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Interactive Audio Lesson
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Introduction to the 8254 Timer and Mode 0
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Today we're going to explore how to use the 8254 timer in Mode 0 to generate time delays. Can anyone tell me why time delays might be needed in a microprocessor system?
We need delays to allow hardware processes, like waiting for an input signal or timing certain operations.
Exactly! In Mode 0, the timer will generate an output pulse once the counter reaches zero, which serves as your timing signal. This is useful for tasks like creating precise delays.
How do we set the timer to count down to generate a specific delay?
Good question! First, we calculate the count based on the desired delay and the timer's clock frequency, which we will cover next.
Counting and Delay Calculation
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To generate a delay, we need to calculate the initial count. If we want a 50 ms delay with a 1 MHz clock, what would be our count?
Wouldn't it be 50,000?
That's correct! You multiply the required delay in seconds by the clock frequency. Now, let's write a control word to set up the timer.
What format do we need for the control word?
The control word is an 8-bit representation where the first two bits select the counter, the next two bits define the read/write mode, followed by the mode selection bits, and finally the binary/BCD selection.
Programming with Assembly Code
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Now that we have our control word and count, let’s program it using assembly code. Can anyone outline the steps we need to follow?
First, we need to load the control word into the timer's control register.
Then we write the LSB and MSB of the count to the timer's counter register.
Exactly! Here’s how the code starts: `MVI A, 30H` to load the control word followed by outputs to the control register and counter register. Let’s proceed to discuss what happens next.
Expected Outcomes and Observations
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After running our timer program, what output should we expect from the OUT pin?
The OUT pin should go high after 50 ms since we set the timer for that delay.
That's correct. If we connect an LED to that pin or use an oscilloscope, we should visually observe this behavior.
And if we programmed it correctly, this pulse would indicate the terminal count reached.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
This section provides an overview of programming the 8254 timer to generate precise time delays using Mode 0. It includes theoretical background, example calculations, assembly code, and expected outcomes of the timer configuration.
Detailed
Overview
This section details the configuration of the 8254 Programmable Interval Timer to produce a specific time delay using Mode 0. The 8254 timer features multiple modes of operation, and Mode 0 is particularly utilized for generating a single pulse delay upon reaching a terminal count. This is crucial in system operations where timed intervals are essential, such as in embedded systems and electronics.
Key Concepts
- Mode 0 (Interrupt on Terminal Count): The counter, upon reaching zero, generates an output signal. This mode is primarily used for single-event timing.
- Initial Count Calculation: The required counts to achieve a desired delay can be calculated based on the timer's input clock frequency. For example, configuring a 50 ms delay with a 1 MHz clock would involve loading a count of 50,000 into the timer.
- Control Word Format: Configuring the timer involves writing an 8-bit control word specifying the operation mode, read/write format, and counter selection.
Practical Application
The section explains how to set up the timer in a microprocessor environment using assembly code, allowing students to observe the timer in action on hardware or simulations. The practical application reinforces theoretical knowledge, fostering a deeper understanding of time delays in computing systems.
Audio Book
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Objective
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To configure 8254 Counter 0 in Mode 0 to generate a specific time delay (e.g., 100 ms) and indicate the end of the delay (e.g., by toggling an LED).
Detailed Explanation
The primary goal of this program is to use the 8254 timer to create a precise delay. In this case, the program aims to create a delay of 100 milliseconds, which is a small window of time that can be crucial for timing-sensitive applications. However, given that the maximum count of a 16-bit counter like those in the 8254 is 65535, the timing may require adjustment if the chosen delay exceeds this value.
Examples & Analogies
Think of it as a timer on your microwave: you set it to ensure your food is heated for exactly the right amount of time. Similarly, the 8254 timer is being programmed to act like that microwave timer, signaling when the specified time (100 ms) has lapsed.
Calculation of Delay
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For 100 ms delay, Count = Delay * F_clk = 0.1 s * 1,000,000 Hz = 100,000. Since 100,000 is greater than 65535 (16-bit max), we need to adjust or use a different counter approach or lower clock frequency. Let's aim for a 50 ms delay instead for a 1MHz clock. Count = 0.050 s * 1,000,000 Hz = 50,000 (0C350H).
Detailed Explanation
To calculate how to set up the timer for the delay, we multiply the desired delay (in seconds) by the frequency of the timer input clock (F_clk). In this scenario, the input clock runs at 1 MHz, meaning it completes 1 million cycles in one second. When we compute the necessary count for a 50 ms delay (which equals 0.050 seconds), we find that we need to load a value of 50,000 into the timer to achieve this delay.
Examples & Analogies
Imagine you're preparing a cake that needs to bake for exactly 50 minutes. You wouldn't just guess the time; instead, you'd use a timer to ensure you’re precise. Similarly, here we utilize calculations to set our timer just right to signal us after 50 ms, ensuring accurate timing in our program.
