Application Brief AB-32
21098765432121098765432109876543210987654321210987654321098765432109876543212109876543210987654321098765432121098765432109876543210987654321
21098765432121098765432109876543210987654321210987654321098765432109876543212109876543210987654321098765432121098765432109876543210987654321
321098765432121098765432109876543210987654321210987654321098765432109876543212109876543210987654321098765432121098765432109876543210987654321
321098765432121098765432109876543210987654321210987654321098765432109876543212109876543210987654321098765432121098765432109876543210987654321
Application Brief AB32
3.3V Bus Switches for PC100 PC133
DIMM Switching
TM
/
TM
by Refugio Jones August 18, 1999
Highlights
In high-end PC and server applications, the Pentium II
TM
processor
memory bus runs at bus speeds of 100 or 133 MHz. Here, bus
switches are used to switch 100/133 MHz SDRAM modules in den-
sities of 128 MBytes and higher.
Pericom鈥檚 3.3V DIMM bus switch series is designed with near zero
delay, low ON Resistance (five ohms), low-capacitance, and ultra-
low quiescent power.
For reliability, the unused module data pins should be connected to
ground. Figure 1 shows the application of Pericom鈥檚 PI3B16226 bus
switches in a Intel Pentium II memory system with bus speeds of
133 MHz and 133 MHz SDRAM DIMMs.
To prevent overloading, a bus switch is required where more than
three SDRAM DIMMs are connected to the bus. The block
diagram shows that the DIMM modules are connected to the
B ports of the bus switch.
The PI3B16226 (see Figure 2), has weak pull-down resistors on
B ports connected to ground. The B port gets enabled when the
switch is disabled. This ensures non-floating connections and
reliable DIMM operation.
Furthermore, PC133 applications require low ON Capacitance to
reduce bus loading under 133 MHz tight timing requirements. The
PI3B16226 provides 9pF of ON Capacitance.
Address Line
RCC
TM
Chipset
Data Line, 72 bits
(133MHz)
Control Signals
Control Signal
A Ports
(B0)
133MHz
SDRAM Module
Bus Switch
24:12 - PI3B16226
32:16 - PI3B16234
B Ports
(B1)
133MHz
SDRAM Module
Intel
TM
Pentium II
Processor
Data Line, 72 bits
(B2)
133MHz
SDRAM Module
Data Line, 72 bits
(B3)
133MHz
SDRAM Module
Figure 1. Using Pericom鈥檚 3.3V Bus Switch in DIMM Switching
1
09/16/99
1
2
