W196
PRELIMINARY
4
Serial Data Interface
The W196 features a two-pin, serial data interface that can be
used to configure internal register settings that control partic-
ular device functions. Upon power-up, the W196 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 applications, SDATA and
SCLOCK are typically driven by two logic outputs of the
chipset. Clock device register changes are normally made
upon system initialization, if required. The interface can also
be used during system operation for power management func-
tions. Table 2 summarizes the control functions of the serial
data interface.
Operation
Data is written to the W196 in ten bytes of eight bits each.
Bytes are written in the order shown in Table 3.
Table 2. 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 un-
used PCI slots.
CPU Clock Frequency
Selection
Provides CPU/PCI frequency selections beyond
the selections that are provided by the FS0:1 pins.
Frequency is changed in a smooth and controlled
fashion.
For alternate microprocessors and power man-
agement options. Smooth frequency transition al-
lows CPU frequency change under normal system
operation.
Output Three-state
Puts all clock outputs into a high-impedance state. Production PCB testing.
Test Mode
All clock outputs toggle in relation to X1 input, in-
ternal PLL is bypassed. Refer to Table 4.
Production PCB testing.
(Reserved)
Reserved function for future device revision or pro-
duction device testing.
No user application. Register bit must be written
as 0.
Table 3. Byte Writing Sequence
Byte
Sequence
Byte Name
Bit Sequence
Byte Description
1
Slave Address
11010010
Commands the W196 to accept the bits in Data Bytes 3–6 for internal
register configuration. Since other devices may exist on the same com-
mon serial data bus, it is necessary to have a specific slave address for
each potential receiver. The slave receiver address for the W196 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 W196, 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 com-
munication 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 W196, 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 communi-
cation protocol and may be used when writing to another addressed slave
receiver on the serial data bus.
4
Data Byte 0
Don’t Care
Refer to Cypress SDRAM drivers.
5
Data Byte 1
6
Data Byte 2
7
Data Byte 3
Refer to Table 4
The data bits in these bytes set internal W196 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 4, Data Byte Serial Configuration Map.
8
Data Byte 4
9
Data Byte 5
10
Data Byte 6
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