Datasheet

 
      
  
SCBS706F − AUGUST 1997 − REVISED OCTOBER 2003
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
The ’LVTH652 devices consist of bus-transceiver circuits, D-type flip-flops, and control circuitry arranged for
multiplexed transmission of data directly from the data bus or from the internal storage registers.
Output-enable (OEAB and OEBA
) inputs are provided to control the transceiver functions. Select-control (SAB
and SBA) inputs are provided to select whether real-time or stored data is transferred. The circuitry used for
select control eliminates the typical decoding glitch that occurs in a multiplexer during the transition between
real-time and stored data. A low input selects real-time data and a high input selects stored data. Figure 1
illustrates the four fundamental bus-management functions that can be performed with the ’LVTH652 devices.
Data on the A or B data bus, or both, can be stored in the internal D-type flip-flops by low-to-high transitions at
the appropriate clock (CLKAB or CLKBA) inputs, regardless of the select- or enable-control pins. When SAB
and SBA are in the real-time transfer mode, it is possible to store data without using the internal D-type flip-flops
by simultaneously enabling OEAB and OEBA
. In this configuration, each output reinforces its input; therefore,
when all other data sources to the two sets of bus lines are at high impedance, each set of bus lines remains
at its last state.
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.
When V
CC
is between 0 and 1.5 V, the device is in the high-impedance state during power up or power down.
However, to ensure the high-impedance state above 1.5 V, OE
should be tied to V
CC
through a pullup resistor
and OE should be tied to GND through a pulldown resistor; the minimum value of the resistor is determined by
the current-sinking/current-sourcing capability of the driver.
This device is 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 device when it is 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
OEAB OEBA CLKAB CLKBA SAB SBA A1−A8 B1−B8
OPERATION OR FUNCTION
L H H or L H or L X X Input Input Isolation
L H ↑↑X X Input Input Store A and B data
X H ↑ H or L X X Input Unspecified
‡
Store A, hold B
H H ↑↑X
‡
X Input Output Store A in both registers
L X H or L ↑ X X Unspecified
‡
Input Hold A, store B
L L ↑↑XX
‡
Output Input Store B in both registers
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
H H X X L X Input Output Real-time A data to B bus
H H H or L X H X Input Output Stored A data to B bus
H L H or L H or L H H Output Output
Stored A data to B bus and
stored B data to A bus
†
The data-output functions can be enabled or disabled by a variety of level combinations at OEAB or OEBA
. 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.
‡
Select control = L; clocks can occur simultaneously.
Select control = H; clocks must be staggered to load both registers.