STPC4EDBI datasheet(80/93 Pages) STMICROELECTRONICS

80 / 93 page

DESIGN GUIDELINES

80/93

Release 1.5 - January 29, 2002

The DIMM sockets should be populated starting

with the furthest DIMM from the STPC device first

(DIMM1). There are two types of DIMM devices;

single-row and dual-row. The dual-row devices

require two chip select signals to select between

the two rows. A STPC device with 4 chip select

control lines could control either 4 single-row

DIMMs or 2 dual-row DIMMs. When only 2 chip

select control lines are activated, only two single-

row DIMMs or one dual-row DIMM can be

controlled.

When using DIMM modules, schematics have to

be done carefully in order to avoid data buses

completely crossing on the board. This has to be

checked at the library level. In order to achieve the

layout shown in Figure 6-21, schematics have to

implement the crossing described in Figure 6-22.

The DQM signals must be exchanged using the

same order.

6.4.3.4. Summary

For unbuffered DIMMs the address/control signals

will be the most critical for timing. The simulations

show that for these signals the best way to drive

them is to use a parallel termination. For

applications where speed is not so critical series

termination can be used as this will save power.

Using a low impedance such as 50

Ω for these

critical traces is recommended as it both reduces

the delay and the overshoot.

The other memory interface signals will typically

be not as critical as the address/control signals.

Using lower impedance traces is also beneficial

for the other signals but if their timing is not as

critical as the address/control signals they could

use the default value. Using a lower impedance

implies using wider traces which may have an

impact on the routing of the board.

The layout of this interface can be validated by an

electrical

simulation

using

the

IBIS

model

available on the STPC web site.

Figure 6-21. Optimum Data Bus Layout for DIMM

Figure 6-22. Schematics for Optimum Data Bus Layout for DIMM

DIMM

STPC

SDRAM I/F

D[15:00]

D[31:16]

D[47:32]

D[63:48]

MD[31:00]

MD[63:32]

MD[15:00],DQM[1:0]

MD[31:16],DQM[3:2]

MD[47:32],DQM[5:4]

MD[63:48],DQM[7:6]

D[15:00],DQM[1:0]

D[31:16],DQM[3:2]

D[47:32],DQM[5:4]

D[63:48],DQM[7:6]

DIMM

STPC

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