Rainbow Electronics ATmega64M1 - Manual (Page 33)

33
8209A–AVR–08/09
ATmega16M1/32M1/64M1
Figure 9-3. PCK Clocking System
9.7 128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3V and 25
°
C. This clock is used by the Watchdog Oscillator.
9.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
9-4 . To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 9-4. External Clock Drive Configuration
8 MHz RC OSCILLATOR
OSCCAL
XTAL1
XTAL2
OSCILLATORS
DIVIDE
BY 8
DIVIDE
BY 2
CK
PLL
64x
PLLE
Lock
Detector
PLOCK
SOURCE
PLLF
DIVIDE
BY 4
CLK
PLL
Table 9-8. External Clock Frequency
CKSEL3..0 Frequency Range
0000 0 - 16 MHz
XTAL2
XTAL1
GND
NC
External
Clock
Signal