Rainbow Electronics ATtiny2313 - Manual (Page 25)

25
ATtiny2313
2543A–AVR–08/03
Notes: 1. These options should only be used when not operating close to the maximum fre-
quency of the device, and only if frequency stability at start-up is not important for the
application. These options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure fre-
quency stability at start-up. They can also be used with crystals when not operating
close to the maximum frequency of the device, and if frequency stability at start-up is
not important for the application.
Calibrated Internal RC
Oscillator The calibrated internal RC Oscillator provides a fixed 8.0 MHz clock. The frequency is
nominal value at 3V and 25
°
C. If 8 MHz frequency exceeds the specification of the
device (depends on V
CC
), the CKDIV8 Fuse must be programmed in order to divide the
internal frequency by 8 during start-up. The device is shipped with the CKDIV8 Fuse
programmed. This clock may be selected as the system clock by programming the
CKSEL Fuses as shown in Table 6. If selected, it will operate with no external compo-
nents. During reset, hardware loads the calibration byte into the OSCCAL Register and
thereby automatically calibrates the RC Oscillator. At 3V and 25
°
C, this calibration gives
a frequency within ± 1% of the nominal frequency. When this Oscillator is used as the
chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the
Reset Time-out. For more information on the pre-programmed calibration value, see the
section “Calibration Byte” on page 162.
Note: 1. The device is shipped with this option selected.
Table 5. Start-up Times for the Crystal Oscillator Clock Selection
CKSEL0 SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
0 00 258 CK (1) 14CK + 4.1 ms Ceramic resonator, fast
rising power
0 01 258 CK (1) 14CK + 65 ms Ceramic resonator,
slowly rising power
0 10 1K CK (2) 14CK Ceramic resonator,
BOD enabled
0 11 1K CK (2) 14CK + 4.1 ms Ceramic resonator, fast
rising power
1 00 1K CK (2) 14CK + 65 ms Ceramic resonator,
slowly rising power
1
01 16K CK 14CK Crystal Oscillator, BOD
enabled
1
10 16K CK 14CK + 4.1 ms Crystal Oscillator, fast
rising power
1
11 16K CK 14CK + 65 ms Crystal Oscillator,
slowly rising power
Table 6. Internal Calibrated RC Oscillator Operating Modes (1)
CKSEL3..0 Nominal Frequency
0010 - 0011 4.0 MHz
0100 - 0101 8.0 MHz