24
8126F–AVR–05/12
ATtiny13A
6.1.4
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.
6.2
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.
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
6.2.1
External Clock
To drive the device from an external clock source, CLKI should be driven as shown in
. To run the device on an external clock, the CKSEL fuses must be programmed to “00”.
Figure 6-2.
External Clock Drive Configuration
Table 6-1.
Device Clocking Options Select
Device Clocking Option
External Clock (see
00
Calibrated Internal 4.8/9.6 MHz Oscillator (see
)
01, 10
Internal 128 kHz Oscillator (see
)
11
Table 6-2.
Number of Watchdog Oscillator Cycles
Typ Time-out
Number of Cycles
4 ms
512
64 ms
8K (8,192)
EXTERNAL
CLOCK
S
IGNAL
CLKI
GND