ATMEGA88 datasheet(31/349 Pages) ATMEL

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ATmega48/88/168

2545D–AVR–07/04

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 cali-

brate to more than 8.8 MHz. 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 frequency ranges are overlapping, in other words a setting of OSCCAL = 0x7F

gives a higher frequency than OSCCAL = 0x80.

The CAL6..0 bits are used to tune the frequency within the selected range. A setting of

0x00 gives the lowest frequency in that range, and a setting of 0x7F gives the highest

frequency in the range. Incrementing CAL6..0 by 1 will give a frequency increment of

less than 2% in the frequency range 7.3 - 8.1 MHz.

128 kHz Internal

Oscillator

The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz.

The frequency is nominal at 3V and 25

°C. This clock may be select as the system clock

by programming the CKSEL Fuses to “11” as shown in Table 13.

Note:

1. The frequency is preliminary value. Actual value is TBD.

When this clock source is selected, start-up times are determined by the SUT Fuses as

shown in Table 14.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

External Clock

The device can utilize a external clock source as shown in Figure 15. To run the device

on an external clock, the CKSEL Fuses must be programmed as shown in Table 15.

Notes:

1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz frequency exceeds the specification of the device (depends on V

CC), the

CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. It

must be ensured that the resulting divided clock meets the frequency specification of

the device.

Table 13. 128 kHz Internal Oscillator Operating Modes

Nominal Frequency

CKSEL3..0

128 kHz

0011

Table 14. Start-up Times for the 128 kHz Internal Oscillator

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset

SUT1..0

BOD enabled

6 CK

14CK(1)

Fast rising power

6 CK

14CK + 4 ms

Slowly rising power

6 CK

14CK + 64 ms

Reserved

Table 15. Full Swing Crystal Oscillator operating modes

(2)

Frequency Range(1) (MHz)

CKSEL3..0

Recommended Range for Capacitors

C1 and C2 (pF)

0 - 100

0000

12 - 22

ATMEGA88 Datasheet(PDF) 31 Page - ATMEL Corporation

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