PCINT8 datasheet(41/294 Pages) ATMEL

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9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

4.4

System Clock Prescaler

4.4.1

Features

The Atmel

® AVR® system clock can be divided by setting the Clock Prescaler 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 4-10 on page 43.

4.4.2

Switching Time

When switching between prescaler settings, the System Clock Prescaler ensures that no

glitches occur in the clock system and 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 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, 2 active clock edges are produced. Here, T1

is the previous clock period, and T2 is the period corresponding to the new prescaler setting.

4.5

4.5.1

OSCCAL – Oscillator Calibration Register

• Bits 7:0 – CAL7:0: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to

remove process variations from the oscillator frequency. The factory-calibrated value is auto-

matically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at

25°C. The application software can write this register to change the oscillator frequency. The

oscillator can be calibrated to any frequency in the range 7.3 - 8.1MHz within ±2% accuracy.

Calibration outside that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write

times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to

more than 8.8MHz. Otherwise, the EEPROM or Flash write may fail.

The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the

lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-

quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher

frequency than OSCCAL = 0x80.

Bit

6543210

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

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