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.

 

 

Rabbit 3000

 

 

CLK

clock out

2

 

f/2

 

 

 

f/1

 

 

 

 

 

XTALB1

113

 

Processor clock

 

 

 

 

 

1 MW

enb

 

 

 

2.5 kW

114

enb

enb

 

XTALB2

enb

 

 

 

 

 

 

Spectrum

Clock

f/(8,6,4,2)

 

 

Spreader

Doubler

 

 

 

R1 and R2 control the

 

 

 

power consumed by the

 

 

 

unbuffered inverter.

 

 

 

 

VBAT

 

 

 

 

R1

 

 

Divider

 

 

 

 

f/(1,2,4,8,16)

 

 

 

SN74AHC1GU04

 

 

 

 

 

 

Cin

 

49

 

 

 

 

U1A

U2A

 

 

 

R2

 

NC7SP14

 

 

 

 

 

 

 

 

 

Rp

 

 

 

 

22 MW

Rs

 

 

 

 

 

 

 

Peripheral clock

CL

= 7 pF

330 kW

 

 

 

Watchdog

Real-Time

Note: Peripherals

 

 

 

32.768 kHz

 

Timer

Clock

cannot be clocked

 

 

 

slower than processor

C1

C2

 

 

 

 

Reference design for 32.768 kHz oscillator

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.

User’s Manual

81

Page 90
Image 90
Jameco Electronics 3000, 2000 manual Rabbit