PRELIMINARY
C9926
Low EMI Clock Generator for Intel 133MHz/ 2DIMM Chipset Systems
2-Wire SMBus Control Interface
The 2-wire control interface implements a read/write slave only interface according to SMBus specification. (See Fig. 7
below). The device can 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 SCLK 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 an 8-bit address. The LSB address Byte = 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. Data is transferred MSB first at a max rate of 100kbits/S. This device
will also respond to a D3 address which sets it in a read mode. It will not respond to any other control interface
conditions, and previously set control registers are retained.
Transmit
ACK
ACK
ACK
ACK
ACK
Receive
COMMAND BYTE
(Don't Care)
BYTE COUNT
(Don't Care)
BYTE 0
(Valid)
BYTE N
(Valid)
1
1
0
1
0
0
1
0
SDATA
MSB
LSB
8
8
8
8
SCLK
START CONDITION
STOP CONDITION
Fig.7a (WRITE)
Transmit
ACK BYTE COUNT
BYTE 0
(Valid)
BYTE1
(Valid)
BYTE N
(Valid)
Receiv
ACK
ACK
ACK
ACK
1
1
0
1
0
1
0
1
(Valid)
SDATA
MSB
LSB
8
8
8
8
SCLK
START CONDITION
STOP CONDITION
Fig.7b (READ)
Figure 7
SMBus Communications Waveforms
Cypress Semiconductor Corporation
525 Los Coches St.
Document#: 38-07070 Rev. **
5/4/2001
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress
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