8255 Programmable Peripheral Interface

(Dated: pre-2000)

• Features
• Pinout
• Block diagram
• BSR mode
• I/O mode
• Mode 1
• Mode 2

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Features:

• 3 8-bit IO ports PA, PB, PC
• PA can be set for Modes 0, 1, 2. PB for 0,1 and PC for mode 0 and for BSR. Modes 1 and 2 are interrupt driven.
• PC has 2 4-bit parts: PC upper (PCU) and PC lower (PCL), each can be set independently for I or O. Each PC bit can be set/reset individually in BSR mode.
• PA and PCU are Group A (GA) and PB and PCL are Group B (GB)
• Address/data bus must be externally demux'd.
• TTL compatible.
• Improved dc driving capability

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Pinout

A1 A0 Select 0 0 PA 0 1 PB 1 0 PC 1 1 Control reg.

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Block diagram

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BSR mode

Bit set/reset, applicable to PC only. One bit is S/R at a time. Control word:

D7	D6	D5	D4	D3	D2	D1	D0
0 (0=BSR)	X	X	X	B2	B1	B0	S/R (1=S,0=R)

Bit select: (Taking Don't care's as 0)

B2	B1	B0	PC bit	Control word (Set)	Control word (reset)
0	0	0	0	0000 0001 = 01h	0000 0000 = 00h
0	0	1	1	0000 0011 = 03h	0000 0010 = 02h
0	1	0	2	0000 0101 = 05h	0000 0100 = 04h
0	1	1	3	0000 0111 = 07h	0000 0110 = 06h
1	0	0	4	0000 1001 = 09h	0000 1000 = 08h
1	0	1	5	0000 1011 = 0Bh	0000 1010 = 0Ah
1	1	0	6	0000 1101 = 0Dh	0000 1100 = 0Ch
1	1	1	7	0000 1111 = 0Fh	0000 1110 = 0Eh

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I/O mode

D7	D6	D5	D4	D3	D2	D1	D0
1 (1=I/O)	GA mode select		PA	PCU	GB mode select	PB	PCL

• D6, D5: GA mode select:
  • 00 = mode0
  • 01 = mode1
  • 1X = mode2
• D4(PA), D3(PCU): 1=input 0=output
• D2: GB mode select: 0=mode0, 1=mode1
• D1(PB), D0(PCL): 1=input 0=output

Mode 0: No interrupts. Plain I/O. Two 8 bit ports PA, PB. Two 4 bit ports PCU and PCL. Outputs latched, inputs buffered.

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Mode 1

(Input and output data are latched)

PC bits in input mode:
D7	D6	D5	D4	D3	D2	D1	D0
PC7	PC6	IBF-A	INTE-A / STB-A-bar	INTR-A	INTE-B / STB-B-bar	IBF-B	INTR-B
PC bits in output mode:
D7	D6	D5	D4	D3	D2	D1	D0
OBF-A-bar	INTE-A / ACK-A-bar	PC5	PC4	INTR-A	INTE-B / ACK-B-bar	OBF-B-bar	INTR-B

Input mode:

• D4, D2: Set/Reset INTE using BSR. STB-bar input is connected to external peripheral's strobe output (i.e. PC2, PC4 pin to external strobe).
• INTE is internal connection. STB-bar is external connection.

Output mode:

• D6, D2: Set/Reset INTE using BSR. ACK-bar input is connected to external peripheral's acknowledge output (i.e. PC2, PC6 pin to external ack).
• INTE is internal connection. ACK-bar is external connection.

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Mode 2

Only for PA

Status:

D7	D6	D5	D4	D3	D2	D1	D0
OBF-A-bar	INTE1(O/P) / ACK-A-BAR	IBF-A	INTE2(I/P) / STB-A-bAR	INTR-A	X	X	X

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