SCBS705H − AUGUST 1997 − REVISED MAY 2004
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
The ’LVTH646 devices consist of bus transceiver circuits, D-type flip-flops, and control circuitry arranged for
multiplexed transmission of data directly from the input bus or from the internal registers. Data on the A or B
bus is clocked into the registers on the low-to-high transition of the appropriate clock (CLKAB or CLKBA) input.
Figure 1 illustrates the four fundamental bus-management functions that can be performed with the ’LVTH646.
Output-enable (OE
) and direction-control (DIR) inputs are provided to control the transceiver functions. In the
transceiver mode, data present at the high-impedance port can be stored in either register or in both.
The select-control (SAB and SBA) inputs can multiplex stored and real-time (transparent mode) data. The
direction control (DIR) determines which bus receives data when OE is low. In the isolation mode (OE high),
A data can be stored in one register and/or B data can be stored in the other register.
When an output function is disabled, the input function still is enabled and can be used to store and transmit
data. Only one of the two buses, A or B, can be driven at a time.
Active bus-hold circuitry is provided to hold unused or floating data inputs at a valid logic level. Use of pullup
or pulldown resistors with the bus-hold circuitry is not recommended.
To ensure the high-impedance state during power up or power down, OE
should be tied to V
CC
through a pullup
resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver.
These devices are fully specified for hot-insertion applications using I
off
and power-up 3-state. The I
off
circuitry
disables the outputs, preventing damaging current backflow through the devices when they are powered down.
The power-up 3-state circuitry places the outputs in the high-impedance state during power up and power down,
which prevents driver conflict.
FUNCTION TABLE
INPUTS
DATA I/Os
OE DIR CLKAB CLKBA SAB SBA A1−A8 B1−B8
X X ↑ X X X Input Unspecified
†
Store A, B unspecified
†
X XX ↑ X X Unspecified
†
Input Store B, A unspecified
†
H X ↑ ↑ X X Input Input Store A and B data
H X H or L H or L X X Input disabled Input disabled Isolation, hold storage
L L X X X L Output Input Real-time B data to A bus
L L X H or L X H Output Input Stored B data to A bus
L H X X L X Input Output Real-time A data to B bus
L H H or L X H X Input Output Stored A data to B bus
†
The data-output functions can be enabled or disabled by various signals at OE
and DIR. Data-input functions always are enabled; i.e., data at
the bus terminals is stored on every low-to-high transition of the clock inputs.
