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Introduction to the 8255 Programmable Peripheral Interface
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Today, we're going to learn about the 8255 Programmable Peripheral Interface, often referred to as PPI. What do you think its main purpose is?
Is it to connect the microprocessor to input/output devices?
Exactly! The 8255 allows us to interface with various peripherals efficiently. Can anyone tell me the advantages of using parallel I/O?
Parallel I/O transfers multiple bits simultaneously, making it faster than serial I/O.
Good point! This speed is vital for applications needing quick and efficient data handling. Remember, for a mnemonic: 'Parallel is Quick, Serial is Slow'—this can help you remember their differences.
Control Word Configuration for 8255
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To operate the 8255, we have to write a control word to it. Can anyone explain what this control word does?
It configures the operational modes of the I/O ports.
Correct! The control word sets which ports are inputs and which are outputs, as well as the mode of operation. Let’s break down how we calculate it for our setup.
For Mode 0, Port A as output and others as inputs, right?
Exactly! That means our control word will be 10001010 in binary, which is 8AH in hexadecimal. Remember: 'Bits tell their role—set them right to control!'
Writing Assembly Code for 8255 Initialization
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Now, let’s talk about how we write the assembly code to initialize and output data to Port A. Has anyone tried writing assembly before?
I have! But I find it tricky to remember the instructions.
Fair enough! Remember, writing the control word and then the data we want to send is fundamental. For instance, we start with 'MVI A, 8AH' followed by 'OUT 83H'.
So, we load the control word, and then we output our static data?
That's right! Then we send the static byte 55H to Port A. Think of it like this: 'Initialize, Then Operate'!
Expected Outcomes and Observations
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Once we execute our program, what should we expect to see on Port A?
The LEDs connected to Port A should represent the data 55H.
Exactly! In binary, that’s 01010101, which means alternating LEDs ON and OFF. Does everyone follow the pattern?
Yes, so PA7 is OFF, PA6 is ON, and so forth.
Great! Remember, when these values are sent, each LED reflects its binary state. Keep this sentence in mind: 'Control leads to Outcome!'
Conclusion and Wrap-Up
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To sum up, what have we learned today about the 8255 and Port A?
We learned how to configure the 8255 and output static data to the port.
And how important the control word is for determining the function of each port.
Exactly! Always remember: 'Control Word for Configuration - Essential for Operation.' Are there any questions before we finish?
No, this was really engaging!
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
In this section, we delve into the specifics of programming the 8255 PPI to output a static byte (55H) to Port A. We cover the control word configuration necessary for the setup, the assembly code to execute, and the expected outcomes when interfacing with LEDs.
Detailed
Detailed Summary
The section focuses on the initialization and programming of the 8255 Programmable Peripheral Interface (PPI) to output a static byte to Port A of the device. The objective is to configure the 8255 in Mode 0, specifying the role of each port: Port A as output, Port B as input, and the lower part of Port C as output, while the upper part serves as input.
The necessary control word is determined based on this configuration, resulting in 8AH as the appropriate hex value for the control register. We then detail the assembly instructions required for setting up this configuration, including loading the control word and outputting a static byte (55H) to the designated port. Additionally, we provide the expected behavior of the connected LEDs when executed correctly, showcasing the binary representation of the data sent to Port A.
Audio Book
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Objective of Program 1
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Objective: To configure 8255 for Mode 0 (Port A as Output, Port B as Input, Port C Lower as Output, Port C Upper as Input) and then output a fixed data byte (55H) to Port A.
Detailed Explanation
The goal of this program is to prepare the 8255 device for specific roles for its ports. In Mode 0, Port A will be set as an output port, meaning it will send signals (data) to LEDs. Port B will be an input port to accept signals from devices like switches, and the lower half of Port C (C_L) will function as output for additional devices (like more LEDs). The upper half of Port C (C_U) is reserved as input, but isn’t actively used in this program. The program also sends a fixed value (55H) to Port A, which would switch the LEDs on and off in a specific pattern.
Examples & Analogies
Think of it like running a theater production where the stage (Port A) has its lights programmed to turn on in a specific pattern. The backstage (Port B and C) controls what switches are pulled or what additional equipment is that can send or receive messages to/from the stage.
