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APPLICATION INFORMATION

SMBus Data Interface

Data Protocol

Slave Receiver Address (7 bits)

CDC906

SCAS828 – SEPTEMBER 2006

To enhance the flexibility and function of the clock synthesizer, a two-signal serial interface is provided. It follows

SMBus specification can be found at http://http://www.smbus.org.

Through the SMBus, various device functions, such as individual clock output buffers, can be individually

enabled or disabled. The registers associated with the SMBus data interface initialize to their default setting

upon power-up; therefore, using this interface is optional. The clock device register changes are normally made

upon system initialization, if any are required.

The clock driver serial protocol accepts Byte Write, Byte Read, Block Write, and Block Read operations from the

controller.

For Block Write/Read operations, the bytes must be accessed in sequential order from lowest to highest byte

(most significant bit first) with the ability to stop after any complete byte has been transferred. For Byte Write and

Byte Read operations, the system controller can access individually addressed bytes.

Once a byte has been sent, it is written into the internal register and effective immediately. With the rising edge

of the ACK bit, this applies to each transferred byte, independent of whether this is a Byte Write or a Block Write

sequence.

The offset of the indexed byte is encoded in the command code, as described in Table 1.

The Block Write and Block Read protocol is outlined in Figure 9 and Figure 10, while Figure 7 and Figure 8

outlines the corresponding Byte Write and Byte Read protocol.

A6

A5

A4

A3

A2

A1

A0

R/W

1

1

0

1

0

0

1

0

Table 1. Command Code Definition

Bit

Description

0 = Block Read or Block Write operation

7

1 = Byte Read or Byte Write operation

Byte Offset for Byte Read and Byte Write operation.

(6:0)

For Block Read and Block Write operation, these bits have to be 000 0000.

11

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