Intel mcs-48 manual Operating Modes, Interrupt Control Functions

8255A18255A·5

CONTROL WORD

BIT SET/RESET

1 "'SET

0= RESET

BIT SELECT

01234567

0101010180

001100118,

00001'118

BIT SET/RESET FLAG 0'" ACTIVE

Figure 5. Bit Set/Reset Format

Operating Modes

MODE 0 (Basic Input/Output). This functional configura· tion provides simple input and output operations for each of the three ports. No "handshaking" is required, data is simply written to or read from a specified port.

When Port C is being used as status/control for Port A or B. these bits can be set or reset by using the Bit Set/Reset op· eration just as if they were data output ports.

Interrupt Control Functions

When the 8255A ie programmed to operate in mode 1 or mode 2, control signals are provided that can be used as interrupt request inputs to the CPU. The interrupt reo quest signals, generated from port C, can be inhibited or enabled by setting or resetting the associated INTE flip· flop, using the bit set/reset function of port C.

This function allows the Programmer to disallow or allow a specific I/O device to interrupt the CPU without affecting any other device in the interrupt structure.

INTE flip-flop definition:

(BIT-SET) -INTE is SET - Interrupt enable (BIT-RESET) - INTE is RESET - Interrupt disable

Note: All Mask flip-flops are automatically reset during mode selection and device Reset.

Mode 0 Basic Functional Definitions:

•Two 8-bit ports and two 4-bit ports.

•Any port can be input or output.

•Outputs are latched.

•Inputs are not latched.

•16 different Input/Output configurations are possible in this Mode.

	.	tRR	0
	-- 'r		-, <-
C IR -			- tHR -!
INPUT
t==..tAR -			-	tRA------:!
CS, Al, AD
---- ------~<			1 I.
		t Ro		toF
MODE 0 (Basic Input)
		'wW---~-
	~r-		7f-
		I' --- tow	'wo--l
	<>
tAW	-I			twA
CS. A1, AD

OUTPUT

l---

-

-

MODE 0 (Basic Output)

9-21 AFN-00744A-05