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Interactive Audio Lesson
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Understanding Port Configuration
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Today, we will learn how to read switch inputs from Port B of the 8255 and display the information on Port C Lower. Can anyone tell me what the first step in configuring the 8255 is?
I think we need to set a control word first, right?
Exactly! The control word configures the ports for input or output. For our program, we need to configure Port B as an input and Port C Lower as an output.
What is the control word for that configuration?
Great question! For this configuration, the control word would be 10011010 in binary or 9AH in hexadecimal. What do those bits represent?
Could we break that down? Like what each part means?
Of course! The first bit indicates it's in I/O mode, the next bits configure the functionality for Groups A and B, specifying directions. For our task, bits for Port B must indicate input. Generally, these kinds of configurations help remember what sections correspond to which ports.
That's helpful! How do we write this control word?
You will load the value into the accumulator and then output it to the control word register at address 83H. Let's summarize: You configure the ports by setting the control word and write it to register using assembly language.
Programming Example and Flow
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Now, let’s look at the assembly code. The program starts at address 2000H. What do you think is the first instruction?
It would be to move the control word into the accumulator.
Correct! We use `MVI A, 9AH`. After that, we output it to the CWR using `OUT 83H`. Can anyone explain the next part?
We would need to read from Port B next using `IN 81H`, right?
Yes! That's right. What does that input instruction do for us?
It brings the state of the switches connected to Port B into the accumulator.
Exactly! After storing the data, we need to ensure we only keep the lower 4 bits for the LEDs. Can anyone remind me how we do that?
We use the `ANI 0FH` instruction to mask out the upper bits.
Great memory! Finally, we will output the result to Port C using `OUT 82H`. This flow is essential for displaying the switch states on the connected LEDs.
Expected Outcomes and Observations
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After executing the program, what kind of behavior do you expect from the LEDs connected to Port C Lower?
They should match the state of the switches connected to Port B.
Exactly! If all switches are off, the LEDs should be off too, and if PB0 is on, PC0 should light up.
Is there a way to test different switch combinations?
Yes! You can record the states of the switches in a table format to verify the outputs against your expectations.
Can we predict how many unique combinations there are for the switches?
Sure! Since we have four switches, we can have 2^4 or 16 combinations of states. Always handy to know as you compare results!
This will really help us make sense of how the inputs affect outputs in our program.
Absolutely! This understanding is crucial for interfacing and control in microcontroller applications.
Introduction & Overview
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Quick Overview
Standard
The objective is to configure Port B as an input port to read the state of switches and output that data to Port C Lower, thereby lighting up LEDs based on switch conditions. The section includes control word calculations, assembly code examples, and expected outcomes.
Detailed
Detailed Summary
In this section, the process of reading switch inputs from Port B and displaying them on Port C Lower using the 8255 Programmable Peripheral Interface (PPI) is outlined. The program objectives include configuring Port B as an input port to capture switch states and outputting this data to Port C Lower, activating corresponding LEDs. The control word calculation is critical, establishing the role of each port and the mode of operation. The detailed assembly code is provided to show how to implement this function using 8085 assembly language, along with an expected outcome, illustrating how LEDs reflect the state of connected switches. This demonstrates the effective utility of the 8255 in simple I/O operations, enhancing the understanding of data interfacing with microcontrollers.
Audio Book
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Objective of Program 3
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Objective: To configure Port B as input and Port C Lower as output (Mode 0). Read the state of 4 switches connected to PB0-PB3, then output this 4-bit data to PC0-PC3 to display on LEDs.
Detailed Explanation
The objective of Program 3 is focused on reading the state of input switches connected to Port B and reflecting their state on output LEDs connected to Port C Lower. The program operates in Mode 0, which allows for basic input/output operations without handshaking.
Examples & Analogies
Think of this program as someone taking a poll by pressing buttons (the switches) to vote. Each button pressed updates a scoreboard (the LEDs), showing the current votes. This way, everyone can see the results in real-time.
