
40
2513L–AVR–03/2013
ATmega162/V
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure20. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 20. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
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. It is required to ensure that the
MCU is kept in reset during such changes in the clock frequency.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
Clock output
buffer
When the CKOUT Fuse is programmed, the system clock will be output on PortB 0. This mode is
suitable when chip clock is used to drive other circuits on the system. The clock will be output
also during Reset and the normal operation of PortB will be overridden when the fuse is pro-
Table 13. Internal RC Oscillator Frequency Range.
OSCCAL Value
Min Frequency in Percentage of
Nominal Frequency
Max Frequency in Percentage of
Nominal Frequency
0x00
50%
100%
0x3F
75%
150%
0x7F
100%
200%
Table 14. Start-up Times for the External Clock Selection
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset (VCC = 5.0V)
Recommended Usage
00
6 CK
–
BOD enabled
01
6 CK
4.1 ms
Fast rising power
10
6 CK
65 ms
Slowly rising power
11
Reserved
EXTERNAL
CLOCK
SIGNAL