ATMEGA261 View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATMEGA261' PDF : 226 Pages View PDF
ATtiny261/ATtiny461/ATtiny861
7.2 Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Table 7-1. Device Clocking Options Select(1) vs. PB4 and PB5 Functionality
Device Clocking Option
External Clock
PLL Clock
Calibrated Internal RC Oscillator 8.0 MHz
Watchdog Oscillator 128 kHz
External Low-Frequency Crystal
External Crystal/Ceramic Resonator (0.4 - 0.9 MHz)
External Crystal/Ceramic Resonator (0.4 - 0.9 MHz)
External Crystal/Ceramic Resonator (0.9 - 3.0 MHz)
External Crystal/Ceramic Resonator (0.9 - 3.0 MHz)
External Crystal/Ceramic Resonator (3.0 - 8.0 MHz)
External Crystal/Ceramic Resonator (3.0 - 8.0 MHz)
External Crystal/Ceramic Resonator (8.0 - 16.0 MHz)
External Crystal/Ceramic Resonator (8.0 - 16.0 MHz)
CKSEL3..0
0000
0001
0010
0011
01xx
1000
1001
1010
1011
1100
1101
1110
1111
PB4
XTAL1
I/O
I/O
I/O
XTAL1
XTAL1
XTAL1
XTAL1
XTAL1
XTAL1
XTAL1
XTAL1
XTAL1
PB5
I/O
I/O
I/O
I/O
XTAL2
XTAL2
XTAL2
XTAL2
XTAL2
XTAL2
XTAL2
XTAL2
XTAL2
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the CPU
wakes up from Power-down or Power-save, the selected clock source is used to time the
start-up, ensuring stable Oscillator operation before instruction execution starts. When the CPU
starts from reset, there is an additional delay allowing the power to reach a stable level before
commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of
the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table
7-2.
Table 7-2.
Number of Watchdog Oscillator Cycles
Typ Time-out
4 ms
64 ms
Number of Cycles
512
8K (8,192)
7.3 Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up
time and an initial system clock prescaling of 8. This default setting ensures that all users can
make their desired clock source setting using an In-System or High-voltage Programmer.
7.4 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure
7-3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
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7753E–AVR–06/10
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