Default Clock Source
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 8.0MHz with longest start-up time and an initial system clock prescaling of 8,
resulting in 1.0MHz system clock. This default setting ensures that all users can make their desired clock source setting
using an in-system or high-voltage programmer.
38 ATA8743 [DATASHEET]
9152D–INDCO–09/14