8155/6 Multifunction Device (memory+IO)

(Dated pre-2000)

• Features
• Pins
• Block diagram
• Registers
• Control word format
• Status register format
• Timer Section

---

Features:

• 2kbits static RAM 256x8
• 2 programmable 8 bit I/O ports
• 1 programmable 6 bit I/O port
• 1 programmable 14 bit binary counter/timer
• Internal address latch to demux AD0-AD7, using ALE line

---

Pinout (40 pins):

AD0-AD7 I/O Addr/Data bus mux'd RESET I Reset input CE-bar or CE I Chip enable (55/56) ALE I Address latch enable RD-bar I Read input WR-bar I Write input IO/Mbar I I/O or memory section PA0-7 I/O Port A (8 bit) PB0-7 I/O Port B (8 bit) PC0-5 I/O Port C (6 bit) TIMER-IN I Timer input TIMER-OUT-bar O Timer output

---

Block diagram:

---

Registers:

(ALE high, AD0=A0 etc)

A2	A1	A0	Port
0	0	0	Command/status reg.
0	0	1	PA
0	1	0	PB
0	1	1	PC
1	0	0	Timer LSB
1	0	1	Timer MSB

---

Control word (command register) format:

D7	D6	D5	D4	D3	D2	D1	D0
Timer command		IEB	IEA	PC		PB	PA

• D0, D1: mode for PA and PB, 0=i/p, 1=o/p
• D2, D3: mode for PC
• D4, D5: interrupt enable for PA and PB, 0=disable 1=enable
• D6, D7: Timer command:
  • 00: No effect
  • 01: Stop if running else no effect
  • 10: Stop after terminal count (TC) if running, else no effect
  • 11: Start if not running, reload at TC if running.

Port C bits (D2, D3):

ALT	D3	D2	PC5	PC4	PC3	PC2	PC1	PC0
1	0	0	I	I	I	I	I	I
2	1	1	O	O	O	O	O	O
3	0	1	O	O	O	STB-Abar	BF-A	INTR-A
4	1	0	STB-Bbar	BF-B	INtr-B	STB-Abar	BF-A	INTR-A

I - input O - output

---

Status register format:

D7	D6	D5	D4	D3	D2	D1	D0
X	Timer	INTE-B	BF-B	INTR-B	INTE-A	BF-A	INTR-A

D6: Timer. Latched high when TC is reached, low when status reg is read or reset is done.

---

Timer section

MSB:
M2	M1	T13	T12	T11	T10	T9	T8
LSB:
T7	T6	T5	T4	T3	T2	T1	T0

M2, M1: mode bits:

• 00: Single square wave of wavelength TC/2 (TC/2,TC/2 if TC even; [TC+1/2],[TC-1/2] if TC odd)
• 01: Square waves of wavelength TC (TC/2,TC/2 if TC even; [TC+1/2],[TC-1/2] if TC odd)
• 10: Single pulse on the TC'th clock pulse
• 11: Single pulse on every TC'th clock pulse.