ATTINY20 データシートの表示（PDF） - Atmel Corporation

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ATTINY20

8-bit AVR Microcontroller with 2K Bytes In-System Programmable Flash

Atmel Corporation 

6.2.4

Switching Clock Source

The main clock source can be switched at run-time using the “CLKMSR – Clock Main Settings Register” on page 20.

When switching between any clock sources, the clock system ensures that no glitch occurs in the main clock.

6.2.5

Default Clock Source

The calibrated internal 8 MHz oscillator is always selected as main clock when the device is powered up or has been

reset. The synchronous system clock is the main clock divided by 8, controlled by the System Clock Prescaler. The Clock

Prescaler Select Bits can be written later to change the system clock frequency. See “System Clock Prescaler”.

6.3 System Clock Prescaler

The system clock is derived from the main clock via the System Clock Prescaler. The system clock can be divided by

setting the “CLKPSR – Clock Prescale Register” on page 21. The system clock prescaler can be used to decrease power

consumption at times when requirements for processing power is low or to bring the system clock within limits of

maximum frequency. The prescaler can be used with all main clock source options, and it will affect the clock frequency

of the CPU and all synchronous peripherals.

The System Clock Prescaler can be used to implement run-time changes of the internal clock frequency while still

ensuring stable operation.

6.3.1

Switching Prescaler Setting

When switching between prescaler settings, the system clock prescaler ensures that no glitch occurs in the system clock

and that no intermediate frequency is higher than neither the clock frequency corresponding 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 main 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 another 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, two active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period

corresponding to the new prescaler setting.

6.4 Starting

6.4.1

Starting from Reset

The internal reset is immediately asserted when a reset source goes active. The internal reset is kept asserted until the

reset source is released and the start-up sequence is completed. The start-up sequence includes three steps, as follows.

1. The first step after the reset source has been released consists of the device counting the reset start-up time. The

purpose of this reset start-up time is to ensure that supply voltage has reached sufficient levels. The reset start-up

time is counted using the internal 128 kHz oscillator. See Table 6-1 for details of reset start-up time.

Note that the actual supply voltage is not monitored by the start-up logic. The device will count until the reset start-

up time has elapsed even if the device has reached sufficient supply voltage levels earlier.

2. The second step is to count the oscillator start-up time, which ensures that the calibrated internal oscillator has

reached a stable state before it is used by the other parts of the system. The calibrated internal oscillator needs to

oscillate for a minimum number of cycles before it can be considered stable. See Table 6-1 for details of the oscil-

lator start-up time.

3. The last step before releasing the internal reset is to load the calibration and the configuration values from the

Non-Volatile Memory to configure the device properly. The configuration time is listed in Table 6-1.

ATtiny20 [DATASHEET] 19

Atmel-8235F-AVR-ATtiny20-Datasheet_09/2014

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