32
AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
Figure 6-4.
External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-9.
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
6.9
Clock Output Buffer
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 opera-
tion 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).
6.10
System Clock Prescaler
The AT90PWM216/316 system clock can be divided by setting the Clock Prescale Register – CLKPR. This feature
can be used to decrease power consumption when the requirement for processing power is low. This can be used
with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals.
clk
I/O, clkADC, clkCPU, and clkFLASH are divided by a factor as shown in Table 6-10. When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the
clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency correspond-
ing to the previous setting, nor the clock frequency corresponding to the new setting. The ripple counter that
implements the prescaler runs at the frequency of the undivided clock, which may be faster than the CPU's clock
Table 6-8.
External Clock Frequency
CKSEL3..0
Frequency Range
0000
0 - 16 MHz
Table 6-9.
Start-up Times for the External Clock Selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Recommended Usage
00
6 CK
14CK
BOD enabled
01
6 CK
14CK + 4.1 ms
Fast rising power
10
6 CK
14CK + 65 ms
Slowly rising power
11
Reserved
XTAL2
XTAL1
GND
NC
External
Clock
Signal