Rainbow Electronics ATmega128L - Manual (Page 39)

39
ATmega128(L)
2467B–09/01
External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 20. To run the device on an external clock, the CKSEL fuses must be pro-
grammed to “0000”. By programming the CKOPT fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND.
Figure 20. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT fuses as
shown in
Table 16.
Timer/Counter Oscillator For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the
crystal is connected directly between the pins. No external capacitors are needed. The
oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external
clock source to TOSC1 is not recommended.
XTAL Divide Control Register
– XDIV The XTAL Divide Control Register is used to divide the Source clock frequency by a
number in the range 2 - 129. This feature can be used to decrease power consumption
when the requirement for processing power is low.
Table 15. Internal RC Oscillator Frequency Range.
OSCCAL Value
Min Frequency in Percentage of
Nominal Frequency (%)
Max Frequency in Percentage of
Nominal Frequency (%)
$00 50 100
$7F 75 150
$FF 100 200
Table 16. Start-up Times for the External Clock Selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4 ms Fast rising power
10 6 CK 64 ms Slowly rising power
11 Reserved
EXTERNAL
CLOCK
SIGNAL
Bit 7 6 5 4 3 2 1 0
XDIVEN XDIV6 XDIV5 XDIV4 XDIV3 XDIV2 XDIV1 XDIV0 XDIV
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0