Datasheet

25
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
Main purpose of the delay is to keep the AVR
®
in reset until it is supplied with minimum V
CC
. The delay will not monitor the
actual voltage and it will be required to select a delay longer than the V
CC
rise time. If this is not possible, an internal or
external brown-out detection circuit should be used. A BOD circuit will ensure sufficient V
CC
before it releases the reset, and
the time-out delay can be disabled. Disabling the time-out delay without utilizing a brown-out detection circuit is not
recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An internal ripple
counter monitors the oscillator output clock, and keeps the internal reset active for a given number of clock cycles. The reset
is then released and the device will start to execute. The recommended oscillator start-up time is dependent on the clock
type, and varies from 6 cycles for an externally applied clock to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when the device starts up from
reset. When starting up from power-save or power-down mode, V
CC
is assumed to be at a sufficient level and only the
start-up time is included.
6.3 Low Power 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. Either a quartz crystal or a ceramic resonator may be used.
This crystal oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the lowest power
consumption, but is not capable of driving other clock inputs, and may be more susceptible to noise in noisy environments. In
these cases, refer to the Section 6.4 “Full Swing Crystal Oscillator” on page 26.
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 6-3. For ceramic resonators, the capacitor values
given by the manufacturer should be used.
Figure 6-2. Crystal Oscillator Connections
The low power oscillator can operate in three different modes, each optimized for a specific frequency range. The operating
mode is selected by the fuses CKSEL3..1 as shown in Table 6-3 on page 25.
Table 6-3. Low Power Crystal Oscillator Operating Modes
(3)
Frequency Range
(1)
(MHz) CKSEL3..1 Recommended Range for Capacitors C1 and C2 (pF)
0.4 - 0.9 100
(2)
–
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. This option should not be used with crystals, only with ceramic resonators.
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. It must be ensured that the resulting divided clock
meets the frequency specification of the device.
C2
XTAL2
XTAL1
GND
C1