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AN1816

USING THE HC912B32 TO IMPLEMENT THE DISTRIBUTED SYSTEMS INTERFACE (DSI) PROTOCOL

Freescale Semiconductor 

Application Note

Freescale Semiconductor, Inc.

control message. The short word response message encoding is shown

in Figure 7 Short Word Response Message.

DATA

CRC

D7 D6 D5 D4 D3 D2 D1 D0 X3 X2 X1 X0

Figure 7 Short Word Response Message

Error Checking

Slave Device

Addressing

The message consists of one 8-bit data byte and the calculated 4-bit

CRC.

Long words are always sent in response to long word commands and

short words are always sent in response to short word commands. When

the word format changes between successive commands, the first

response sent during the new format will be invalid since it will not have

the proper number of bits.

The master and slaves calculate a CRC on the information portion of their

received messages. The message is valid only if the calculated CRC

matches the CRC sent as part of the message. An error bit is set in the

master when it receives an invalid message. The slaves discard and ignore

all invalid received messages and in addition do not respond to them.

Each slave device on the bus must be given a unique 4-bit address (from

1 (0001) to 15 (1111)) and assigned to one of four groups. Address 0

(0000) is used to address all 15 slave nodes at once.

Programmable

Devices

6

After system power up the master must set the address of all daisy chain

slave devices with programmable addresses before network

communications can commence. These devices have a bus switch on

the power/signal line. At power up the programmable device bus

switches are open and only close once an initialisation message has

been received.

With the first bus switch open, the bus only goes as far as the first slave.

When the master sends a slave initialisation command, the first slave

device stores its address information and closes its bus switch. The

second daisy chain slave is now connected to the network. When the

master initialises the second slave’s address, the first device responds

with an initialisation response message. The response message echoes

the programming information back to the master so that it knows that the

address was successfully established. Each slave sends the

initialisation response message only once after receiving a program

address command message.

AN1816

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