1FEATURES
2· Member of the Texas Instruments
Widebus+ ™Family
· Pinout Optimizes DDR2 DIMM PCB Layout
· 1-to-2 Outputs Supports 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
· 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 SDRAM loads or two devices per DIMM are required to drive up to 36 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 74SSTUB32868 operates from a differential clock (CLK and CLK). Data are registered at the crossing of
CLK going high and CLK going low.
The 74SSTUB32868 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, i.e., 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 74SSTUB32868 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.