PRELIMINARY
B9852
High Performance Pentium III Clock Buffer
2-Wire SMBUS Control Interface
The 2-wire control interface implements a write slave only interface according to SMBUS specification. (See fig5,
page6). The device can be read back by using standard SMBUS command bytes. Sub addressing is not supported,
thus all preceding bytes must be sent in order to change one of the control bytes. The 2-wire control interface allows
each clock output to be individually enabled or disabled. 100 Kbits/second (standard mode) data transfer is supported.
During normal data transfer, the SDATA signal only changes when the SCLK signal is low, and is stable when SCLK is
high. There are two exceptions to this. A high to low transition on SDATA while SDCLK is high is used to indicate the
start of a data transfer cycle. A low to high transition on SDATA while SCLK is high indicates the end of a data transfer
cycle. Data is always sent as complete 8-bit bytes, after which an acknowledge is generated. The first byte of a
transfer cycle is a 7-bit address with a Read/Write bit (R/W#) as the LSB. R/W# = 0 in write mode.
The device will respond to writes to 10 bytes (max) of data to address D2 by generating the acknowledge (low) signal
on the SDATA wire following reception of each byte. The device will not respond to any other control interface
conditions, and previously set control registers are retained.
Serial Control Registers
NOTE: The Pin# column lists the affected pin number where applicable. The “@ Power Up” (@Pup) column gives the
state at true power up. Bytes are set to the values shown only on true power up.
The serial bits will be read by the clock driver in the following order:
Byte 0 – Bits 7, 6, 5, 4, 3, 2, 1, 0
Byte 1 – Bits 7, 6, 5, 4, 3, 2, 1, 0
Byte N – Bits 7, 6, 5, 4, 3, 2, 1, 0
All unused register bits (reserved and N/A) will be designated “don’t care”. They must be sent and will be
acknowledged
Cypress Semiconductor Corporation
525 Los Coches St.
Document#: 38-07076 Rev. **
5/9/2001
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
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