MEGA128CAN Datasheet, PDF (40/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

Figure 22. External Clock Drive Configuration
NC
External
Clock
Signal
XTAL2
XTAL1
GND
Clock Output Buffer
Timer/Counter2
Oscillator
Table 14. External Clock Frequency
CKSEL3..0
Frequency Range
0000
0 - 16 MHz
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 15.
Table 15. Start-up Times for the External Clock Selection
SUT1..0
00
Start-up Time from Power-
down and Power-save
6 CK
Additional Delay from
Reset (V_CC = 5.0V)
14CK
Recommended Usage
BOD enabled
01
6 CK
14CK + 4.1 ms
Fast rising power
10
6 CK
14CK + 65 ms
Slowly rising power
11
Reserved
When applying an external clock, it is required to avoid sudden changes in the applied
clock frequency 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 frequency while still ensuring stable operation. Refer to “System Clock
Prescaler” on page 41 for details.
When the CKOUT Fuse is programmed, the system Clock will be output on CLKO. 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 I/O pin will be overridden
when the fuse is programmed. Any clock source, including internal RC Oscillator, can be
selected when CLKO serves as clock output. If the System Clock Prescaler is used, it is
the divided system clock that is output (CKOUT Fuse programmed).
For AVR microcontrollers with Timer/Counter2 Oscillator pins (TOSC1 and TOSC2), the
crystal is connected directly between the pins. The Oscillator is optimized for use with a
32.768 kHz watch crystal. 12-22 pF capacitors may be necessary if the parasitic imped-
ance (pads, wires & PCB) is very low.
40 AT90CAN128
4250E–CAN–12/04