Datasheet

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DESCRIPTION/ORDERING INFORMATION (CONTINUED)
SN74LVCH16646A
16-BIT BUS TRANSCEIVER AND REGISTER
WITH 3-STATE OUTPUTS
SCAS318K – NOVEMBER 1993 – REVISED MARCH 2005
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 SN74LVCH16646A.
Output-enable ( OE) and direction-control (DIR) inputs 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 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.
This device is fully specified for partial-power-down applications using I
off
. The I
off
circuitry disables the outputs,
preventing damaging current backflow through the device when it is powered down.
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.
Inputs can be driven from either 3.3-V or 5-V devices. This feature allows the use of this device as a translator in
a mixed 3.3-V/5-V system environment.
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.
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.
FUNCTION TABLE
INPUTS DATA I/O
(1)
OPERATION OR FUNCTION
OE DIR CLKAB CLKBA SAB SBA A1–A8 B1–B8
X X ↑ X X X Input Unspecified Store A, B unspecified
(1)
X X X ↑ X X Unspecified Input Store B, A unspecified
(1)
H X ↑ ↑ X X Input Input Store A and B data
H X H or L H or L X X Input Input 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
(1) 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.
2