Datasheet

SN54HC646, SN74HC646
OCTAL BUS TRANSCEIVERS AND REGISTERS
WITH 3-STATE OUTPUTS
SCLS150C – DECEMBER 1982 – REVISED MARCH 2003
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description/ordering information (continued)
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.
When an output function is disabled, the input function is still enabled and can be used to store data. Only one
of the two buses, A or B, may be driven at a time.
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 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.