Interface

VT5376

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4.3

Message interpretation

All serial interface communications with the sensor must begin with a start condition. If the

start condition is followed by a valid address byte then further communications can take

place. The sensor will acknowledge the receipt of a valid address by driving the sda wire

low. The state of the read/~write bit (lsb of the address byte) is stored and the next byte of

data, sampled from sda, can be interpreted.

During a write sequence the second byte received is an address index and is used to point

to one of the internal registers. The serial interface will automatically increment the index

address by one location after each slave acknowledge. The master can therefore send data

bytes continuously to the slave until the slave fails to provide an acknowledge or the master

terminates the write communication with a stop condition or sends a repeated start, (Sr).

As data is received by the slave it is written bit by bit to a serial/parallel register. After each

data byte has been received by the slave, an acknowledge is generated, the data is then

stored in the internal register addressed by the current index.

During a read message, the content of the addressed register is then parallel loaded into the

serial/parallel register and clocked out of the device by scl.

At the end of each byte, in both read and write message sequences, an acknowledge is

issued by the receiving device. A message can only be terminated by the bus master, either

by issuing a stop condition, a repeated start condition or by a negative acknowledge (NAck)

after reading a complete byte during a read operation.

4.4

Type of messages

Single location, single data write

When a random value is written to the sensor, the message will look like this:

Figure 5.

Single location, single write

The r/w bit is set to zero for writing. The write message is terminated with a stop condition

from the master.

Single location read

When a location is to be read, but the value of the stored index is not known, a write

message with no data byte must be written first, specifying the index. The read message

then completes the message sequence. To avoid relinquishing the serial to bus to another

master a repeated start condition is asserted between the write and read messages.

S

A

A

A

P

Start

Device

Ack

address

Index

Data

Stop