Control Word Calculation
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Control Word Calculation (I/O Mode):
- D7 = 1 (I/O Mode)
- D6, D5 = 00 (Group A - Mode 0)
- D4 (Port A Direction) = 1 (Input - arbitrary as not used)
- D3 (Port C Upper Direction) = 1 (Input - arbitrary)
- D2 = 0 (Group B - Mode 0)
- D1 (Port B Direction) = 1 (Input)
- D0 (Port C Lower Direction) = 0 (Output)
- Resulting Control Word (Binary): 10011010b = 9AH (Hex)
Detailed Explanation
The Control Word defines how the ports of the 8255 PPI will operate. In this case, the control word is set such that Port B is configured as input, Port C Lower as output, and Port A is configured as input (though it won't be used). The control word, represented in binary as 10011010, translates to 9AH in hexadecimal, which the program sends to the 8255 to set these configurations.
Examples & Analogies
Imagine this control word as a remote control's settings. When you press a button on the remote, it sends specific settings to your television, like changing the channel or adjusting the volume. The control word assigns specific functions to the 8255's ports just like the remote assigns functions to the TV.
Assembly Code Explanation
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; Program to read switches from Port B and display on Port C lower
; Starting Address: 2000H
ORG 2000H
; --- 8255 Initialization ---
MVI A, 9AH ; Load 8255 Control Word (Port A=In, B=In, C_L=Out, C_U=In)
OUT 83H ; Write Control Word to 8255 CWR
; --- Main Loop to Read and Display ---
READ_LOOP:
IN 81H ; Read data from Port B (connected to switches) into Accumulator
ANI 0FH ; Mask out higher nibble (keep only lower 4 bits for switches PB0-PB3)
OUT 82H ; Output Accumulator content to Port C (PC0-PC3 will show switch states)
JMP READ_LOOP; Jump back to READ_LOOP to continuously read
Detailed Explanation
This assembly code consists of several key sections: First, it initializes the 8255 PPI with the control word for the ports. Then, it enters a loop where it continuously reads the input from Port B using the 'IN' instruction, which captures the states of the switches into the Accumulator. The 'ANI' instruction masks any higher bits so only the relevant states of the first four switches are taken into account, and it outputs that data to Port C Lower to control the LEDs.
Examples & Analogies
Consider this loop as a teacher checking student attendance at a school. The teacher goes around the classroom (READ_LOOP) and checks if each student is present (the switches). Each student's presence or absence modifies the class roster displayed on the board (reflecting the lights on LEDs). The teacher keeps checking until class ends.
Expected Outcomes
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Expected Outcomes:
● LEDs connected to PC0-PC3 should reflect the exact binary state of the switches connected to PB0-PB3. For example, if PB0 and PB2 are ON (switches closed, assuming active high or pull-up/low logic), and PB1, PB3 are OFF, the LEDs on PC0 and PC2 should be ON.
Detailed Explanation
The expected outcome of the program is that the states of the switches directly correlate to the states of the LEDs on Port C Lower. If a switch is pressed (logic high), the corresponding LED should light up and if no switch is pressed (logic low), the LED should remain off. This real-time feedback allows users to see which switches are activated.
Examples & Analogies
Think of it like a light switch in your home. When you flip the switch, a light turns on in your room (like the LED) indicating that the switch is active. If you don't flip the switch, the light remains off, showing that the switch is inactive.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Port Configuration: Setting the appropriate bits in the control word to define each port's function.
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I/O Operations: Reading input from Port B and outputting to Port C.
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Switch States: Understanding how switch positions affect the LEDs connected to Port C.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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An example is reading the state of four switches attached to PB0 through PB3 to control the LEDs on PC0 to PC3.
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When switches PB0 and PB2 are closed (ON), the corresponding LEDs on PC0 and PC2 should illuminate.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To read switches, set the word like so, / Output to LEDs where they glow.
📖 Fascinating Stories
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Imagine a party where the lights only turn on when someone presses a switch. Each switch corresponds to a different LED, lighting up in response to the guests’ actions.
🧠 Other Memory Gems
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Remember: CBOSS - Control word, B for input, O for output to C.
🎯 Super Acronyms
PICO – Port Input, Control Output – helps to recall that ports are configured based on input and control settings.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Control Word
Definition:
An 8-bit command written to the 8255's Control Word Register to configure its ports.
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Term: Accumulator
Definition:
The register in the microprocessor where intermediate arithmetic and logic results are stored.
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Term: Input/Output (I/O)
Definition:
The process of sending or receiving data to or from external devices.
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Term: LED
Definition:
Light Emitting Diode, a type of electronic component that produces light when an electric current passes through it.
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Term: Masking
Definition:
A technique used to select certain bits of a binary value while ignoring others.