Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
26
The CKSEL0 fuse together with the SUT1..0 fuses select the start-up times as shown in Table 6-4.
6.4 Full Swing Crystal Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an
on-chip oscillator, as shown in Figure 6-2 on page 25. Either a quartz crystal or a ceramic resonator may be used.
This crystal oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving other clock
inputs and in noisy environments. The current consumption is higher than the Section 6.3 “Low Power Crystal Oscillator” on
page 25. Note that the full swing crystal oscillator will only operate for V
CC
= 2.7 to 5.5V.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends on the
crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment. Some initial
guidelines for choosing capacitors for use with crystals are given in Table 6-6 on page 27. For ceramic resonators, the
capacitor values given by the manufacturer should be used.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-5.
Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection
Oscillator Source / Power
Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) CKSEL0 SUT1..0
Ceramic resonator, fast rising
power
258CK 14CK + 4.1ms
(1)
0 00
Ceramic resonator, slowly rising
power
258CK 14CK + 65ms
(1)
0 01
Ceramic resonator, BOD enabled 1KCK 14CK
(2)
0 10
Ceramic resonator, fast rising
power
1KCK 14CK + 4.1ms
(2)
0 11
Ceramic resonator, slowly rising
power
1KCK 14CK + 65ms
(2)
1 00
Crystal oscillator, BOD enabled 16KCK 14CK 1 01
Crystal oscillator, fast rising power 16KCK 14CK + 4.1ms 1 10
Crystal oscillator, slowly rising
power
16KCK 14CK + 65ms 1 11
Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and
only if frequency stability at start-up is not important for the application. These options are not suitable for
crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up.
They can also be used with crystals when not operating close to the maximum frequency of the device, and if
frequency stability at start-up is not important for the application.
Table 6-5. Full Swing Crystal Oscillator operating modes
(2)
Frequency Range
(1)
(MHz) CKSEL3..1 Recommended Range for Capacitors C1 and C2 (pF)
0.4 - 20 011 12 - 22
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. 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. It must be ensured that the resulting divided clock
meets the frequency specification of the device.