Operating Modes

82C55A

The modes for Port A and Port B can be separately deﬁned, while Port C is divided into two portions as required by the Port A and Port B deﬁnitions. All of the output registers, including the status ﬂip-ﬂops, will be reset whenever the mode is changed. Modes may be combined so that their functional deﬁnition can be “tailored” to almost any I/O structure. For instance: Group B can be programmed in Mode 0 to monitor simple switch closings or display compu- tational results, Group A could be programmed in Mode 1 to monitor a keyboard or tape reader on an interrupt-driven basis.

The mode deﬁnitions and possible mode combinations may seem confusing at ﬁrst, but after a cursory review of the complete device operation a simple, logical I/O approach will surface. The design of the 82C55A has taken into account things such as efﬁcient PC board layout, control signal deﬁ- nition vs. PC layout and complete functional ﬂexibility to sup- port almost any peripheral device with no external logic. Such design represents the maximum use of the available pins.

Single Bit Set/Reset Feature (Figure 5)

Any of the eight bits of Port C can be Set or Reset using a single Output instruction. This feature reduces software requirements in control-based applications.

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 operation just as if they were output ports.

				CONTROL WORD

D7	D6			D5	D4	D3	D2	D1	D0

										BIT SET/RESET
	X			X	X					BIT SET/RESET
	X			X	X					1 = SET
										1 = SET
		DON’T								0 = RESET
		DON’T
			CARE
										BIT SELECT
											0	1	2	3	4	5	6	7
											0	1	0	1	0	1	0	1	B0
											0	1	0	1	0	1	0	1	B0
											0	0	1	1	0	0	1	1	B1
											0	0	1	1	0	0	1	1	B1
											0	0	0	0	1	1	1	1	B2
											0	0	0	0	1	1	1	1	B2


										BIT SET/RESET FLAG
										0 = ACTIVE
										0 = ACTIVE

FIGURE 5. BIT SET/RESET FORMAT

Interrupt Control Functions

When the 82C55A is 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 request 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 enable or disable a CPU interrupt by a speciﬁc I/O device 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 se- lection and device Reset.

Mode 0 (Basic Input/Output). This functional conﬁguration provides simple input and output operations for each of the three ports. No handshaking is required, data is simply writ- ten to or read from a speciﬁc port.

Mode 0 Basic Functional Deﬁnitions:

•Two 8-bit ports and two 4-bit ports

•Any Port can be input or output

•Outputs are latched

•Input are not latched

•16 different Input/Output conﬁgurations possible

MODE 0 PORT DEFINITION

	A			B		GROUP A			GROUP B

							PORTC			PORTC
D4		D3	D1		D0	PORT A	(Upper)	#	PORT B	(Lower)

0		0	0		0	Output	Output	0	Output	Output

0		0	0		1	Output	Output	1	Output	Input

0		0	1		0	Output	Output	2	Input	Output

0		0	1		1	Output	Output	3	Input	Input

0		1	0		0	Output	Input	4	Output	Output

0		1	0		1	Output	Input	5	Output	Input

0		1	1		0	Output	Input	6	Input	Output

0		1	1		1	Output	Input	7	Input	Input

1		0	0		0	Input	Output	8	Output	Output

1		0	0		1	Input	Output	9	Output	Input

1		0	1		0	Input	Output	10	Input	Output

1		0	1		1	Input	Output	11	Input	Input

1		1	0		0	Input	Input	12	Output	Output

1		1	0		1	Input	Input	13	Output	Input

1		1	1		0	Input	Input	14	Input	Output

1		1	1		1	Input	Input	15	Input	Input

Diamond Systems RUBY-MM-1612 user manual Operating Modes, Single Bit Set/Reset Feature Figure