Control Word for 8254
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Control Word for Counter 0 (Mode 0, LSB/MSB, Binary): SC1 SC0 = 00 (Counter 0), RW1 RW0 = 11 (Read/Load LSB then MSB), M2 M1 M0 = 000 (Mode 0), BCD = 0 (Binary). Resulting Control Word: 00110000b = 30H (Hex).
Detailed Explanation
The control word is an essential part of configuring the timer's behavior. It consists of several bits indicating which counter to use, how to load the count value, and what mode to operate in. In this case, we select Counter 0, indicate we want to load both the least significant byte (LSB) and most significant byte (MSB), and set it to Mode 0 – which is designed for generating a delay based on terminal counts. After interpreting these settings, we arrive at the hexadecimal value 30H to configure the timer.
Examples & Analogies
Think of a control word like setting the dials on an oven. You turn the dials to specify which oven you want to use, what temperature you need, and how long to cook. Each bit in the control word tells the 8254 exactly how to operate, just like the dials tell an oven how to cook your meal.
Assembly Code Overview
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; Program to generate a 50ms delay using 8254 Counter 0 in Mode 0
; Starting Address: 2000H
ORG 2000H
START:
; --- Configure 8254 Counter 0 ---
MVI A, 30H ; Load Control Word for Counter 0, Mode 0, LSB/MSB load, Binary
OUT 93H ; Write to 8254 CWR (assume CWR at 93H)
MVI A, 50H ; Load LSB of count (50000 decimal = C350H) -> 50H
OUT 90H ; Write LSB to Counter 0 register (assume Counter 0 at 90H)
MVI A, 0C3H ; Load MSB of count -> C3H
OUT 90H ; Write MSB to Counter 0 register
; --- Wait for Timer to Finish ---
MVI A, 01H ; Assume LED connected to PA0, turn it ON initially
OUT 80H ; Output to 8255 Port A (assuming 8255 configures PA0 as output)
HLT ; Halt processor. The timer is now running.
Detailed Explanation
This assembly code details the steps required to configure the 8254 timer to create a 50 ms delay. It begins by writing the control word to set it in the desired mode, proceeding to load the low and high bytes of the count value into the counter registers. The code also indicates that the program will raise an LED at the appropriate port, signaling that the delay period has started. The final command halts the processor, allowing the timer to operate independently until its count reaches zero.
Examples & Analogies
Consider writing a recipe where you prepare ingredients and set an oven to bake. This code is similar to the steps laid out for preparing the ingredients, where each line has a specific purpose—just like the recipe does for baking your dish. The processor, like you waiting by the oven, pauses until the timer signals it's time to act.
Expected Outcomes
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Expected Outcomes: After the program executes, the 8254 Counter 0 is configured to count down from 50,000. If GATE0 is high and CLK0 is active, its OUT0 pin will go high after 50 milliseconds from when the count was loaded.
Detailed Explanation
After executing the program, the 8254 timer should begin counting from the preset value of 50,000 down to zero. Assuming that the gate input for the counter is enabled, the timer's output pin will reflect this operation by going high after 50 milliseconds. This change can be perceived by an attached LED, which will light up at the expected time, confirming that the delay was successfully implemented.
Examples & Analogies
It's like setting a timer on your phone to alert you after 50 minutes. Once the time elapses, your phone vibrates or rings to signal you. Similarly, the 8254 outputs a signal in the form of a high signal at its output pin once the 50 ms have passed, indicating the timer has counted down as expected.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Mode 0 (Interrupt on Terminal Count): The counter, upon reaching zero, generates an output signal. This mode is primarily used for single-event timing.
-
Initial Count Calculation: The required counts to achieve a desired delay can be calculated based on the timer's input clock frequency. For example, configuring a 50 ms delay with a 1 MHz clock would involve loading a count of 50,000 into the timer.
-
Control Word Format: Configuring the timer involves writing an 8-bit control word specifying the operation mode, read/write format, and counter selection.
-
Practical Application
-
The section explains how to set up the timer in a microprocessor environment using assembly code, allowing students to observe the timer in action on hardware or simulations. The practical application reinforces theoretical knowledge, fostering a deeper understanding of time delays in computing systems.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Calculating counts for a 50 ms delay with a 1 MHz clock results in loading a count of 50,000.
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Configuration example for Mode 0: Control Word set to 30H initializes the timer appropriately.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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When counting down to zero, here’s the trick, a pulse will fire when the count is quick.
📖 Fascinating Stories
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Imagine a race where the timer counts down to call the finish line; at precisely the right moment, a flag waves to signal a count!
🧠 Other Memory Gems
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C.O.U.N.T - Control Word, Output, Update Count, necessary Timer setup.
🎯 Super Acronyms
P.U.L.S.E - Program the timer, Use the control word, Load counts, Signal output, Ensure timing.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: 8254 Timer
Definition:
A programmable interval timer that can be configured to generate delays, square waves, and count events.
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Term: Mode 0
Definition:
An operational mode of the 8254 timer that generates a single pulse output when the counter reaches zero.
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Term: Control Word
Definition:
An 8-bit instruction sent to the timer to configure its operating mode, counter selection, and read/write settings.