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| 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 IITM 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 densities 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 ultralow 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. 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 RCCTM 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's 3.3V Bus Switch in DIMM Switching 1 09/16/99 Application Brief AB-32 21098765432121098765432109876543210987654321210987654321098765432109876543212109876543210987654321098765432121098765432109876543210987654321 1A 1B 1B 12A 12B 12B SF Figure 2. PI3B16226 Block Diagram Pericom has recently introduced several bus switch products for DIMM switching in computer, networking and multi-media applications with configuration up to 32 bits. It should be noted that ultralow quiescent power (0.2A typically) makes these products ideally suited for notebook application. A detail of these products and their compatibility to standard chipsets is shown in Figure 3. Pericom BusSwitch PI3B16226 PI3B16234 PI5C16292 PI5C16225 PI5C16226 PI5C16234 Configuration 24:12 Mux/Demux 32:16 Mux/Demux 24:12 Mux/Demux 24:12 Mux/Demux 24:12 Mux/Demux 32:16 Mux/Demux Generic Chipset PC133 & RCC PC133 & RCC PC100 & 440BX PC133 & RCC PC133 & RCC PC133 & RCC TM TM Pin Count 40 56 Packages BQSOP TSSOP TSSOP TSSOP BQSOP TSSOP / 440GX TM 56 56 40 56 Figure 3. Pericom's DIMM Switching Bus Switch Selection Guide 2 09/16/99 |
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