CPC5903

8

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R02

3

Functional Description

3.1 Introduction

The CPC5903 combines the features of multiple logic

isolation barrier, and unidirectionally buffers the clock

from Side A to Side B. If different supply voltage levels

are used at each side, then the part, in conjunction

with its external pull-up resistors, will perform logic

either side. Due to the unidirectional nature of the

clock buffer it is required that the bus master be

connected to Side A of the CPC5903.

Configured with one bidirectional channel and one

unidirectional channel, the CPC5903 is ideal for

systems that do not implement clock stretching or

have bus masters on the Side B bus. This provides a

savings in supply current compared with using a dual

bidirectional isolator, but at the cost of losing the ability

to implement a Side B bus master or clock-stretching

in the future.

repeaters, the CPC5903 has a full-drive side (Side A)

and a limited-drive side (Side B).

On Side B, IOB is a voltage-limited output driver with a

internally set voltage limit prevents IOB from driving to

guarantees the bidirectional buffer cannot drive itself

into a latched logic low condition, which would cause

exhibits a proportionately larger self-drive margin with

IOA, the bidirectional buffer on Side A, is rated as a full

range with input thresholds specified as FAST-mode

compliant; thus the IOA output will drive the full 400pF

to a logic low.

3.2 Fast-mode Operation

Fast-mode operation of the CPC5903 bidirectional

interface on Side A is available over the full operational

range of the device. While Side B operation is

Standard-mode compliant over the full operational

range of the device, it is Fast-mode speed capable

whenever the bus loading is limited to 200pF. Full

Fast-mode compatible operation of the Side B bus is

3.3 Logic Input Thresholds and Output Levels

Because Side A is Fast-mode compliant, it’s inputs

IOA and IA have logic threshold levels and frequency

interface devices. Additionally, the output capability of

IOA is Fast-mode compliant over the entire operational

range.

The output levels of OB and IOB are compatible with

voltage-limited. The input logic low threshold level of

and lower than it’s own output logic low level. This

eliminates the possibility of the IOB output driver

acquiring an IOB input logic low, which would result in

a latched logic low state.

Because Side B of the CPC5903 utilizes a modified

logic low threshold level, only one such device is

allowed on the Side B bus. Side A has no such

restriction as this side of the CPC5903 uses traditional

logic thresholds. This allows for cascaded isolation by

connecting the Side B of one CPC5903 to Side A of

the next.

correctly accept the Side B driven data, thereby

enabling Side B bidirectional communication.

3.4 Pull-Up Resistor Selection

Pull-up resistors are required on both sides of the

barrier. Selecting the value of the pull-up resistors is