Datasheet

8246A–AVR–11/09

ATtiny2313A/4313

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, 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.

6.2.1 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.

For low-voltage devices it should be noted that unprogramming the CKDIV8 fuse may result in

overclocking. At low voltages (below 2.7V) the devices are rated for maximum 4 MHz operation

(see Section 21.3 on page 196), but routing the clock signal from the internal oscillator directly to

the system clock line will run the device at 8 MHz.

6.2.2 External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

6-2. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Table 6-1. Device Clocking Select

Device Clocking Option CKSEL3..0

(1)

External Clock (see page 25) 0000

Calibrated Internal RC Oscillator 4 MHz (see page 26) 0010

Calibrated internal RC Oscillator 8 MHz (see page 26) 0100

128 kHz Internal Oscillator (see page 27)0110

External Crystal/Ceramic Resonator (see page 28) 1000 - 1111

Reserved 0001/0011/0101/0111