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Interactive Audio Lesson
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Introduction to the 8255 PPI
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Today, we'll explore the 8255 Programmable Peripheral Interface. Can anyone tell me what the main purpose of the 8255 is?
It connects microprocessors to peripheral devices!
Exactly! It acts as an interface between the CPU and peripherals. Now, what are some of the ports available in the 8255?
There are three 8-bit ports: Port A, Port B, and Port C.
Great! Now let's remember that with the acronym 'A-B-C', which stands for 'All Bits Connected'. This will help us recall the ports when needed.
What kind of operational modes does the 8255 support?
Good question! The 8255 operates in modes like Mode 0 for simple I/O, Mode 1 for strobed I/O, and Mode 2 for bidirectional communication. We will cover these in more detail later!
To summarize, the 8255 connects CPUs to peripherals via three ports. Remember: A-B-C for Port A, B, and C!
Control Word and Configuration
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Now, let's look at the control word format. What part of the control word determines the port's direction?
The Direction bits in the control word.
Exactly! The higher bits control whether ports are input or output. Can anyone give me an example of a control word that configures Port A as output?
If Port A is output, then I think the control word is 83H, right?
Almost! Remember, we also need to configure the other ports. The correct control word for Port A as output, Port B as input, and both halves of Port C would be 83H. This indicates the direction and mode!
So the bit settings are crucial for configuring the ports correctly?
Exactly! Bit settings determine how each port operates. Let's review: The bits D7-D4 control port modes and directions. Always double-check your settings!
Writing and Reading from Ports
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Next, how do we actually interact with the 8255 in assembly programming? What are the steps involved?
We need to send the control word first, then we can write or read data!
Correct! After we configure the device with the control word, writing to Port A would look like this in assembly: 'MOV AL, 0AAH' and then 'OUT 70H, AL'. What does this code do?
It writes the hexadecimal value AAH to Port A!
That's right! And how do we read data from Port B?
We use the 'IN' instruction to read from Port B, such as 'IN AL, 71H'!
Excellent! So to summarize, to interact with the 8255, we send a control word, write data to output ports, and read from input ports using specific assembly instructions.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
In this section, the programming example illustrates how to set up the 8255 PPI using 8086 assembly language. It covers configuring ports and demonstrates sending data to specified ports, highlighting the steps involved in writing control words and operating the device.
Detailed
Detailed Summary
In this section, we focus on the programming example of configuring the 8255 Programmable Peripheral Interface (PPI) using the 8086 microprocessor. The example outlines the steps required to manage the configuration of the PPI's ports for input and output operations.
Key Highlights:
- Overview of the 8255 PPI: The 8255 is a widely used interface chip that facilitates communication between the microprocessor and peripheral devices, providing three 8-bit data ports.
- Control Word Format: Understanding the control word structure is crucial. The control word determines the operating mode of the device and the direction of each port (input or output).
- Programming Steps:
- Send the control word to set the operational modes of the ports.
- Write data to output ports and read from input ports, demonstrating both output and input operations.
- Example Code: The provided assembly code showcases how to configure the 8255 and utilizes the appropriate ports to execute specified tasks, emphasizing practical application.
Significance:
This programming example encapsulates the practical implementation of interfacing techniques, solidifying the theoretical knowledge provided throughout the chapter and enhancing the learning experience by providing a real-world application.
Audio Book
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Overview of the Programming Example
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Assume the 8255 CWR is at 0073H.
Numerical Programming Example (8086): Configure 8255: Port A = Output (Mode 0), Port B = Input (Mode 0), Port C Upper = Output (Mode 0), Port C Lower = Input (Mode 0). Then, write AAH to Port A, and conceptually 55H to Port C.
Detailed Explanation
This chunk introduces the programming example using the 8086 microprocessor and the 8255 programmable peripheral interface (PPI). The example involves configuring 8255 where Port A is set up as an output port, while Port B and the lower nibble of Port C are set as input ports. It also illustrates sending specific data to these ports, with 'AAH' being sent to Port A and '55H' conceptually to Port C. The control word used to set these configurations is important as it defines how each port operates.
