Datasheet

29
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
6.2.1 Default Clock Source ATmega16U4 and ATmega32U4
The device is shipped with Low Power Crystal Oscillator (8.0 - 16MHz) enabled and with the fuse CKDIV8
programmed, resulting in 1.0MHz system clock with an 8MHz crystal. See Table 28-5 on page 355 for an
overview of the default Clock Selection Fuse setting.
6.2.2 Default Clock Source ATmega16U4RC and ATmega32U4RC
The device is shipped with Calibrated Internal RC oscillator (8.0MHz) enabled and with the fuse CKDIV8
programmed, resulting in 1.0MHz system clock. See Table 28-5 on page 355 for an overview of the default
Clock Selection Fuse setting.
6.2.3 Clock Startup Sequence
Any clock source needs a sufficient V
CC
to start oscillating and a minimum number of oscillating cycles before it
can be considered stable.
To ensure sufficient V
CC
, the device issues an internal reset with a time-out delay (t
TOUT
) after the device reset is
released by all other reset sources. “On-chip Debug System” on page 46 describes the start conditions for the
internal reset. The delay (t
TOUT
) is timed from the Watchdog Oscillator and the number of cycles in the delay is
set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the following table. The frequency of
the Watchdog Oscillator is voltage dependent as shown in this table.
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 six 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.
Table 6-2. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
0ms 0ms 0
4.1ms 4.3ms 512
65ms 69ms 8K (8,192)