VT5366

Serial control bus

21/30

6.3.3

No data write followed by same 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. In this

example, the

Figure 12.

No data write followed by same location read

Note that the read message must be terminated with a

master. A positive acknowledge at this point would indicate that a multiple read was required

and the slave would put the first bit of the next byte onto the SDA line. If this was a 0 then the

SDA would be held low and the master would not be able to issue a STOP.

6.3.4

Multiple data read

Figure 13.

Multiple data read

This example assumes that a write message has already taken place. Note that the read

message is terminated with a negative acknowledge (A) from the master: it is not

guaranteed that the master will be able to issue a stop condition at any other time during a

read message. This is because if the data sent by the slave is all zeros, the

rise, which is part of the stop condition.

SA

Sr

A

A

P

20h

36

21h

15

A

No data write

Read data

from the master

NAck

S

21h

A

aa

A

85

A

P

Start

Device

Ack from slave

address

Data (@current index+1)

Stop

Ack

from the master

from the master

NAck

Data (@current index)