Datasheet

29
7799D–AVR–11/10
ATmega8U2/16U2/32U2
8.3 Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
8.3.1 Default Clock Source
The device is shipped with internal RC oscillator at 8.0 MHz and with the fuse CKDIV8 pro-
grammed, resulting in 1.0 MHz system clock. The startup time is set to maximum and time-out
period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that
all users can make their desired clock source setting using any available programming interface.
8.3.2 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 45
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 Table 8-2. The frequency of the Watchdog Oscillator is voltage
dependent as shown in “Typical Characteristics” on page 273.
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum Vcc. The
delay will not monitor the actual voltage and it will be required to select a delay longer than the
Vcc rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be
used. A BOD circuit will ensure sufficient Vcc 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.
Table 8-1. Device Clocking Options Select
(1)
Device Clocking Option CKSEL3:0
Low Power Crystal Oscillator 1111 - 1000
Full Swing Crystal Oscillator 0111 - 0110
Reserved 0101 - 0100
Reserved 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001
Table 8-2. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
0 ms 0 ms 0
4.1 ms 4.3 ms 512
65 ms 69 ms 8K (8,192)