ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 36

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

9.11
36
System Clock Prescaler
Atmel ATmega48PA/88PA/168PA [Preliminary]
The Atmel ATmega48PA/88PA/168PA has a system clock prescaler, and the system clock
can be divided by setting the
can be used to decrease the system clock frequency and the 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
clk
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
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 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 previous 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 be followed
to change the CLKPS bits:
Interrupts must be disabled when changing prescaler setting to make sure the write procedure
is not interrupted.
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
CPU
CLKPR to zero.
, and clk
FLASH
are divided by a factor as shown in
“CLKPR – Clock Prescale Register” on page
Table 29-5 on page
318.
377. This feature
9223B–AVR–09/11
I/O
, clk
ADC
,