EKT 222

MICROPRESSOR SYSTEM

LAB 4 : IO PORTS USING THE

8255 PPI DEVICE

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LAB 4: I/O Ports using the 8255 PPI Device

Objectives:

1) Ability to define the 8255 PPI device 2) Ability to create programs using the I/O ports 3) Ability to execute and demonstrate simple instructions 4) Ability to analyze the 8085 register and memory map conditions

Equipments :

1. Computer station with Windows OS and MY1 8085 simulation program 2. 8085 Instruction Sets (Assembly Code) 3. 8085 Instruction Sets (Machine Code)

Introduction

The 8255 Programmable Peripheral Interface (PPI) is the general purpose interface device which is widely used in Intel 8085 microprocessor design. It contains three independent 8 bit ports named Port A, B and C. Port A and B can be programmed as either input or output (all eight line must be same), while port C is split into two 4 bit halves (Port C upper (PC4-PC7) and Port C lower (PC0-PC3) that can be separately programmed as input or output. There are four register that control the operations of the PPI and there are mapped to four address locations in the 8085 Development System as shown below: Port Address Port A Register 80H Port B Register 81H Port C Register 82H Control Register 83H

PPI 8255

A0

A1

RD

WR

CS

Data Bus (8 Bit)

Port A (8 Bit)

Port B (8 Bit)

Port C (8 Bit)

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Table 1 shows the details of signals involved in controlling the PPI.

A0 A1 CS RD=1 WR=0

0 0 0 Port A to Data

bus Data bus to Port A

0 1 0 Port B to Data

bus Data bus to Port B

1 0 0 Port C to Data

bus Data bus to Port C

1 1 0 - Data bus to Control

Register

Table 1 : 8255 Port control signals

The control register is used to configure the PPI into a variety of operation modes. There are three basic modes:

mode 0 : basic input/output

mode 1 : strobe input/output

mode 2 : bidirectional bus We will only concentrate at mode 0. Further descriptions about mode 1 and 2 can be referred to the 8255 datasheet. Mode 0 provides for simple input output operations with no handshaking. This means that data either to or from the port does not depends to other signal for data transfer. Basically, to configure the PPI, a control word must first be sent to the control register @ port 83H. Figure 1 summarizes the control word format.

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

1 Port A and upper half of Port C Port B & lower half of Port C

MODE Port A Port CU MODE Port B Port CL

Figure 1 : 8255 Port control signals

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Mode Definition Control Byte. Indicate by bit b7=1. Group A - Port A and upper half of Port C Bit b3 to a bit b6 control the mode and direction of Group A b6 & b5 mode operation 0 0 (mode 0) 0 1 (mode 1) 1 0 (mode 2) b4 Port A direction, 0=output, 1=input b3 upper half of Port C direction, 0=output, 1=input Group B - Port B and lower half of Port C Bit b0 to bit b2 control the mode and direction of Group B b2 mode operation, 0=mod 0, 1=mod 1 b1 Port B direction, 0=output, 1=input b0 lower half of Port C direction, 0=output, 1=input Therefore to enable these ports to function as input or output, the microprocessor needs to configure the PPI by sending the control word as describe above to the control register. The instruction below is an example used to configure the PPI so that port A functions as input and port B & C as output. MVI A, 90H ; Set Port A= I/P and Port B & C=O/P. OUT 83H ; Send control word to control register. When the control port is configured, the data can be sent to any of the output port using OUT instruction. For example, if a data is to be sent to the Port B. The data must first be loaded into the accumulator and OUT instruction is used as below. MVI A, 55H OUT 81H ; where 81H is the address of Port B To read a data from Port A to the accumulator, IN instruction is used. IN 80H ; where 80H is the address of Port A

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Delay Programming Instructions The processor speed is very fast in dealing the fetch and execute cycles. Normally humans tend to ignore internal processes especially when a faster operation gives quicker result than slower processors. But when dealing with inputs and outputs, certain conditions do require a more moderate speed for humans to accommodate. An example where slower tempo is required for humans to accommodate can be seen on a running light. A program to set the lights running without any delay will cause a human to observe that all lights are turned on if the processor clock speed is high. With the inclusion of a delay cycle to reduce the frequency will then allow humans to observe the running light operation. Basically, the NOP instruction code can be used as a simple delay cycle for the microprocessor. A longer delay program can be seen as : MVI C, 255 LOOP: DCR C JNZ LOOP This program forms a loop which will decrease the contents of register C by 1 until gradually reaches 0. Any instructions after JNZ will be executed after register C equals to zero. This loop will create a delay that can be used for certain microprocessor application. The time delay depend’s on the instruction set which determines the T-state for each instruction. (Please refer to Instructions Code Summary). The following example shows the calculation of the software delay routine.

MVI C, 255 ; 7 states LOOP: DCR C ; 4 states JNZ LOOP ; 7/10 states (10 states if condition is met or 7 ; states if the condition not met)

The total number of state, Ts = 7 + 254(4 + 10) + 1(4 + 7) = 3574. Where

T state, Ts=1 / (0.5 x crystal frequency) For 6.144 MHz crystal,

T state, Ts= 325 ns. Time delay, td=3574 x 325 ns = 1.16 ms.

