8085 Microprocessor

D7	D6	D5	D4	D3	D2	D1	D0
Timer command		IEB	IEA	PC (ALT1 and ALT0)		PB	PA

Control register also called command register.
It is a 8-bit which defines different functions.
It defines the function of different ports such as input, output or handshaking mode.
It also defines timer command for timer to start and stop.

Timer command:

00 -No effect
01 -Stop counting if timer is running. No operation if counter is not running.
10 -Stop after terminal counter is reached.
11 -Start timer if timer is not running. If timer is running reload the TC and then continue.


Interrupt Enable for PA and PB(D4 and D5):

1- enable
0- disable

Input or output for PA and PB respectively:

1-output
0-input

                     


This section has 256x8 Memory. It means 256 location of 8 bit each.

When CE-bar=0, IO/M-bar=0 and ALE is activated the address in AD0-AD7 are latched and will be provided to 256 memory location.

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

                                       


1. AD0-AD:- Address/Data bus multiplexed

2. RESET:- RESET input

3. CE bar:- chip enable

4. ALE:- Arithmetic latch enable

5. RD-bar:- Read input

6. WR-bar:- Write input

7. IO/M-bar:- I/O or memory

8. PA0-PA7:- Port A

9. PB0-PB7:- Port B

10. PC0-PC7:- Port C

11. TIMER-IN: Timer input signal

12. TIMER-OUT-bar: Timer output

8155 is a multifunction device. It contains RAM, i/o port and timer.

8155 features:

1. Two programmable 8 bit i/o ports
2. One programmable 6 bit i/o ports
3. One programmable 14 bit binary counter and timer.
4. 2kbits static RAM 256x8.
5. Address bus AD0- AD7
6. It contains an internal select logic for memory and i/o using a command register and two i/o ports.
7. It can be easily interfaced with 8085 microprocessor.