User`s manual

14 rabbit.com Core Design and Components

Figure 3-1 Main Oscillator Circuit

The 32.768 kHz oscillator is slow to start oscillating after power-on. For this reason, a wait loop in the

BIOS waits until this oscillator is oscillating regularly before continuing the startup procedure. The startup

delay may be as much as 5 seconds, but will usually be about 200 ms. Crystals with low series resistance

(R < 35 k

) will start faster.

For more information on the 32.768 kHz oscillator please see Technical Note 235, “External 32.768 kHz

Oscillator Circuits.” This document is available on our website: www.rabbit.com.

3.2 Floating Inputs

Floating inputs or inputs that are not solidly either high or low can draw current because both N and P

FETs can turn on at the same time. To avoid excessive power consumption, floating inputs should not be

included in a design (except that some inputs may float briefly during power-on sequencing). Most unused

inputs on the Rabbit 4000 can be made into outputs by proper software initialization to remove the floating

property. Pull-up resistors will be needed on a few inputs that cannot be programmed as outputs. An alter-

native to a pull-up resistor is to tie an unused output to the unused inputs. If pull-up (or pull-down) resis-

tors are required, they should be made as large as possible if the circuit in question has a substantial part of

its duty cycle with current flowing through the resistor.

VBAT

33pF

29.49 MHz

NC7SZU04

FDV303N

CLKI

CLKIEN