CS82C55A datasheet(9/26 Pages) HARRIS

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INTR (Interrupt Request)

A “high” on this output can be used to interrupt the CPU

when and input device is requesting service. INTR is set by

the condition: STB is a “one”, IBF is a “one” and INTE is a

“one”. It is reset by the falling edge of RD. This procedure

allows an input device to request service from the CPU by

simply strobing its data into the port.

INTE A

Controlled by bit set/reset of PC4.

INTE B

Controlled by bit set/reset of PC2.

Output Control Signal Deﬁnition

(Figure 8 and 9)

OBF - Output Buffer Full F/F). The OBF output will go “low”

to indicate that the CPU has written data out to be speciﬁed

port. This does not mean valid data is sent out of the part at

this time since OBF can go true before data is available.

Data is guaranteed valid at the rising edge of OBF, (See

Note 1). The OBF F/F will be set by the rising edge of the

WR input and reset by ACK input being low.

ACK - Acknowledge Input). A “low” on this input informs the

82C55A that the data from Port A or Port B is ready to be

accepted. In essence, a response from the peripheral device

indicating that it is ready to accept data, (See Note 1).

INTR - (Interrupt Request). A “high” on this output can be

used to interrupt the CPU when an output device has

accepted data transmitted by the CPU. INTR is set when

ACK is a “one”, OBF is a “one” and INTE is a “one”. It is

reset by the falling edge of WR.

INTE A

Controlled by Bit Set/Reset of PC6.

INTE B

Controlled by Bit Set/Reset of PC2.

NOTE:

1. To strobe data into the peripheral device, the user must operate

the strobe line in a hand shaking mode. The user needs to send

OBF to the peripheral device, generates an ACK from the pe-

ripheral device and then latch data into the peripheral device on

the rising edge of OBF.

FIGURE 7. MODE 1 (STROBED INPUT)

tST

STB

INTR

INPUT FROM

IBF

PERIPHERAL

tSIB

tSIT

tPH

tPS

tRIT

tRIB

FIGURE 8. MODE 1 OUTPUT

1/0

D3 D2 D1 D0

CONTROL WORD

MODE 1 (PORT A)

PC7

ACKA

PC6

PA7-PA0

OBFA

INTRA

PC3

PC4, PC5

1 = INPUT

0 = OUTPUT

D7 D6 D5 D4 D3 D2 D1 D0

CONTROL WORD

MODE 1 (PORT B)

PC1

ACKB

PC2

INTE

PB7-PB0

OBFB

INTRB

PC0

INTE

82C55A

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