RAM/Peripheral Interfacing device for 8085 and 8088

Presented By-

Hitesh Patel

The 8155 is a RAM and I/O chip to be used in the 8085 and 8088 MPU system. The RAM is designed with 2K bit static cells organized as 256*8. They have a maximum access time of 440ns.

INTRODUCTION

INTRODUCTION

There are 3 basic I/O ports. One of the three can be programmed to be status pins, thus allowing to the other two ports to operate in handshake mode

A 14 bit counter/timer is also included in the chip to provide either a square wave or terminal count pulse for the CPU system depending on the timer mode.

The programmable I/O ports can be operated by BASIC Mode and STROBE Mode

FEATURES

Single +5V Power Supply. Access Time 330ns. Internal Address Latch. 2 Programmable 8bit I/O ports and 1

programmable 6bit I/O port. 256*8 bit RAM. Programmable 8bit binary Counter/Timer. Multiplexed 8bit Address/Data bus. Chip Enable active low. 40 pin DIP.

BLOCK DIAGRAM

PIN NAMES AND DESCRIPTION

RESET(I/P): The width of reset high should be two clock cycles in 8155. AD 0~7(I/P,O/P and tri-state): Theses are three state address/data lines

that interface with the lower order MPU address/data bus. The 8 bit address can be applied to the memory section or the I/O section depending upon the polarity of the IO/M` signal.

CE: Chip Enable is active low. RD: reading input from the memory or the Input peripheral. Active low. WR: writing output to the memory or the O/P peripheral. Active low. ALE: Address Latch Enable. Works on the falling edge. PA 0~7 / PB 0~7 : These 8 pins are general purpose I/O pins. The input

output direction is set by the command register. PC 0~5: These 6 pins can function either as input port, output port or

the control signals for Port A and Port B. Programming is done via command register.

Pin Names And Description…

The control Signals are :-

Timer In: Input to timer counter. Timer Out: Output to timer counter. Vcc Vss (Ground)

The Command Register

The command register consists of latches. Four bits define the mode of the ports (0~3) 2 bits (4~5) define the enable or disable

interrupts from Port C when its acts as control port.

The last 2 bits are for the timer (6~7) The contents of the command register can be

altered at any time by using I/O address XXXX000 during write operation

COMMAND REGISTER Its also called the control word. 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.

D7 D6 D5 D4 D3 D2 D1 D0

Timer command IEB IEA PC PB PA

I/O SECTION PA Register: This register can either be programmed to input or

output port depending upon the status of the contents of the C/S register. Also depending upon the command it can work in the BASIC or STROBE Mode. The address of the register is XXXXX001.

PB Register: It’s the same as PA register. Its address is XXXXX010. PC Register: It’s a 6Bit register with the address XXXXX011. It can

be programmed to be either Input/output Port or to be the control register for Port A and Port B by properly programming the AD2 and AD3 bits of the C/S Register. When PC 0~5 is used as control port, 3 bits are assigned to Port A and

3 to Port B. The 1st bit is an interrupt. The 2nd is an output signal indicating weather the buffer is full or

empty. 3rd is an in put signal to accept a Strobe from the Strobed Input Mode.

Structure of Ports A & B within 8155

I/O Section …

When Port is programmed in either Mode 3 or Mode 4, the control signals for PA and PB as initialized as below:

I/O Addressing Scheme

PORT Control Assignment

Port Control Assignment

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

TIMER The Timer is a 14 bit down-counter that counts the “timer

input” pulses and provides either a square wave or pulse when the terminal count is reached.

Address for the Low order byte of register : XXXXX100 Address for the high order byte of register : XXXXX101 To Program the Timer, the COUNT LENGTH REGISTER is

loaded first, one byte at a time, by selecting the timer addresses. Bits 0~13 specify the count of next count Bits 14~15 specify the Timer Output Mode. The value of the count length register may have any value

from 2H through 3FFFH on bits 0~13.MSB:

M2 M1 T13 T12 T11 T10 T9 T8

LSB:

T7 T6 T5 T4 T3 T2 T1 T0

TIMER MODES There are 4 basic Timer Modes which may be defined by M1 and M2 :- 0 0: Single square wave of wavelength TC/2 (TC/2,TC/2 if TC even; [TC+1/2],[TC-1/2] if TC odd) 0 1: Square waves of wavelength TC (TC/2,TC/2 if TC even; [TC+1/2],[TC-1/2] if TC odd) 1 0: Single pulse on the TC'th clock pulse 1 1: Single pulse on every TC'th clock pulse.

Asymmetrical Count 5 In case of an odd numbered count, the first half cycle of the

square wave output, which is high, is one count longer than the second (low) half cycle.

Timer Mode Select

Bits 6~7 (TM2 and TM1) of command register contents are used to start or stop the counter.

Mode 0 with count = 4

TWICE COUNTER

The timer circuit on the 8155 chip is designed to be square wave timer, not an event counter. To achieve this it counts down by two in completing one cycle.

Its registers don’t contain the values directly representing the number of TIMER IN pulses being received,

Counter value ‘1’ can’t be loaded as the initial value in the count register as the timer operates with the terminal count value of 10.

#NOTE: when operating as a square wave generator, it operates as a frequency divider with the output frequency equal to the input frequency divided by count number.

HANDSHAKING MODE

The ports A and B can be used in handshaking mode with port C providing the handshaking signals.

(STB)’ : STROBE is an input signal from the peripheral to the 8155. It indicates the MPU that the data to be read is already sent on the port lines of 8155. Upon activation of the signal 8155 loads the data from the input port lines to the input buffer of the port.

BF: This is an active high signal, indicating the presence of data byte in the port.

INTR: It’s a high signal sent from the 8155 to the 8085 indicating that the data from the input device is ready on the input buffer. The signal is reset with the falling edge of read signal.

IE: Interrupt enable signal is vital to activate the interrupt.

HANDSHAKING MODE

TIMING DIAGRAMS

STROBED INPUT MODE

INPUT MODE

Port A in input mode with handshake signal.

TIMING DIAGRAMS…

STROBED OUTPUT MODE

OUTPUT MODE

Port B in output mode with handshake signal.