[ 254

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For a much longer delay, the following program can be used : DELAY: MVI B, x1 LOOP: MVI C, x2 LOOP1: DCR C JNZ LOOP1 DCR B JNZ LOOP

Or

DELAY: LXI B, N LOOP: DCX B MOV A, B ORA A JNZ LOOP

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MY1 Sim85 : INTERFACING THE I/O

1. The 8085 Simulator, MY1 Sim85, also provides hardware devices to the user. By default once the software is opened, an 8-bit LED device is assigned to Port A @ 80H and an 8-bit SWITCH device is assigned to Port B @ 81H.

2. Input output device selections can also be done by choosing Devices from the Main Menu.

3. To change the ports : a. Right mouse click on an existing port and remove the port assignment b. Right mouse click on the desired port and assign the free port

Example : replacing LED with a 7-segment display

8-bit LED @ 80H

8-bit Switch @ 81H

Remove PA0 from LED and reassign PA0 to 7-segment

1 2

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LABORATORY TASK Part A : Introduction to PPI IO Ports on MY1 Sim85 In this task, we will observe how to assign an output to an LED. Given is the program to display 1-bit LED : 1 ORG 0000H 2 MVI A,80H 3 OUT 83H 4 MVI A,0 5 OUT 80H 6 MVI A,1 7 OUT 80H 8 HLT DISCUSSION

1. Step simulate the program above and observe the LED display. Which LED lights up?

2. Remove PA0 and PA4 from the LED0 and LED4 display and swap the ports assignment. LED0 assign with PA4 while LED4 assign with PA0. Step simulate again and explain your observation.

3. Reset the LEDs with the proper port assignments. Change the instruction at line 6 to:

MVI A,16. Step simulate again and explain your observation

7 6 4 5 3 2 0 1

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Part B : Interfacing the 8255 PPI Device Given a following program of an 8-bit running light : ORG 0000H MVI A,10011000B OUT 83H MVI B,01H LOOP: MOV A,B OUT 80H RLC MOV B,A JMP LOOP HLT DISCUSSION

4. Step simulate the program and observe the operation. What can you say about the running lights coming out at Port A of the PPI?

5. Set the PPI control register so that Port B is an input port switch. What is the suitable program to show the running light when the switch at PB0 is turned on but pauses when it is turned off? Step simulate your modified program.

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Part C : Delay program for the running light

6. Instead of step simulate as in steps 4 and 5, run your simulation and observe the running light. Since the running light is moving at high fequency, you will need to reduce the clocking frequency to adjust to your eye movement. Create a delay subroutine to delay the running light by at least 5 clock cycles. Remember to set the stack memory location when using the CALL/RETURN instructions.

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EXERCISE

1. Eight LEDs are assigned to represent eight push-pull switches respectively. Write a program that will display the switches on/off value.

2. Figure 1 shows the connection between LED and switch with 8255 I/O port.

Write a program to make the LED blink when the switch is ON and stop blinking when the switch is OFF.

Figure 1

3. Figure 2 below shows the connection interface of four switches and LED with Port A and Port C. The switches represent as a 4-bit data input. Write a program to read 4 bit data input from the switches and the value will be used to determine how many time the LED should blink.

Figure 2

VCC

SW2

SW DIP-8

12345678

161514131211109

VCC

U3

8255

3433323130292827

53698

356

432140393837

1819202122232425

1415161713121110

D0D1D2D3D4D5D6D7

RDWRA0A1RESETCS

PA0PA1PA2PA3PA4PA5PA6PA7

PB0PB1PB2PB3PB4PB5PB6PB7

PC0PC1PC2PC3PC4PC5PC6PC7

SW2

SW DIP-8

12345678

161514131211109

VCC

VCC

U3

8255

3433323130292827

53698

356

432140393837

1819202122232425

1415161713121110

D0D1D2D3D4D5D6D7

RDWRA0A1RESETCS

PA0PA1PA2PA3PA4PA5PA6PA7

PB0PB1PB2PB3PB4PB5PB6PB7

PC0PC1PC2PC3PC4PC5PC6PC7

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4. Using two push-pull switches and eight LEDs, write a program of an 8-bit

running light that will have the following specifications shown in Table 1 :

SW1 SW0 Action

0 0 No lights ON

0 1 Running light MSB-LSB

1 0 Running light LSB-MSB

1 1 All lights ON

Table 1

5. Write a program that will display a MOD-16 counter onto a 4-bit LED.

6. A BCD counter is to be displayed onto a 4-bit LED. Write a program that will

count the BCD counter on every push button.

7. Figure 3 below show the operation of a certain running light :

Figure 3 On every clock pulse, the upper 4-bit will shift to the right while the lower 4-

bit will shift towards the left. When the upper 4-bit reaches its LSB it will change direction and shift to the left. The same will happen to the lower 4-bit once it reaches its MSB, it will change direction and shifts to the right.

Write a program to show the operation of this running light.