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Interactive Audio Lesson
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Introduction to Control Word Format
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Today, we're going to explore the control word format for the 8255 PPI. Can anyone tell me why it's essential to understand the control word?
Is it because it determines how we configure the ports?
Exactly! The control word allows us to configure the ports for input or output operations, which is fundamental in interfacing processes.
Can you explain what bits control the mode and direction?
Of course! The control word has specific bits allocated for mode selection and port direction. For instance, D6 and D5 control Port A's mode.
What happens if the mode is not set correctly?
If the mode is incorrect, the communication won't function as expected. You might not receive data or control signals properly.
In summary, understanding the control word format is key to successfully interfacing the 8255.
I/O Mode Set Control Word
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Let's delve deeper into the I/O mode set control word. Who remembers what D7 represents?
D7 must always be 1 for the I/O mode set, right?
Correct! This indicates that we are setting the I/O mode. D6 and D5 configure the mode of Port A. Can anyone recall the three modes?
Mode 0 is for basic I/O, Mode 1 for strobed I/O, and Mode 2 for bidirectional I/O.
Great job! Now let's breakdown how the direction bits work. For instance, if we set D4 to 0, what does that signify?
That means Port A is configured as an output.
Exactly! And this is crucial for how we interact with the LEDs connected to the ports. In summary, the I/O Mode Set control word gives full control over the configuration.
Bit Set/Reset (BSR) Mode Control Word
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Now, let’s focus on the Bit Set/Reset mode control word. Why is it useful?
It allows us to manipulate individual bits on Port C without affecting others.
Exactly! This mode is crucial when we need to signal specific devices. Can anyone explain how the bits in this control word are structured?
D7 is always 0, and D6-D4 are don't care bits.
Perfect! D3-D1 allow us to select which bit of Port C we want to set or reset. For example, if we want to set PC4, what would that look like in binary?
It would be 0 000 100 1, which is 00001001 in binary.
Correct! So, understanding this mode can greatly enhance the interactivity of our system. Always remember the significance of manipulating bits individually.
Introduction & Overview
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Quick Overview
Standard
The control word format for the 8255 PPI specifies how to configure its operational modes and port directions. It consists of an 8-bit structure, with different bits designated for mode selection and port direction settings. Understanding this format is crucial for effectively interfacing peripherals with the 8085 microprocessor.
Detailed
The 8255 Programmable Peripheral Interface (PPI) is a versatile device used for interfacing with the 8085 microprocessor through its parallel input/output ports. The operation of the 8255 is dependent on the control word written to its Control Word Register (CWR). The control word is an 8-bit binary number, where specific bits are allocated for different functions, such as configuring the operational modes of the ports and setting the input/output direction of each port. There are two primary categories of control words: I/O mode set control words and Bit Set/Reset (BSR) mode control words.
- I/O Mode Set Control Word (D7 = 1): This control word is used when configuring the ports for I/O operations. The individual bits signify:
- D6 and D5 for selecting the operational mode of Port A (e.g., Mode 0 for basic I/O, Mode 1 for strobed I/O, Mode 2 for bidirectional I/O).
- D4 for defining the direction of Port A (input or output).
- D3 for the direction of Port C Upper (PC4-PC7).
- D2 for the group B mode.
- D1 for the direction of Port B.
- D0 for the direction of Port C Lower (PC0-PC3).
An example control word for configuring Port A as output could be 88H, representing specific configurations as per the bits defined.
- Bit Set/Reset (BSR) Mode Control Word (D7 = 0): This control word focuses on setting or resetting individual bits of Port C without affecting others. It allows for operations like an individual bit manipulation in applications where fine control of port operations is vital.
By understanding the structure and application of the control word format, users can effectively leverage the 8255 PPI in various interfacing scenarios, ensuring proper communication between the 8085 microprocessor and connected peripherals.
Audio Book
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Overview of Control Word Format
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The functionality of the 8255 is configured by writing a specific 8-bit Control Word to its Control Word Register (CWR). The format of this word depends on whether you are setting up I/O modes or using the Bit Set/Reset feature.
Detailed Explanation
The Control Word is an important component for configuring the 8255 PPI. It determines how the ports will operate—whether they will act as input or output, and in which mode. By writing a specific 8-bit binary word into the Control Word Register, you tell the 8255 how you want to use its ports. The structure of this word changes based on your chosen operation mode.
Examples & Analogies
Think of the Control Word as a set of instructions you give to a machine. Just like you might give specific commands to a coffee maker to brew coffee your way, you send specific control commands to the 8255 to configure its actions.
I/O Mode Set Control Word (D7 = 1)
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This control word is written to address 83H (or corresponding CWR address) to configure the modes and directions of Ports A, B, and C.
D7 D6 D5 D4 D3 D2 D1 D0 -------------------------- 1 M1 M0 PA PCU M2 PB PCL
- D7 (Mode Set Flag): Always 1 for I/O mode set.
- D6, D5 (Group A Mode Select - M1 M0):
- 00 = Mode 0 (Basic I/O for Port A)
- 01 = Mode 1 (Strobed I/O for Port A)
- 1X = Mode 2 (Bidirectional I/O for Port A)
- D4 (Port A Direction): 1 = Input, 0 = Output
- D3 (Port C Upper Direction - PC4-PC7): 1 = Input, 0 = Output
- D2 (Group B Mode Select - M2):
- 0 = Mode 0 (Basic I/O for Port B)
- 1 = Mode 1 (Strobed I/O for Port B)
- D1 (Port B Direction): 1 = Input, 0 = Output
- D0 (Port C Lower Direction - PC0-PC3): 1 = Input, 0 = Output
Detailed Explanation
When setting up the I/O mode, you start by writing to the Control Word Register at address 83H. The specific bits in the Control Word dictate how each port operates: Whether the ports are in input or output mode, and in which group mode. For example, if you set D6 and D5 to '00', Port A will operate in Basic I/O mode. Other bits define the direction of each port, which is essential for proper data flow.
