Jameco Electronics Rabbit 3000 - Manual (Page 90)

User’s Manual 81
32.768 kHz Clock
The 32.768 kHz clock is primarily used to clock the on-chip real-time clock. In addition, it
is also used to support remote cold boot via Serial Port A, driving the 2400 baud commu-
nications used to initiate the cold boot. Another function of the 32.768 kHz oscillator is to
drive the low power sleepy mode with the main oscillator shut down to reduce power. The
32.768 kHz clock can be left out of a system provided that its functions are not required.
Figure 7-1. Clock Distribution
TN235, External 32.768 kHz Oscillator Circuits, provides further information on oscilla-
tor circuits and selecting the values of components to use in the oscillator circuit.
Spectrum
Spreader
Clock
Doubler
f/(8,6,4,2)
22 MW
R
s
330 kW
C1 C2
32.768 kHz
C
L
= 7 pF
R
p
C
in
U1A U2A
SN74AHC1GU04
NC7SP14
Divider
f/(1,2,4,8,16)
XTALB2
XTALB1
1 MW
2.5 kW
Watchdog
Timer
Real-Time
Clock
f/2
f/1
Rabbit 3000
CLK
clock out 2
113
114
49
Reference design for
32.768 kHz oscillator
Peripheral clock
Note: Peripherals
cannot be clocked
slower than processo r
Processor clock
enb enb enb
enb
R2
R1
R1 and R2 control the
power consumed by the
unbuffered inverter.
VBAT