
23
2543L–AVR–08/10
ATtiny2313
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, the selected clock source is used to time the start-up, ensuring sta-
ble 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 nor-
mal 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 3. The frequency
Default Clock
Source
The device is shipped with CKSEL = “0100”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is the Internal RC Oscillator 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 Parallel programmer.
Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
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
Table 4 on page 24. For ceramic resonators, the capacitor values
given by the manufacturer should be used.
Table 2. Device Clocking Select
(1) Device Clocking Option
CKSEL3..0
External Clock
0000
Calibrated Internal RC Oscillator 4MHz
0010
Calibrated internal RC Oscillator 8MHz
0100
Watchdog Oscillator 128kHz
0110
External Crystal/Ceramic Resonator
1000 - 1111
Reserved
0001/0011/0101/0111
Table 3. Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V)
Typ Time-out (VCC = 3.0V)
Number of Cycles
4.1 ms
4.3 ms
512
65 ms
69 ms
8K (8,192)