ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 29

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Oscillator Calibration Register
– OSCCAL
External Clock
2570A–AVR–09/04
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 10 on page 29.
Table 10. Start-up times for the internal calibrated RC Oscillator clock selection
Note:
• 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 automatically 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 - 8.1 MHz
within ±1% 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 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.
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 14. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Bit
Read/Write
Initial Value
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
2.
14CK + 4.1 ms to ensure programming mode can be entered.
The device is shipped with this option selected.
CAL7
R/W
7
CAL6
R/W
6
Start-up Time from Power-
down and Power-save
CAL5
R/W
5
Device Specific Calibration Value
Reserved
6 CK
6 CK
6 CK
ATmega325/3250/645/6450
CAL4
R/W
4
CAL3
R/W
3
Additional Delay from
CAL2
R/W
Reset (V
2
14CK + 65 ms
14CK + 4.1 ms
14CK
CAL1
R/W
CC
1
(1)
= 5.0V)
(2)
CAL0
R/W
0
SUT1..0
OSCCAL
00
01
10
11
29