Philips Semiconductors

Product specification

74F786

4-bit asynchronous bus arbiter

February 14, 1991

3

FUNCTIONAL DESCRIPTION

The BRn inputs have no inherent priority. The arbiter assigns priority

to the incoming requests as they are received, therefore, the first BR

asserted will have the highest priority. When a bus request is

received its corresponding bus grant becomes active, provided that

EN is low. If additional bus requests are made during this time they

are queued. When the first request is removed, the arbiter services

the bus request with the next highest priority. Removing a request

while a previous request is being serviced can cause a grant to be

changed when arbitrating between three or four requests. For that

reason, the user should not remove ungranted requests when

arbitrating between three or four requests. This does not apply to

arbitration between two requests.

If two or more BRn inputs are asserted at precisely the same time,

one of them will be selected at random, and all BGn outputs will be

held in the high state until the selection is made. This guarantees

that an erroneous BGn will not be generated even though a

metastable condition may occur internal to the device. When the EN

is in the high state the BGn outputs are forced high.

PIN CONFIGURATION

16

15

14

13

12

11

10

7

6

5

4

3

2

1

9

8

VCC

B

C

D

BR0

BR1

BR2

BR3

GND

A

YOUT

BG0

BG1

BG2

BG3

EN

SF00441

PIN DESCRIPTION

SYMBOL

PINS

TYPE

NAME

FUNCTION

BR0 – BR3

4, 5, 6, 7

Input

Bus request inputs (active low)

The logic of this device arbitrates between these four inputs.

Unused inputs should be tied high.

A, B, C, D

15, 1, 2, 3

Input

Inputs of the 4–input AND gate

EN

9

Input

Enable input

When low it enables the BG0 – BG3 outputs.

BG0 – BG3

13, 12, 11, 10

Output

Bus grant outputs (active low)

These outputs indicate the selected bus request. BG0 corre-

sponds to BR0, BG1 to BR1, etc.

YOUT

14

Output

Output of the 4–input AND gate

GND

8

Ground

ground (0V)

16

Power

Positive supply voltages