Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
28
6.5 Low Frequency Crystal Oscillator
The device can utilize a 32.768kHz watch crystal as clock source by a dedicated low frequency crystal Oscillator. The crystal
should be connected as shown in Figure 6-2 on page 25. When this oscillator is selected, start-up times are determined by
the SUT fuses and CKSEL0 as shown in Table 6-7.
6.6 Calibrated Internal RC Oscillator
The calibrated internal RC oscillator by default provides a 8.0MHz clock. The frequency is nominal value at 3V and 25°C.
The device is shipped with the CKDIV8 fuse programmed. See Section 6.11 “System Clock Prescaler” on page 31 for more
details. This clock may be selected as the system clock by programming the CKSEL fuses as shown in Table 6-8. If selected,
it will operate with no external components. During reset, hardware loads the calibration byte into the OSCCAL register and
thereby automatically calibrates the RC oscillator. At 3V and 25°C, this calibration gives a frequency of 8MHz ±1%. The
tolerance of the internal RC oscillator remains better than ±10% within the whole automotive temperature and voltage ranges
(2.7V to 5.5V, –40°C to +125°C). The oscillator can be calibrated to any frequency in the range 7.3 - 8.1MHz within ±1%
accuracy, by changing the OSCCAL register. When this Oscillator is used as the chip clock, the Watchdog Oscillator will still
be used for the watchdog timer and for the reset time-out. For more information on the pre-programmed calibration value,
see Section 25.4 “Calibration Byte” on page 245.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in Table 6-9.
Table 6-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1KCK 14CK
(1)
0 00
Fast rising power 1KCK 14CK + 4.1ms
(1)
0 01
Slowly rising power 1KCK 14CK + 65ms
(1)
0 10
Reserved 0 11
BOD enabled 32KCK 14CK 1 00
Fast rising power 32KCK 14CK + 4.1ms 1 01
Slowly rising power 32KCK 14CK + 65ms 1 10
Reserved 1 11
Note: 1. These options should only be used if frequency stability at start-up is not important for the application.
Table 6-8. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) CKSEL3..0
7.3 - 8.1 0010
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8MHz 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.
Table 6-9. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Power Conditions
Start-up Time from Power-down and
Power-save
Additional Delay from Reset
(V
CC
= 5.0V) SUT1..0
BOD enabled 6CK 14CK
(1)
00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(2)
10
Reserved 11
Notes: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1ms to ensure programming mode can be entered.
2. The device is shipped with this option selected.