Clock sources, Attiny26(l) – Rainbow Electronics ATtiny26L User Manual

Page 27

ATtiny26(L)

1477B–AVR–04/02

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as
shown below on Table 4. The clock from the selected source is input to the AVR clock
generator, and routed to the appropriate modules.The use of pins PB5 (XTAL2), and
PB4 (XTAL1) as I/O pins is limited depending on clock settings, as shown below in
Table 5.

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 stable oscillator operation before instruction execution starts. When the CPU
starts from Reset, there is as 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

Table 4. Device Clocking Options Select

Device Clocking Option

PLLCK

CKSEL3..0

External Crystal/Ceramic Resonator

1111 - 1010

External Low-frequency Crystal

1001

External RC Oscillator

1000 - 0101

Calibrated Internal RC Oscillator

0100 - 0001

External Clock

0000

PLL Clock

0001

Table 5. PB5, and PB4 Functionality vs. Device Clocking Options

(1)

Device Clocking Option

PLLCK

CKSEL [3:0]

PB4

PB5

External Clock

0000

XTAL1

I/O

Internal RC Oscillator

0001

I/O

Internal RC Oscillator

0010

I/O

Internal RC Oscillator

0011

I/O

Internal RC Oscillator

0100

I/O

External RC Oscillator

0101

XTAL1

I/O

External RC Oscillator

0110

XTAL1

I/O

External RC Oscillator

0111

XTAL1

I/O

External RC Oscillator

1000

XTAL1

I/O

External Low-frequency Oscillator

1001

XTAL1

XTAL2