Examples & Analogies
Think of this programming example like setting up buttons and lights in a room: Port A is like a light switch that you can turn on, Port B represents a button that you can press to register a command, and Port C acts like a control panel that can monitor multiple settings.
Control Word for Mode Set
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Control Word for Mode Set:
○ D7=1, G_A_Mode=00, PA_Dir=0, PCU_Dir=0, G_B_Mode=0, PB_Dir=1, PCL_Dir=1
○ Control Word = 10000011 binary = 83H
Detailed Explanation
The control word is an 8-bit configuration that specifies how the 8255 device should behave. Each bit holds a specific function: D7 tells the device whether the mode is being set, while the next bits determine the modes for Group A and Group B, the direction for each port (input/output), and the operation mode for the ports. The final binary output '10000011' represents this configuration in hexadecimal as '83H'. This binary representation allows the machine to understand how to manage data flow across the connected ports.
Examples & Analogies
Consider the control word as the instructions for a vending machine. Just as you select different buttons to determine whether you want candy or soda (input and output), the bits in the control word tell the 8255 what kind of data it should read or output through each port.
Code Snippet Explanation
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; 8086 Assembly Code Snippet
ASSUME CS:CODE
CODE SEGMENT
START:
; Configure 8255
MOV AL, 83H ; Control word: PA=OUT, PB=IN, PC(upper)=OUT, PC(lower)=IN, all Mode 0
OUT 73H, AL ; Send control word to 8255 CWR (0073H)
; Write AAH to Port A
MOV AL, 0AAH
OUT 70H, AL ; Write to Port A (0070H)
; Write 55H to Port C (conceptual: only affects output-configured bits)
MOV AL, 55H
OUT 72H, AL ; Write to Port C (0072H)
; Read from Port B
IN AL, 71H ; Read data from Port B (0071H) into AL
; Read from Port C
IN AL, 72H ; Read data from Port C (0072H) into AL
; AL will contain current input from PC0-PC3, and output from PC4-PC7.
HLT
CODE ENDS
END START
Detailed Explanation
This code snippet demonstrates how to configure the 8255 chip and perform read and write operations using the 8086 assembly language. Starting with setting up the segment and configuration, the code uses specific assembly instructions: 'MOV' to move data into registers and 'OUT' to send data to the device at its designated ports (like 0070H for Port A). Similarly, 'IN' is used to read back data from Port B and Port C, where 'AL' register eventually holds that incoming data, confirming the successful execution of the configured commands.
Examples & Analogies
Think of this code as programming a smart home system. Each command (MOV, OUT, IN) is like telling different devices (like a thermostat or a light) what to do. You move numbers to the systems to turn them on or off, and you can check the current settings of each device, just as you can with this assembly code directing the PPI.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Control Word Configuration: The control word is crucial in setting up the mode of operation and direction of data flow on the 8255.
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Assembly Language Operations: Writing to and reading from the 8255 requires specific assembly commands, showcasing the interaction between software and hardware.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Configuring Port A as output involves setting the control word appropriately and writing data to Port A using the OUT command.
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Reading from Port B can be performed using the IN instruction, allowing the program to access external signals.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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For the 8255, ports you must know, A, B, and C—where data flows!
📖 Fascinating Stories
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Imagine the 8255 as a traffic officer, guiding data cars through Port A, Port B, and Port C, ensuring smooth communication between the CPU and peripherals.
🧠 Other Memory Gems
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A-B-C stands for All Bits Connected - remember the three ports of the 8255!
🎯 Super Acronyms
P-A-C stands for Parallel Access Control - a reminder of the PPI's role in managing ports.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: 8255 Programmable Peripheral Interface (PPI)
Definition:
An integrated circuit that facilitates communication between a microprocessor and peripheral devices.
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Term: Control Word
Definition:
An 8-bit command sent to the 8255 to define operational modes and port directions.
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Term: Port
Definition:
A channel through which data is sent and received, such as Port A, Port B, or Port C on the 8255.