Data Sheet

User manual Rev. 6 — 4 April 2014 14 of 64

NXP Semiconductors

UM10204

C-bus specification and user manual

Possible data transfer formats are:

• Master-transmitter transmits to slave-receiver. The transfer direction is not changed

(see Figure 11

). The slave receiver acknowledges each byte.

• Master reads slave immediately after first byte (see Figure 12). At the moment of the

first acknowledge, the master-transmitter becomes a master-receiver and the

slave-receiver becomes a slave-transmitter. This first acknowledge is still generated

by the slave. The master generates subsequent acknowledges. The STOP condition

is generated by the master, which sends a not-acknowledge (A

) just before the STOP

condition.

• Combined format (see Figure 13). During a change of direction within a transfer, the

START condition and the slave address are both repeated, but with the R/W

bit

reversed. If a master-receiver sends a repeated START condition, it sends a

not-acknowledge (A

) just before the repeated START condition.

Notes:

1. Combined formats can be used, for example, to control a serial memory. The internal

memory location must be written during the first data byte. After the START condition

and slave address is repeated, data can be transferred.

2. All decisions on auto-increment or decrement of previously accessed memory

locations, etc., are taken by the designer of the device.

3. Each byte is followed by an acknowledgment bit as indicated by the A or A

blocks in

the sequence.

4. I

C-bus compatible devices must reset their bus logic on receipt of a START or

repeated START condition such that they all anticipate the sending of a slave

address, even if these START conditions are not positioned according to the proper

format.

5. A START condition immediately followed by a STOP condition (void message) is an

illegal format. Many devices however are designed to operate properly under this

condition.

6. Each device connected to the bus is addressable by a unique address. Normally a

simple master/slave relationship exists, but it is possible to have multiple identical

slaves that can receive and respond simultaneously, for example in a group

broadcast. This technique works best when using bus switching devices like the

PCA9546A where all four channels are on and identical devices are configured at the

same time, understanding that it is impossible to determine that each slave

acknowledges, and then turn on one channel at a time to read back each individual

device’s configuration to confirm the programming. Refer to individual component

data sheets.