ATmega48/88/168
Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is
used, it is the divided system clock that is output.
8.10 Timer/Counter Oscillator
The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a exter-
nal clock source. The Timer/Counter Oscillator Pins (TOSC1 and TOSC2) are shared with
XTAL1 and XTAL2. This means that the Timer/Counter Oscillator can only be used when an
internal RC Oscillator is selected as system clock source. See Figure 8-2 on page 31 for crystal
connection.
Applying an external clock source to TOSC1 requires EXTCLK in the ASSR Register written to
logic one. See “Asynchronous Operation of Timer/Counter2” on page 152 for further description
on selecting external clock as input instead of a 32 kHz crystal.
8.11 System Clock Prescaler
The ATmega48/88/168 has a system clock prescaler, and the system clock can be divided by
setting the “CLKPR – Clock Prescale Register” on page 387. This feature can be used to
decrease the system clock frequency and the power consumption when the requirement for pro-
cessing 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. clkI/O, clkADC, clkCPU, and clkFLASH are
divided by a factor as shown in Table 28-3 on page 308.
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 corresponding to the previous setting, nor the clock frequency corre-
sponding 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 frequency. Hence, it
is not possible to determine the state of the prescaler - even if it were readable, and the exact
time it takes to switch from one clock division to the other cannot be exactly predicted. From the
time the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new
clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is the pre-
vious clock period, and T2 is the period corresponding to the new prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must befollowed to
change the CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bitsin
CLKPR to zero.
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
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2545M–AVR–09/07
ATMEL [ ATMEL ]
