ATMEGA48V_11 ATMEL [ATMEL Corporation], ATMEGA48V_11 Datasheet - Page 35

• Manufacturer Part Number: ATMEGA48V_11
• Description: 8-bit Atmel Microcontroller with 4/8/16K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

9.8
9.9
2545T–AVR–05/11
External clock
Clock output buffer
To drive the device from an external clock source, XTAL1 should be driven as shown in
9-4. To run the device on an external clock, the CKSEL fuses must be programmed to “0000”
(see
Table 9-12.
Figure 9-4.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table
Table 9-13.
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
36
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT
Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other cir-
cuits on the system. The clock also will be output during reset, and the normal operation of I/O
pin will be overridden when the fuse is programmed. Any clock source, including the internal RC
BOD enabled
Fast rising power
Slowly rising power
for details.
Power conditions
Table
9-13.
9-12).
Crystal oscillator clock frequency.
External clock drive configuration.
Start-up times for the external clock selection.
Frequency
0 - 20MHz
EXTERNAL
NC / PB7
power-down and power-save
SIGNAL
CLOCK
Start-up time from
Reserved
6CK
6CK
6CK
XTAL2
XTAL1
GND
Additional delay from
ATmega48/88/168
“System clock prescaler” on page
reset (V
14CK + 4.1ms
14CK + 65ms
14CK
CC
CKSEL3..0
= 5.0V)
0000
SUT1..0
01
Figure
00
10
11
35