Examples & Analogies
Imagine preparing a recipe where you specify the ingredients and their quantities. Similar to how a chef follows a written recipe to know what to do with each ingredient, the 8255 uses the Control Word to determine the role of its ports during operation.
Numerical Example for I/O Mode Control Word
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Let's configure the 8255 as follows:
- Port A = Output (Mode 0)
- Port B = Output (Mode 0)
- Port C Lower (PC0-PC3) = Output (Mode 0)
- Port C Upper (PC4-PC7) = Input (Mode 0)
Breaking down the bits for the Control Word:
- D7 = 1 (I/O Mode Set)
- D6, D5 = 00 (Group A - Mode 0)
- D4 = 0 (Port A Output)
- D3 = 1 (Port C Upper Input)
- D2 = 0 (Group B - Mode 0)
- D1 = 0 (Port B Output)
- D0 = 0 (Port C Lower Output)
Combining these bits: 1 00 0 1 0 0 0 (binary) = 10001000b = 88H (hexadecimal). So, to achieve this configuration, you would write 88H to the Control Word Register (e.g., address 83H).
Detailed Explanation
In this example, we break down the required settings for each port. Each binary digit in the Control Word corresponds to specific configurations—‘0’ means output for Port A, whereas ‘1’ signifies input for the upper half of Port C. The combination of these bits in binary translates to the hexadecimal value 88H, which is then written into the Control Word Register to set up the ports accordingly.
Examples & Analogies
Consider programming a television remote. You have different buttons that ensure the TV functions in various modes, like volume increase, channel change, etc. Similarly, each binary bit in the Control Word is like a button that sets up the 8255’s ports to function in the desired mode.
Bit Set/Reset (BSR) Mode Control Word (D7 = 0)
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This control word is used to individually set or reset any one of the 8 bits of Port C. It is also written to the Control Word Register (e.g., address 83H).
D7 D6 D5 D4 D3 D2 D1 D0 -------------------------- 0 X X X B2 B1 B0 S/R
- D7 (Mode Set Flag): Always 0 for BSR mode.
- D6, D5, D4 (Don't Care): Value does not matter (often set to 0).
- D3, D2, D1 (Bit Select - B2 B1 B0): These three bits select which of the 8 bits of Port C (PC0-PC7) is to be set or reset.
- D0 (Set/Reset): 1 = Set the selected bit, 0 = Reset the selected bit.
Detailed Explanation
In BSR mode, the Control Word format changes significantly. The D7 bit is set to 0 indicating BSR mode. The bits that follow specify which individual bit of Port C will be affected—both to either set it high or low. For example, if you want to set PC4, you would represent it in the Control Word, indicating PC4 should be high with an additional bit that says whether to set or reset it.
Examples & Analogies
Think of this mode like turning on or off specific lights in a room; instead of switching all lights at once, you can control them individually. Similarly, BSR mode allows for precise control over each bit of Port C.
Numerical Example for BSR Mode Control Word
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- To Set PC4 (make PC4 high):
- D7 = 0
- D6, D5, D4 = X (e.g., 000)
- Bit Select (for PC4) = 100b
- S/R = 1 (Set)
- Control Word: 0 000 100 1 (binary) = 00001001b = 09H.
- To Reset PC0 (make PC0 low):
- D7 = 0
- D6, D5, D4 = X (e.g., 000)
- Bit Select (for PC0) = 000b
- S/R = 0 (Reset)
- Control Word: 0 000 000 0 (binary) = 00000000b = 00H.
Detailed Explanation
In the first example, we set PC4 by crafting the Control Word appropriately, determining which bit to manipulate and its direction (set high). In the second example, we reset PC0 by simply altering the bits to reflect that action. The conversion to binary and then to hexadecimal makes it suitable for the Control Word Register.
Examples & Analogies
Consider a situation where you have an app on your phone controlling your room lights. You can individually turn on or off different lights depending on your needs. This is similar to how the BSR mode allows controlling of specific bits for creating bit-specific outputs or inputs on the 8255.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Control Word: A configuration tool for setting operational modes in the 8255 PPI.
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I/O Modes: Includes multiple operational setups for parallel I/O management.
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Bit Set/Reset Mode: Allows for individual bit manipulation within 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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Example of an I/O Mode Set Control Word: For configuring Port A as output, the control word may be 88H.
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In BSR mode, to set PC4, the control word could be 09H, where D0 is set to 1.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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D7 is a star, it shines so bright, for I/O Mode, it's always right.
📖 Fascinating Stories
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Once a controller needed to organize its castle (the 8255). It used the control word like a key to open the gates and let villagers in or out, determining who would enter the courtyard (input/output).
🧠 Other Memory Gems
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I see MOPs in binary! (M-Mode, O-Output, P-Port direction; a reminder for the control word breakdown).
🎯 Super Acronyms
B-S-R
- Bit Set/Reset Mode where each bit can get its own custom command (like superheroes with individual powers).
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Control Word
Definition:
An 8-bit word used to configure operational parameters of the 8255 PPI.
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Term: PPI
Definition:
Programmable Peripheral Interface; a device that allows microprocessors to interface with peripheral devices.
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Term: Mode Set Flag
Definition:
The flag in the control word that indicates whether the word is for setting the I/O mode or BSR mode.
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Term: Bit Set/Reset Mode
Definition:
A mode that allows individual bits of a port to be set or reset without affecting other bits.