1FEATURES

23· Member of the Texas Instruments

Widebus+™ Family

· Pinout Optimizes DDR2 DIMM PCB Layout

· 1-to-2 Outputs Support Stacked DDR2 DIMMs

· One Device Per DIMM Required

· Chip-Select Inputs Gate the Data Outputs from Changing State and Minimizes System Power

Consumption

· Output Edge-Control Circuitry Minimizes

Switching Noise in an Unterminated Line

· Supports SSTL_18 Data Inputs

· Differential Clock (CLK and CLK) Inputs

· Supports LVCMOS Switching Levels on the

Chip-Select Gate-Enable, Control, and RESET

Inputs

· Checks Parity on DIMM-Independent Data

Inputs

· Supports industrial temperature range

(-40°C to 85°C)

· RESET Input Disables Differential Input

Receivers, Resets All Registers, and Forces

All Outputs Low, Except QERR

APPLICATIONS

· Heavily loaded DDR2 registered DIMM

DESCRIPTION

This 28-bit 1:2 configurable registered buffer is designed for 1.7-V to 1.9-V VCC operation. One device per DIMM

is required to drive up to 18 stacked SDRAM loads or two devices per DIMM are required to drive up to 36

stacked SDRAM loads.

All inputs are SSTL_18, except the chip-select gate-enable (CSGEN), control (C), and reset (RESET) inputs,

which are LVCMOS. All outputs are edge-controlled circuits optimized for unterminated DIMM loads, and meet

SSTL_18 specifications, except the open-drain error (QERR) output.

The 74SSTUB32868A operates from a differential clock (CLK and CLK). Data are registered at the crossing of

CLK going high and CLK going low.

The 74SSTUB32868A accepts a parity bit from the memory controller on the parity bit (PAR_IN) input, compares

it with the data received on the DIMM-independent D-inputs (D1−D5, D7, D9−D12, D17−D28 when C = 0; or

D1−D12, D17−D20, D22, D24−D28 when C = 1) and indicates whether a parity error has occurred on the

open-drain QERR pin (active low). The convention is even parity; that is, valid parity is defined as an even

number of ones across the DIMM-independent data inputs combined with the parity input bit. To calculate parity,

all DIMM-independent D-inputs must be tied to a known logic state.

The 74SSTUB32868A includes a parity checking function. Parity, which arrives one cycle after the data input to

which it applies, is checked on the PAR_IN input of the device. Two clock cycles after the data are registered,

the corresponding QERR signal is generated.