W149

Rev 1.0, November 21, 2006

Page 5 of 15

Serial Data Interface

The W149 features a two-pin, serial data interface that can be

used to configure internal register settings that control

particular device functions. Upon power-up, the W149

initializes with default register settings, therefore the use of this

serial data interface is optional. The serial interface is

write-only (to the clock chip) and is the dedicated function of

device pins SDATA and SCLOCK. In motherboard applica-

tions, SDATA and SCLOCK are typically driven by two logic

outputs of the chipset. Clock device register changes are

normally made upon system initialization, if any are required.

The interface can also be used during system operation for

power management functions. Table 3 summarizes the control

functions of the serial data interface.

Operation

Data is written to the W149 in eleven bytes of eight bits each.

Bytes are written in the order shown in Table 4.

Table 3. Serial Data Interface Control Functions Summary

Control Function

Description

Common Application

Clock Output Disable

Any individual clock output(s) can be disabled.

Disabled outputs are actively held LOW.

Unused outputs are disabled to reduce EMI

and system power. Examples are clock

outputs to unused PCI slots.

CPU Clock Frequency

Selection

Provides CPU/PCI frequency selections through

software. Frequency is changed in a smooth and

controlled fashion.

For alternate microprocessors and power

management options. Smooth frequency

transition allows CPU frequency change

under normal system operation.

Output Three-state

Puts clock output into a high-impedance state.

Production PCB testing.

(Reserved)

Reserved function for future device revision or

production device testing.

No user application. Register bit must be

written as 0.

Table 4. Byte Writing Sequence

Byte

Sequence

Byte Name

Bit Sequence

Byte Description

1

Slave Address

11010010

Commands the W149 to accept the bits in Data Bytes 0–6 for internal

register configuration. Since other devices may exist on the same

common serial data bus, it is necessary to have a specific slave

address for each potential receiver. The slave receiver address for the

W149 is 11010010. Register setting will not be made if the Slave

Address is not correct (or is for an alternate slave receiver).

2

Command Code

Don’t Care

Unused by the W149, therefore bit values are ignored (“Don’t Care”).

This byte must be included in the data write sequence to maintain

proper byte allocation. The Command Code Byte is part of the standard

serial communication protocol and may be used when writing to

another addressed slave receiver on the serial data bus.

3

Byte Count

Don’t Care

Unused by the W149, therefore bit values are ignored (“Don’t Care”).

This byte must be included in the data write sequence to maintain

proper byte allocation. The Byte Count Byte is part of the standard

serial communication protocol and may be used when writing to

another addressed slave receiver on the serial data bus.

4

Data Byte 0

Refer to Table 5

The data bits in Data Bytes 0–7 set internal W149 registers that control

device operation. The data bits are only accepted when the Address

Byte bit sequence is 11010010, as noted above. For description of bit

control functions, refer to Table 5, Data Byte Serial Configuration Map.

5

Data Byte 1

6

Data Byte 2

7

Data Byte 3

8

Data Byte 4

9

Data Byte 5

10

Data Byte 6

11

Data Byte 7