Data Sheet

User manual Rev. 6 — 4 April 2014 18 of 64

NXP Semiconductors

UM10204

C-bus specification and user manual

There are two cases to consider:

• When the least significant bit B is a ‘zero’.

• When the least significant bit B is a ‘one’.

When bit B is a ‘zero’, the second byte has the following definition:

• 0000 0110 (06h): Reset and write programmable part of slave address by

hardware. On receiving this 2-byte sequence, all devices designed to respond to the

general call address reset and take in the programmable part of their address.

Precautions must be taken to ensure that a device is not pulling down the SDA or SCL

line after applying the supply voltage, since these low levels would block the bus.

• 0000 0100 (04h): Write programmable part of slave address by hardware.

Behaves as above, but the device does not reset.

• 0000 0000 (00h): This code is not allowed to be used as the second byte.

Sequences of programming procedure are published in the appropriate device data

sheets. The remaining codes have not been fixed and devices must ignore them.

When bit B is a ‘one’, the 2-byte sequence is a ‘hardware general call’. This means that

the sequence is transmitted by a hardware master device, such as a keyboard scanner,

which can be programmed to transmit a desired slave address. Since a hardware master

does not know in advance to which device the message has to be transferred, it can only

generate this hardware general call and its own address — identifying itself to the system

(see Figure 17

The seven bits remaining in the second byte contain the address of the hardware master.

This address is recognized by an intelligent device (for example, a microcontroller)

connected to the bus which then accepts the information from the hardware master. If the

hardware master can also act as a slave, the slave address is identical to the master

address.

Fig 16. General call address format

Fig 17. Data transfer from a hardware master-transmitter

mbc623

LSB

second byte

0 0 0 0 0 0 0 0 A X X X X X X X BA

first byte

(general call address)

mbc624

general

call address

(B)

A A

second

byte

A A

(n bytes + ack.)

S 00000000 MASTER ADDRESS 1 PDATA DATA