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Address Decoding for 8254
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Today, we're going to discuss how we connect the 8254 Programmable Interval Timer to the 8086 microprocessor. Can anyone tell me why address decoding is crucial in interfacing?
I think it's to ensure the microprocessor can communicate with devices connected to it correctly.
That's correct! Address decoding helps us identify which peripheral we are addressing. For the 8254, if it is placed at base address 0040H, the register addresses would be 0040H for Counter 0, 0041H for Counter 1, and so on. Remember, all communications occur at these addresses.
So, 0043H would be for the Control Word Register?
Exactly! Nice catch. This allows the 8086 to know exactly which action to perform. Can you remember the address for Counter 1?
Yes, it’s 0041H!
Great job! Remember, the control word that we send through these addresses sets the operation of the PIT.
How do we know what to include in the control word?
Good question! The control word format includes bits that determine the counter selection and operational configuration. We will learn more about that next.
Control Word Configuration
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Now that we have our addresses set, let’s dive into the control word format. Can anyone explain what the bits in the control word signify?
Isn’t it about which counter we want to select and what mode we want to use?
Exactly! The first two bits select the counter, while the next bits set the read/write mode and the operation mode. For example, if we want to configure Counter 0 in Mode 3, we will have a specific binary sequence.
What does Mode 3 do?
Mode 3 is a square wave generator. It will continuously output a square wave signal. Remember the control bits—like the first two bits representing which counter we are addressing? Anyone remember what the HEX equivalent would look like?
It becomes 36H for Counter 0 in Mode 3.
Correct! So let’s summarize: The control word is crucial for configuring each counter specifically. Who can tell me what the control word looks like in binary?
It’s 00110110!
Well done! Understanding how to configure this will let us effectively use the timer in various applications.
Programming Example of 8086 with 8254
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Let’s work through a programming example for configuring the 8254. How do we start off the Assembly code for setting it up?
We first set the control word, right?
Yes. We use `MOV AL, 36H` to load the control word. What comes next?
We would then send it to the Control Word Register with `OUT 43H, AL`.
Exactly! After that, we need to load in the count value. Which part confirms that we’re ready to generate the desired frequency?
That's right! We write the LSB first, which is followed by the MSB for the count value.
Perfect! And what was our count value for this example?
10000, which is 2710 in hexadecimal.
Well done! After executing the code, the counter generates a 100 Hz square wave on its output pin. This is how you directly control timing events using the 8254.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section covers the specific interfacing process for the 8254 Timer with an 8086 microprocessor, presenting address decoding, control word formats, and programming examples. It highlights a numerical example of configuring Counter 0 in Mode 3 (Square Wave) for signal generation.
Detailed
Interfacing and Programming Example (8086)
In this section, we explore the interfacing of the 8254 Programmable Interval Timer (PIT) with the 8086 microprocessor. The 8254 operates at a base address, which can be decoded into specific register selections necessary for communication.
Address Decoding
Assuming the 8254 is placed at a base address of 0040H, the register selections are demonstrated:
- Counter 0 Data Register: 0040H
- Counter 1 Data Register: 0041H
- Counter 2 Data Register: 0042H
- Control Word Register: 0043H
These addresses facilitate communication between the microprocessor and the timer.
Control Word Format
The control word used to configure the PIT is an 8-bit binary instruction that encodes crucial information about the operation of the timer.
Format Breakdown:
- D7 D6: Select Counter (SC1 SC0)
- D5 D4: Read/Write Mode (RW1 RW0)
- D3 D2 D1: Mode Selection (M2 M1 M0)
- D0: Binary or BCD mode (BCD)
Programming Example: Configuring Counter 0
To illustrate how to interface the 8254 PIT, let’s consider a numerical example where we configure Counter 0 for Mode 3 (Square Wave) using a desired count value of 10000 (decimal) or 2710 (hex).
Steps:
- Control Word Configuration:
- Counter 0 selected (00)
- Read/Write LSB first, then MSB (11)
- Mode 3 (011)
- Binary (0)
- Control Word in binary: 00110110 → 36H
- Assembly Code Implementation:
In this code snippet, the timer is configured to generate a 100 Hz square wave by writing to respective registers of the PIT.
Understanding this interfacing method is essential for leveraging programmable timers in various applications like real-time clocks and event counting.
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Interfacing an 8253/8254
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Interfacing an 8253/8254 involves connecting its data bus, address lines, and control signals to the microprocessor.
Detailed Explanation
Interfacing an 8253/8254 Programmable Interval Timer (PIT) means establishing a connection between the PIT and a microprocessor like the 8086. This involves connecting various lines for data, address, and control. The data bus carries actual data to and from the PIT, while address lines specify which register or component of the PIT we want to access, and control signals direct the operation (like reading or writing data). This setup is crucial for the microprocessor to communicate effectively with the PIT.
Examples & Analogies
Think of interfacing like connecting a new smart home device to your home network. You need to plug it into the correct outlet (data bus), set its IP address (address lines), and configure it via an app (control signals) to make it work with your existing system.
