ATTINY88_14 Datasheet, PDF (30/302 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 8K Bytes In-System

Figure 6-2. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
CLKI
GND
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-2.
Table 6-2.
SUT[1:0]
00
01
10
11
Start-up Times for the External Clock Selection
Power Conditions
Start-up Time
from Power-down
BOD enabled
6 CK
Fast rising power
6 CK
Slowly rising power
6 CK
Reserved
Additional Delay
from Reset (V_CC = 5.0V)
14CK
14CK + 4 ms
14CK + 64 ms
6.2.2
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from
one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is
required, ensure that the MCU is kept in Reset during the changes.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page
31 for details.
Calibrated Internal 8MHz Oscillator
By default, the Internal Oscillator provides an approximate 8.0 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 22-1
on page 208 for more details. The device is shipped with the CKDIV8 Fuse programmed. See
“System Clock Prescaler” on page 31 for more details.
When selected, this oscillator operates with no external components. During reset, hardware
loads the pre-programmed calibration value into the OSCCAL Register and thereby automati-
cally calibrates the oscillator. The accuracy of this calibration is shown as Factory calibration in
Table 22-1 on page 208.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 34, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 22-1 on page 208.
30 ATtiny48/88
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