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09/16/99

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Application Brief AB-32

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DIMM Switching

by Refugio Jones August 18, 1999

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’s 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’s PI3B16226 bus

switches in a Intel Pentium II memory system with bus speeds of

133 MHz and 133 MHz SDRAM DIMMs.

Figure 1. Using Pericom’s 3.3V Bus Switch in DIMM Switching

Application Brief AB32

Highlights

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.

(B0)

133MHz

SDRAM Module

Intel

Pentium II

Processor

(B1)

133MHz

SDRAM Module

(B2)

133MHz

SDRAM Module

(B3)

133MHz

SDRAM Module

TM

RCC

Chipset

A Ports

(133MHz)

Address Line

Data Line, 72 bits

Control Signals

Control Signal

Data Line, 72 bits

Data Line, 72 bits

B Ports

Bus Switch

24:12 - PI3B16226

32:16 - PI3B16234

TM