LTC4331 datasheet(13/22 Pages) AD

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LT3960

Rev. B

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Data Transmission Detail

The timing diagram in Figure 8 shows how a byte of data

is sent by the I2C master and acknowledged by the I2C

slave in the single-master single-slave system described

in Figure 7. The I2C master issues a start command to

initiate a communication frame. The LT3960 connected to

the master drives the CANSCL and CANSDA buses dom-

inant in response to the change in state on the SCL and

SDA pins without interpretation or delay. The LT3960 con-

nected to the I2C slave receives the dominant signals on

the CANSCL and CANSDA buses and drives the slave SCL

and SDA pins dominant without interpretation or delay.

The result on the slave I2C bus is an I2C Start command

nearly identical to that generated by the master, delayed by

propagation delays of the master LT3960, twisted pairs,

and slave LT3960.

As additional clock and data edges are written by the I2C

master, they too are recreated, first on the CANSCL and

CANSDA buses and then on the slave I2C bus. Once the

entire byte of data is written on the slave I2C bus, the I2C

slave device issues an ACK, pulling down SDA to acknowl-

edge receipt of a valid byte. The slave LT3960 recognizes

that a slave I2C device is driving the SDA line dominant

and switches from receiving to transmitting to drive the

CANSDA bus dominant. The master LT3960 then receives

the ACK on the CANSDA bus and pulls the master SDA

low, communicating the ACK to the master.

Note that clock data is always transmitted from master to

slave, but the LT3960 dynamically switches the direction

of SDA communication based primarily on the time of

arrival of dominant signals on its inputs.

Bidirectional Arbitration of SDA

The LT3960 facilitates bidirectional SDA communication

between master and slave I2C devices by dynamically

controlling the direction of traffic between SDA and the

CANSDA bus. The primary factor determining the direction

of communication is the time of arrival of dominant sig-

nals on SDA and CANSDA. The first of SDA and CANSDA

to be asserted dominant by an external device will cause

the LT3960 to drive the other dominant, establishing the

direction of communication until it is released and returns

to a recessive state. The transmitter which opposes the

established direction of communication will be blocked

until it can be safely re-enabled without locking up a bus

or misinterpreting the direction of communication. To

fully describe the method of arbitration, communication

in each direction is described in detail below.

In the default state, SDA and CANSDA are in a recessive

state and no direction of communication is set. If SDA is

asserted dominant (low) by an external I2C device from

a default state, the LT3960 will drive CANSDA dominant

and the LT3960’s receiver on CANSDA is blocked from

driving SDA. When the SDA line is eventually released by

the external I2C device and returns to a recessive state,

the LT3960 stops driving the CANSDA bus dominant.

After allowing the CANSDA bus sufficient time to return

to a passive state as required by the CAN physical layer

specifications, the LT3960 reopens the possibility of bidi-

rectional traffic and waits for a dominant signal on SDA or

CANSDA to once again set a direction for communication.

If, from the default state, CANSDA is asserted dominant

by another LT3960 on the bus while SDA remains reces-

sive (high), the LT3960 will drive SDA dominant (low)

and the CAN transmitter is blocked from driving CANSDA

based on its input while CANSDA is held dominant. When

the CANSDA bus returns to a recessive state, the LT3960

stops driving the other dominant. When it is safe to do so

without causing glitches or latch up, the LT3960 reopens

the possibility of bidirectional traffic and waits for a dom-

inant signal on SDA or CANSDA to once again set a direc-

tion for communication.

OPERATION

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