Rainbow Electronics ATtiny2313 - Manual (Page 23)

23
ATtiny2313
2543A–AVR–08/03
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 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. The frequency of the Watchdog Oscillator is voltage
dependent as shown in “ATtiny2313 Typical Characteristics – Preliminary Data” on page
181.
Default Clock Source The device is shipped with CKSEL = “0010”, 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. This default setting ensures that all users can
make their desired clock source setting using an In-System or Parallel programmer.
Table 2. Device Clocking Select (1)
Device Clocking Option CKSEL3..0
External Clock 0000 - 0001
Calibrated Internal RC Oscillator 4MHz 0010 - 0011
Calibrated internal RC Oscillator 8MHz 0100 - 0101
Watchdog Oscillator 128kHz 0110 - 0111
External Crystal/Ceramic Resonator 1000 - 1111
Table 3. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
4.1 ms 4.3 ms 4K (4,096)
65 ms 69 ms 64K (65,536)