Datasheet

22
ATtiny13
2535E–AVR–10/04
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 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 3.
Default Clock Source The device is shipped with CKSEL = “10”, SUT = “10”, and CKDIV8 programmed. The
default clock source setting is therefore the Internal RC Oscillator running at 9.6 MHz
with longest start-up time and an initial system clock prescaling of 8. This default setting
ensures that all users can make their desired clock source setting using an In-System or
High-voltage Programmer.
Table 2. Device Clocking Options Select
(1)
Device Clocking Option CKSEL1..0
Calibrated Internal RC Oscillator 01, 10
External Clock 00
128 kHz Internal Oscillator 11
Table 3. Number of Watchdog Oscillator Cycles
Typ Time-out Number of Cycles
4 ms 512
64 ms 8K (8,192)