42
7679H–CAN–08/08
AT90CAN32/64/128
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Note:
1. The device is shipped with this option selected.
5.6.1
Oscillator Calibration Register – OSCCAL
• Bit 7 – Reserved Bit
This bit is reserved for future use.
• Bits 6..0 – CAL6..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the internal Oscillator to remove process vari-
ations 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 regis-
ter 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 calibrate to more than 10% above the nominal fre-
quency. 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
5.7
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in
. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Table 5-8.
Start-up times for the internal calibrated RC Oscillator clock selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V)
Recommended Usage
00
6 CK
14 CK
BOD enabled
01
6 CK
14 CK + 4.1 ms
Fast rising power
6 CK
14 CK + 65 ms
Slowly rising power
11
Reserved
Bit
7
6
5
4
3
2
1
0
–
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
OSCCAL
Read/Write
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
<----- -----------
Device Specific Calibration Value
----------- ----->
Table 5-9.
Internal RC Oscillator Frequency Range.
OSCCAL Value
Min Frequency in Percentage of
Nominal Frequency
Max Frequency in Percentage of
Nominal Frequency
0x00
50%
100%
0x3F
75%
150%
0x7F
100%
200%
