ATMEGA169 ATMEL [ATMEL Corporation], ATMEGA169 Datasheet - Page 28

• Manufacturer Part Number: ATMEGA169
• Description: 8-bit Microcontroller with 16K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Oscillator Calibration Register
– OSCCAL
External Clock
28
ATmega169/V
• Bits 6..0 – CAL6..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the internal Oscillator to remove pro-
cess variations from the Oscillator frequency. This is done automatically during Chip
Reset. When OSCCAL is zero, the lowest available frequency is chosen. Writing non-
zero values to this register will increase the frequency of the internal Oscillator. Writing
0x7F to the register gives the highest available frequency. The calibrated Oscillator is
used to time EEPROM and Flash access. If EEPROM or Flash is written, do not cali-
brate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash
write may fail. Note that the Oscillator is intended for calibration to 8.0 MHz. Tuning to
other values is not guaranteed, as indicated in Table 10.
Table 10. Internal RC Oscillator Frequency Range.
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 13. To run the device on an external clock, the CKSEL Fuses must be pro-
grammed to “0000”.
Figure 13. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 12.
Table 11. Crystal Oscillator Clock Frequency
Bit
Read/Write
Initial Value
OSCCAL Value
CKSEL3..0
0000
0x00
0x3F
0x7F
R
7
–
Min Frequency in Percentage of
EXTERNAL
CAL6
R/W
6
SIGNAL
CLOCK
Nominal Frequency
NC
CAL5
R/W
100%
5
50%
75%
Device Specific Calibration Value
Frequency Range
0 - 16 MHz
CAL4
R/W
4
CAL3
R/W
3
XTAL2
XTAL1
GND
Max Frequency in Percentage of
CAL2
R/W
2
Nominal Frequency
CAL1
R/W
1
100%
150%
200%
CAL0
R/W
0
2514P–AVR–07/06
OSCCAL