Address Decoding
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Address Decoding Example: Assume the 8254 is at base address 0040H.
| A1 | A0 | Register Selection | Address (if base is 0040H) |
| --- | --- | ------------------ | -------------------------- |
| 0 | 0 | Counter 0 Data Register | 0040H |
| 0 | 1 | Counter 1 Data Register | 0041H |
| 1 | 0 | Counter 2 Data Register | 0042H |
| 1 | 1 | Control Word Register (CWR) | 0043H |
Detailed Explanation
Address decoding is a method where the microprocessor determines which device or register to communicate with. In our example, the PIT (8254) has a base address of 0040H. By using the address lines A1 and A0, the microprocessor can uniquely identify which of the four registers it wants to access, allowing it to either read data from a counter or send control commands. Each combination of A1 and A0 represents a different register, ensuring the correct communication happens.
Examples & Analogies
Imagine sorting mail in a post office. The base address is like the post office location. Each letter has a specific code (like A1 and A0) that indicates which compartment the mail should go into, ensuring it gets delivered to the right recipient.
Control Word Format
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Control Word Format (8-bit):
| D7 D6 | D5 D4 | D3 D2 D1 | D0 |
| ------ | ------ | ---------- | --- |
| SC1 SC0 | RW1 RW0 | M2 M1 M0 | BCD |
| (Counter) | (R/W Mode) | (Mode) | (Binary/BCD) |
Detailed Explanation
The control word format dictates how the PIT behaves. It consists of an 8-bit word where certain bits are set aside for specific types of information. For example, bits SC1 and SC0 determine which counter (0, 1, or 2) is being configured. RW1 and RW0 establish whether we're reading from or writing to the counter as well as the operation mode (like which counting mode to use). Lastly, the BCD bit can specify if the counting format is in binary or BCD (Binary-Coded Decimal). This structure is essential for proper setup and operation of the PIT.
Examples & Analogies
Imagine a vending machine where you have to press specific buttons to get a snack. Each button represents a different function (like a counter), and the vending machine needs to know which button was pressed (RW bits) and what kind of snack you want (binary or BCD). This way, the machine can correctly dispense the product you are trying to buy.
Numerical Programming Example
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Numerical Programming Example (8086): Configure Counter 0 of an 8254 (at base 0040H) for Mode 3 (Square Wave), 16-bit binary count of 10000 (decimal), with a 1MHz input clock.
• Desired Count: 10000_decimal=2710_hex. (LSB: 10_hex, MSB: 27_hex)
• Output Frequency: 1,000,000textHz/10000=100textHz.
• Control Word:
○ Counter 0 (00)
○ Read/Write LSB then MSB (11)
○ Mode 3 (011)
○ Binary (0)
○ Control Word = 00110110 binary = 36H
Code snippet
; 8086 Assembly Code Snippet
ASSUME CS:CODE
CODE SEGMENT
START:
MOV AL, 36H ; Control word: Counter 0, R/W LSB then MSB, Mode 3, Binary
OUT 43H, AL ; Send control word to 8254 CWR (0043H)
MOV AX, 2710H ; 16-bit count (10000 decimal)
OUT 40H, AL ; Send LSB (10H) to Counter 0 Data Register (0040H)
MOV AL, AH ; Get MSB (27H)
OUT 40H, AL ; Send MSB to Counter 0 Data Register (0040H)
; Counter 0 is now generating a 100 Hz square wave on its OUT0 pin.
HLT
CODE ENDS
END START
Detailed Explanation
This programming example demonstrates how to configure Counter 0 of the PIT (8254) using 8086 assembly language. The goal is to generate a square wave at 100 Hz. First, we set up a control word to define the counter's configuration (Counter 0, write LSB first, Mode 3, binary counting). The desired count (10000 in decimal) is converted into hexadecimal. It's crucial to write the least significant byte (LSB) and then the most significant byte (MSB) to the data register to correctly set the counter value. Once the code is executed, Counter 0 will start generating a square wave output at the specified frequency.
Examples & Analogies
Consider a chef who is setting a timer for a recipe. To do this correctly, the chef must first select the timer (Control Word) and then set the correct time (10000 count). Just like writing the LSB first, the chef might set the minutes and then the seconds. Once the timer is set, it will start counting down, just like the counter generates the square wave.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Interfacing: The connection and communication between the microprocessor and peripheral devices.
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Control Word: An encoded instruction sent to the PIT to configure its operation.
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Programmable Timer: A timer that can be programmed to generate specific timing signals.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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When configuring the 8254 Timer for a square wave generation at 100 Hz, the count value is set to 10000 (decimal) or 2710 (hex).
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In Assembly language, the control word for Counter 0 configured in Mode 3 is 00110110, which is written as 36H.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To set the timer with great ease, write the word that fits like a breeze.
📖 Fascinating Stories
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Think of a race car (the clock) slowly counting laps (the timer) and each lap needs a signal (the output). Control words keep the race in line.
🧠 Other Memory Gems
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COW – Control, Output, Write. Remember to manage your timer with the correct commands.
🎯 Super Acronyms
C-DMO – Counter, Decode, Mode, Output. This acronym will help you recall the key steps in setting up the timer.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Programmable Interval Timer (PIT)
Definition:
A device that provides precise timing and waveform generation for microprocessors by offloading timing tasks from the CPU.
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Term: Control Word Register (CWR)
Definition:
An 8-bit register in the PIT where the CPU writes a control word to configure the operation of the timer.
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Term: Counter
Definition:
A component in the PIT that decrements its value with each clock pulse and generates signals based on programmed settings.
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Term: Mode
Definition:
The operational state configured in the PIT to dictate how the timer behaves, such as generating square waves or counting events.
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Term: Address Decoding
Definition:
The process of determining the specific address of registers or components in a microprocessor architecture.