8254 Internal Addressing
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Overview of the 8254 Internal Addressing
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Today, we'll explore the internal addressing of the 8254 timer. Why do you think clear addressing is important when interfacing with a timer?
I think it helps ensure that we communicate with the correct part of the hardware.
Absolutely! Clear addressing helps us avoid confusion by specifying which counter or the control register we're referring to. For the 8254, if we set the base address to 90H, what addresses correspond to Counter 0, 1, 2, and the Control Word Register?
Counter 0 would be 90H, Counter 1 is 91H, Counter 2 is 92H, and the Control Word Register is 93H.
Correct! Each counter and register has its unique address which allows precise control.
Understanding the Control Word Format
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Let's move on to the control word format. The 8254 is programmed using an 8-bit control word. Does anyone remember what these bits control?
They control which counter is selected, the mode of operation, and whether it's binary or BCD counting.
Exactly! The control word consists of different bits representing the selected counter, read/write options, and the counting mode. Can anyone tell me what the meaning of the SC1 and SC0 bits are?
They select which counter we're programming!
Good job! This ensures we can directly interact with the desired counter efficiently.
Modes of Operation in 8254
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Now let's discuss the operational modes of the 8254. We have several modes. Can anyone name one of them?
There's Mode 0, which generates an interrupt on terminal count!
Correct! Mode 0 signals when the counter reaches zero. What about another mode?
Mode 3 generates a square wave signal, right?
Exactly! Mode 3 creates a square wave with a specified frequency. Understanding these modes is critical for applying the 8254 effectively in real projects.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The 8254 internal addressing section provides a thorough overview of how to communicate with the 8254 timer by explaining its I/O addressing, control word structure, and operational modes. It emphasizes the programming aspects needed to utilize the timer effectively in microprocessor applications.
Detailed
Detailed Summary of 8254 Internal Addressing
The 8254 Programmable Interval Timer presents specific I/O addressing and control word structures essential for its operation in microprocessor systems. The 8254's I/O addresses are set by external address decoding logic and internal A0/A1 pins. If the base address is configured to 90H, the addressing structure is as follows:
- Counter 0: 90H (A1=0, A0=0)
- Counter 1: 91H (A1=0, A0=1)
- Counter 2: 92H (A1=1, A0=0)
- Control Word Register (CWR): 93H (A1=1, A0=1)
To program the 8254, an 8-bit control word is written to its Control Word Register, which determines various parameters such as counter selection, read/write control, modes of operations, and counting method (BCD or binary). The control word consists of bits that signify the selected counter, the read/write function, mode of operation, and whether the counter operates in binary or BCD format. The operating modes relevant to the 8254 include:
- Mode 0: Interrupt on Terminal Count
- Mode 2: Rate Generator / Divide-by-N Counter
- Mode 3: Square Wave Generator
These operational modes enable specific functionalities such as generating delays or continuous signals, making the 8254 a crucial component in embedded systems for timing and control applications.
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I/O Addressing of 8254
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Chapter Content
The 8254's I/O addresses are determined by external address decoding logic and its internal A0/A1 pins.
If the 8254 is decoded to a base address of 90H:
β Counter 0: 90H (A1=0, A0=0)
β Counter 1: 91H (A1=0, A0=1)
β Counter 2: 92H (A1=1, A0=0)
β Control Word Register (CWR): 93H (A1=1, A0=1)
Detailed Explanation
The I/O addressing mechanism allows the microprocessor to communicate with the 8254 timer. Address lines A0 and A1 form the basis for selecting which specific register of the 8254 is being accessed. For example, if the 8254 is set up with a base address of 90H, the microprocessor can access:
- Counter 0 at address 90H,
- Counter 1 at address 91H,
- Counter 2 at address 92H,
- And the Control Word Register at address 93H. This organized addressing is crucial because it enables precise communication and configuration of the device.
Examples & Analogies
Think of the addresses like different rooms in a building with a specific address. Each room (or register) serves a different function. Just like you would need to know the exact address to find a specific room, the microprocessor needs to know these I/O addresses to access the correct counters or registers in the 8254.
Purpose of Control Addresses
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Chapter Content
The 8254 is programmed by writing an 8-bit control word to its Control Word Register (CWR).
D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 RW1 RW0 M2 M1 M0 BCD
Detailed Explanation
The Control Word Register (CWR) is essential for configuring the operation of the 8254. When the control word (8-bit instruction) is written to the CWR, it defines parameters such as which counter to operate (SC1, SC0), how the count value will be loaded or read (RW1, RW0), the mode of operation (M2, M1, M0), and whether the counting mode is binary or BCD (BCD). Each bit of the control word is crucial for specific functionality, enabling the timer to function for various applications.
Examples & Analogies
Imagine the CWR like a remote control for a television. Each button on the remote corresponds to different functionsβchanging the channel vs. adjusting the volume, etc. Similarly, each bit of the control word configures how the 8254 will operate, just as pressing different buttons modifies the televisionβs behavior.
Configuring the 8254
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To configure Counter 0 for Mode 3 (Square Wave Generator), to load LSB then MSB, and to count in Binary:
β SC1 SC0 = 00 (Counter 0)
β RW1 RW0 = 11 (Read/Load LSB then MSB)
β M2 M1 M0 = 011 (Mode 3)
β BCD = 0 (Binary)
β Resulting Control Word (Binary): 00110110b = 36H
Detailed Explanation
Configuring the 8254 involves setting the appropriate bits in the control word based on the desired operation. For example, to set up Counter 0 for generating a square wave (Mode 3), you specify the counter to be used (00 for Counter 0), indicate that you want to load the least significant byte (LSB) and then the most significant byte (MSB), set the mode to square wave mode, and select binary counting. The resulting binary control word is 00110110, which in hexadecimal translates to 36H.
Examples & Analogies
Configuring the 8254 can be likened to programming a microwave. You need to specify the power level and duration; similarly, with the control word, you define how the timer should behave. Just as different settings produce varied heating results, the different configurations in the control word lead to distinct timer operations.
Key Concepts
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I/O Addressing: Defines memory locations for accessing timer functions.
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Control Word: An essential instruction format used to program the timer.
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Modes of Operation: The various predefined functions of the timer that determine its behavior.
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BCD vs Binary Counting: Different methods employed for counting which affect output.
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Square Wave Generation: A functional mode aimed at producing continuous waveforms.
Examples & Applications
If the 8254 is assigned a base address of 90H, then Counter 0 is accessed at 90H, Counter 1 at 91H, and so forth, facilitating organized communication.
A sample control word to set Counter 0 in Mode 3 could be represented as 00110110b, indicating it's ready for square wave generation.
Memory Aids
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Rhymes
To time and control the way we play, the 8254 helps us every day.
Stories
Imagine a chef in a kitchen who uses a special timer (8254) to ensure every dish is cooked to perfection. Each counter represents a different dish, and the control words help the chef manage cooking times precisely.
Memory Tools
Remember 'I-C-M' for I/O Address, Control Word, and Modes of Operation of the 8254.
Acronyms
Use 'BMS' to remember Binary, Mode, and Square wave for operational functionalities.
Flash Cards
Glossary
- I/O Addresses
Memory addresses used to communicate with peripheral devices, defining where data can be read from or written to.
- Control Word
An 8-bit code sent to the control register of a device to configure its operational parameters.
- Mode of Operation
Specific operational settings that define how a timer functions and responds to inputs.
- BCD Counting
Binary-Coded Decimal counting method where each digit is represented by its own binary sequence.
- Square Wave Generator
Mode that generates continuous square waves by altering output states based on a counter's value.
Reference links
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