Intel mcs-48 manual 8255A18255A·5, Group a and Group B Controls, Ports A, B, and C

8255A18255A·5

(RESET)

Reset. A "high on this input clears the control register and all ports (A, C, C) are set to the Input mode.

Group A and Group B Controls

The functional configuration of each port is program· med by the systems software. In essence, the CPU "out· puts" a control word to the 8255A. The control word con- tains information such as "mode", "bit set", "bit reset", etc., that initializes the functional configuration of the

8255.-

Each of the Control blocks (Group A and Group 8) accepts "commands" from the ReadlWrite Control Logic, receives "control words" from the internal data bus and issues the proper commands to its associated ports.

Control Group A - Port A and Port C upper (C7·C4) Control Group B - Port B and Port Clower (C3-CO)

The Control Word Register can Only be written into. No Read operation of the Control wo;:d Register is allowed.

Ports A, B, and C

The 8255A contains three 8-bit ports (A, B, and C). All can be configured in a wide variety of functional charac· teristlcs by the system software but each has its own special features or "personality" to further enhance the power and flexibility of the 8255A.

Port A. One 8-bit data output latch/buffer and one 8-bit data input latch.

Port B. One 8-bit data input/output latch/buffer and one 8·bitdata input buffer.

Port C. One 8-bit data output latch/buffer and one 8·bit data input buffer (no latch for input). This port can be divided into two 4-bit ports under the mode control. Each 4-bit port contains a 4-blt latch and It can be used for the control signal outputs and status signal inputs in conjunction with ports A and B.

PIN CONFIGURATION

....		PIN NAMES
"	D7 00	·DATA BUS (BI·DIRECTIONALI
	RESET	RESET INPUT
	CS	CHIP SELECT	--
		READ INPUT
	"e		--
	W"	WRITE INPUT
CI ___-- '	AG,A1	PORT ADDRESS	--
	PA7·PAD	PORT A 181Tl
	PB7·P80	PORT BIBITI
	Pe7·pca	PORT C (BITI
	Vee	+5 VOL15
	GNe	'VOLTS

Figure 2. 8225A Block Diagram Showing Group A and

Group B Control Functions

9-19 AFNoOO744A-03