1FEATURES

2· Member of the Texas Instruments Widebus+™

Family

· Pinout Optimizes DDR2 DIMM PCB Layout

· Configurable as 25-Bit 1:1 or 14-Bit 1:2

Registered Buffer

· 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

Control and RESET Inputs

· Supports Industrial Temperature Range

(-40°C to 85°C)

· RESET Input Disables Differential Input

Receivers, Resets All Registers, and Forces

All Outputs Low

DESCRIPTION

This 25-bit 1:1 or 14-bit 1:2 configurable registered buffer is designed for 1.7-V to 1.9-V VCC operation. In the

1:1 pinout configuration, only one device per DIMM is required to drive nine SDRAM loads. In the 1:2 pinout

configuration, two devices per DIMM are required to drive 18 SDRAM loads.

All inputs are SSTL_18, except the reset (RESET) and control (Cn) inputs, which are LVCMOS. All outputs are

edge-controlled circuits optimized for unterminated DIMM loads and meet SSTL_18 specifications.

The SN74SSTUB32864 operates from a differential clock (CLK and CLK). Data are registered at the crossing of

CLK going high and CLK going low.

The C0 input controls the pinout configuration of the 1:2 pinout from register-A configuration (when low) to

register-B configuration (when high). The C1 input controls the pinout configuration from 25-bit 1:1 (when low) to

14-bit 1:2 (when high). C0 and C1 should not be switched during normal operation. They should be hard-wired to

a valid low or high level to configure the register in the desired mode. In the 25-bit 1:1 pinout configuration, the

A6, D6, and H6 terminals are driven low and are do-not-use (DNU) pins.

In the DDR2 RDIMM application, RESET is specified to be completely asynchronous with respect to CLK and

CLK. Therefore, no timing relationship can be ensured between the two. When entering reset, the register is

cleared, and the data outputs are driven low quickly, relative to the time required to disable the differential input

receivers. However, when coming out of reset, the register becomes active quickly, relative to the time required

to enable the differential input receivers. As long as the data inputs are low, and the clock is stable during the

time from the low-to-high transition of RESET until the input receivers are fully enabled, the design of the

SN74SSTUB32864 ensures that the outputs remain low, thus ensuring there will be no glitches on the output.

To ensure defined outputs from the register before a stable clock has been supplied, RESET must be held in the

low state during power up.

The device supports low-power standby operation. When RESET is low, the differential input receivers are

disabled, and undriven (floating) data, clock, and reference voltage (VREF) inputs are allowed. In addition, when

RESET is low, all registers are reset and all outputs are forced low, except QERR. The LVCMOS RESET and Cn

inputs always must be held at a valid logic high or low level.