Datasheet
SCBS698G − JULY 1997 − REVISED MAY 2004
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
These devices can be used as two 8-bit transceivers or one 16-bit transceiver. 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 ’LVTH16646
devices.
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 may 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 circuitry
used for select control eliminates the typical decoding glitch that occurs in a multiplexer during the transition
between stored and real-time data. 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 holds unused or undriven inputs at a valid logic state. 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/O
OPERATION OR FUNCTION
OE DIR CLKAB CLKBA SAB SBA A1−A8 B1−B8
OPERATION OR FUNCTION
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 bus
†
The data-output functions can be enabled or disabled by various signals at OE
or 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.