AT90PWM2(2005) View Datasheet(PDF) - Atmel Corporation
Part Name
Description
Manufacturer
AT90PWM2
(Rev.:2005)
(Rev.:2005)
Atmel Corporation
AT90PWM2 Datasheet PDF : 365 Pages
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128 kHz Internal
Oscillator
External Clock
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz.
The frequency is nominal at 3V and 25°C. This clock is used by the Watchdog
Oscillator.
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 16. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Figure 16. External Clock Drive Configuration
NC
External
Clock
Signal
XTAL2
XTAL1
GND
Clock Output Buffer
Table 9. 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 10.
Table 10. Start-up Times for the External Clock Selection
SUT1..0
00
01
10
11
Start-up Time from Power-
down and Power-save
6 CK
6 CK
6 CK
Additional Delay from
Reset (VCC = 5.0V)
14CK
14CK + 4.1 ms
14CK + 65 ms
Reserved
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
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 37 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).
36 AT90PWM2/3
4317B–AVR–02/05
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