Control Word Calculation
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Control Word Calculation (I/O Mode):
- D7 = 1 (I/O Mode)
- D6, D5 (Group A Mode) = 00 (Mode 0)
- D4 (Port A Direction) = 0 (Output)
- D3 (Port C Upper Direction) = 1 (Input)
- D2 (Group B Mode) = 0 (Mode 0)
- D1 (Port B Direction) = 1 (Input)
- D0 (Port C Lower Direction) = 0 (Output)
- Resulting Control Word (Binary): 10001010b = 8AH (Hex)
Detailed Explanation
The control word is an 8-bit value set before using the 8255 to configure its ports. Each bit in the control word corresponds to a different setting. Bit D7 indicates whether we are setting the I/O mode (1 means 'yes'). Bits D6 and D5 set the functional mode of Port A (00 means Mode 0). The direction of Port A (D4), the upper part of Port C (D3), and Port B (D1) are configured as output, input, and input respectively. The complete binary string (10001010) translates to hexadecimal (8AH) which we will write to the 8255's control register.
Examples & Analogies
Think of the control word like a list of instructions in a recipe that tell you how to prepare a dish. Each bit is like a specific ingredient or action you need to take—for instance, whether to chop an onion (input), fry it (output), or leave a part of preparation waiting (configuring).
Assembly Code Structure
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Assembly Code:
Code snippet
; Program to initialize 8255 and output static data to Port A
; Starting Address: 2000H
ORG 2000H
MVI A, 8AH ; Load Accumulator with 8255 Control Word for desired configuration
OUT 83H ; Write Control Word to 8255 CWR (Port A=Out, B=In, C_L=Out, C_U=In)
MVI A, 55H ; Load Accumulator with data to be sent to Port A
OUT 80H ; Output data (55H) to Port A (connected to LEDs)
HLT ; Halt processor execution
Detailed Explanation
This assembly code begins execution at memory address 2000H. The first command, 'MVI A, 8AH', moves the calculated control word into a register called the Accumulator. The second line outputs this control word to the control register of the 8255 using the OUT command to set the ports as specified. After setting up the control word, the next instruction loads the value 55H into the Accumulator, which represents an ON/OFF pattern for the LEDs connected to Port A. The final OUT instruction sends this value to Port A, and the process is halted at 'HLT'. This entire sequence allows the setup and then sends a fixed pattern to be displayed on the connected LEDs.
Examples & Analogies
Imagine giving step-by-step instructions to a new employee on how to set up a display for a sports event: first, you give them the layout plan (control word), then tell them what items to place and where (sending the fixed data). Each instruction corresponds with actions they must take to ensure the display looks correct.
Expected Outcomes
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Expected Outcomes:
- Port A LEDs should display the binary pattern for 55H (01010101b).
- Registers: A = 55H, PC = 2009H.
Detailed Explanation
After running the program, you would expect to see the connected LEDs corresponding to the pattern of 55H, which in binary is 01010101. This means that every alternate LED would light up: PA7 and PA5 (OFF), PA6 and PA4 (ON), and so on in that checkered pattern. Furthermore, the program counter and the accumulator register are expected to be set to values indicating the program has run successfully, showing where it stopped executing.
Examples & Analogies
It's akin to flipping a sign board where every other letter gets illuminated simply by following the program; in the same way, executing the code has illuminated specific LEDs based on the instruction set, confirming everything is functioning as intended.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Static Output: A predetermined value sent to the output port.
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Control Word Register: The register where the control word is written to configure the 8255.
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Port Functions: Understanding the role of Port A, Port B, and Port C in a given mode.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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To configure the 8255 for Mode 0, a control word of 8AH is written to the Control Word Register, allowing Port A to output static data.
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When the static byte 55H is sent to Port A, it will cause the connected LEDs to light up in an alternating pattern, corresponding to its binary representation.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To control the port and make it right, write your words both day and night.
📖 Fascinating Stories
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Once there was a microprocessor that wanted to communicate clearly with its friends (the peripherals). It learned that writing a control word would help set things right for a fun game of lights - that's how it lit up the LEDs!
🧠 Other Memory Gems
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COW: Control word sets Operations Worldwide.
🎯 Super Acronyms
PPI
- Points Processing Inputs.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: 8255 Programmable Peripheral Interface
Definition:
A versatile chip used for interfacing microprocessors with parallel input/output devices.
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Term: Control Word
Definition:
An 8-bit word that configures the operational modes of the 8255's ports.
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Term: Mode 0
Definition:
A mode of operation allowing basic input/output without handshaking signals.
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Term: Static Output
Definition:
A fixed value sent to a port, as opposed to dynamic or variable outputs.