TRUTH TABLE

HC646 (The truth table for HC648 is the same as this, but with the outputs inverted)

G

DIR

CAB CBA SAB SBA

A

B

FUNCTION

HX

INPUTS

INPUTS

Both the A bus and the B bus are inputs

X

X

X

X

Z

Z

The output functions of the A and B bus are disabled

X

X

INPUTS

INPUTS

Both the A and B bus are used for inputs to the

internal flip-flops. Data at the bus will be stored on

low to high transition of the clock inputs

LH

INPUTS

OUTPUTS

The A bus are inputs and the B bus are outputs

X

X*

L

X

L

L

The data at the A bus are displayed at the B bus

HH

X*

L

X

L

L

The data at the A bus are displayed at the B bus.

The data of the A bus are stored to the internal

flip-flop on low to high transition of th clock pulse.

HH

X

X*

H

X

X

Qn

The data stored to the internal flip-flop are dispayed

at the B bus

X*

H

X

L

L

The data at the A bus are stored to the internal flip-

flop on low to high transition of the clock pulse. The

states of the internal flip-flops output directly to the

B bus

HH

LL

OUTPUTS

INPUTS

The B bus are inputs and the A bus are outputs

X*

X

X

L

L

L

The data at the B bus are displayed at the A bus

HH

X*

X

L

L

L

The data at the B bus are displayed at the A bus.

The data of the B bus are stored to the internal flip-

flop on low to high transition of the clock pulse

HH

X*

X

X

H

Qn

X

The data stored to the internal flip-flops are

displayed at the A bus

x*

X

H

L

L

the data at the B bus are stored to the internal flip-

flop on low to high transition of the clock pulse. The

states of the internal flip-flops output directly to the

A bus

HH

X

: DON’T CARE

Z

: HIGH IMPEDANCE

Qn

: THE DATA STORED TO THE INTERNAL FLIP-FLOPS BY MOST RECENT LOW TO HIGH TRANSITION OF THE CLOCK INPUTS

*

: THE DATA AT THE A AND B BUS WILL BE STORED TO THE INTERNAL FLIP-FLOPS ON EVERY LOW TO HIGH TRANSITION OF

THE CLOCK INPUTS

M74HC646/648

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