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Introduction to the 8254 Timer
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Today, we will explore the 8254 Programmable Interval Timer. Can anyone describe what a timer does in microprocessors?
I think a timer keeps track of time, but how does it do that?
Great question! The 8254 timer can generate accurate delays and square waves for essential timing applications. It has three independent counters. Each of these can be programmed for various modes of operation.
So, how do we actually program these counters?
We use an 8-bit control word! This control word is what we write to the Control Word Register to configure the timer. Let’s dive deeper into the bits of this control word.
Understanding the Control Word Structure
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The structure of the control word has the bits SC1, SC0 for counter selection. For instance, what do you think happens if we set SC1, SC0 to '00'?
'00' would select Counter 0, right?
Exactly! And RW1, RW0 determines how we read or load the counter. Can anyone tell me what '11' means for RW1, RW0?
That means we do a read/load for both the least and most significant byte.
Correct! Remember, each combination defines how our timer will behave, which is critical in timing applications. Now let’s summarize this segment.
Operational Modes of the 8254 Timer
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Now let’s discuss the operational modes. Can anyone name one of the modes available?
Mode 3, the Square Wave Generator.
Right! In Mode 3, the timer generates a continuous square wave signal. What do you think would happen if we wanted to generate a simple delay?
I believe we would use Mode 0, right?
Absolutely correct! Mode 0 allows for an interrupt upon terminal count. It's important to know which mode to select based on what we want to achieve with the timer.
BCD and Binary Counting
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The last part of our control word is the BCD bit. Can anyone explain what this means?
It specifies whether we are counting in binary or BCD, right?
Exactly! A binary counter counts from 0 to 65535, while a BCD counter counts from 0 to 99. This is useful in applications where we need decimal count representation.
So, if we set BCD to 1, we can use it for applications that require decimal display?
Yes! BCD counting is suitable for devices like digital clocks and calculators. Now, let’s summarize what we’ve learned about the control formats.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section elaborates on the 8254 timer's configuration through an 8-bit control word that dictates which counter is to be programmed, how data is read or written, the operational mode of the counter, and whether the counting will use binary or BCD format. This formatting is critical for achieving precise timing operations in various applications.
Detailed
Detailed Summary of 8254 Control Word Format
The 8254 Programmable Interval Timer (PIT) is configured through an 8-bit control word written to its Control Word Register (CWR). The control word not only specifies which of the three independent 16-bit counters is being programmed but also defines how the counting will occur, including select options for read/write operations and the mode of operation.
The format of the control word can be represented as follows:
| D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
|---|---|---|---|---|---|---|---|
| SC1 | SC0 | RW1 | RW0 | M2 | M1 | M0 | BCD |
- SC1, SC0: These bits select the counter to be programmed (Counter 0, Counter 1, or Counter 2) or trigger a read-back command (specific to 8254).
- RW1, RW0: These bits determine how the count value will be handled. Options include: latching the count, reading/loading the least significant byte only, the most significant byte only, or both.
- M2, M1, M0: These bits select the operational mode of the counter ranging from interrupt on terminal count to square wave generation, among others.
- BCD: This bit specifies whether the counting will be in Binary (0) or BCD (1).
Significance
Understanding the 8254 Control Word Format is essential for engineers and Technicians that work with microcontrollers and embedded systems, as accurate timing operations are critical in applications ranging from simple switches to complex timing processes in machinery.
Audio Book
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Overview of Control Word Register
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The 8254 is programmed by writing an 8-bit control word to its Control Word Register (CWR).
Detailed Explanation
The 8254 timer is configured using a control word, which is an 8-bit value that is written into a specific register known as the Control Word Register (CWR). Each bit in this control word has a specific role in determining how the timer operates. This programming allows the timer to function in various modes based on the needs of the application.
Examples & Analogies
Think of the control word like a remote control for a TV; different buttons on the remote correspond to different functions (like volume, channel change, etc.). Similarly, each bit in the control word 'tells' the timer what to do in a specific way.
Control Word Format Breakdown
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D7 D6 D5 D4 D3 D2 D1 D0
SC1 SC0 RW1 RW0 M2 M1 M0 BCD
Detailed Explanation
The control word is organized into different fields, each corresponding to specific functions:
- SC1 and SC0 (Select Counter): These bits choose which counter (0, 1, or 2) you want to configure.
- RW1 and RW0 (Read/Write Control): These bits determine how the timer reads from or writes to the counter. For example, they might specify whether you want to read just the least significant byte or both the least and most significant bytes.
- M2, M1, M0 (Mode Select): These bits define the operation mode of the selected counter. There are several modes available, such as a square wave generator or interrupt on terminal count.
- BCD (BCD/Binary Mode): This bit indicates whether the counting should be done in Binary or Binary-Coded Decimal (BCD) format. If the BCD bit is set, the counter will work in a 4-decade mode.
Examples & Analogies
Imagine you’re setting the timer on an oven. Each button you press corresponds to a different setting. You press a button to choose the oven temperature (like choosing a counter), another to choose whether you want a quick bake or slow roast (mode selection), and so forth. The control word is like the combination of buttons pressed to set up the oven just the way you want.
Selecting the Counter
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SC1, SC0 (Select Counter): Selects which counter is being programmed.
○ 00 = Counter 0
○ 01 = Counter 1
○ 10 = Counter 2
○ 11 = Read-Back Command (8254 only)
Detailed Explanation
In the control word, the first two bits (SC1 and SC0) are used to select which of the three possible counters you want to program. There are four possible combinations:
- 00 selects Counter 0.
- 01 selects Counter 1.
- 10 selects Counter 2.
- 11 is a special command used only for the 8254, allowing you to read back the current configuration or count of the counters.
Examples & Analogies
Consider a multi-lane highway where each lane corresponds to a different counter. If you signal to go into the left lane, you're choosing to use that lane for your travel (programming that specific counter). You can only use one lane at a time for a specific journey, just like you can program one counter at a time.
Read/Write Control Settings
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RW1, RW0 (Read/Write Control): Specifies how the count value is loaded/read.
○ 00 = Latch count (read-back command only)
○ 01 = Read/Load LSB byte only
○ 10 = Read/Load MSB byte only
○ 11 = Read/Load LSB then MSB byte
Detailed Explanation
The next two bits in the control word are RW1 and RW0. They dictate how the timer will handle data:
- 00 indicates you want to latch the count, which is used primarily during read-back operations.
- 01 means you want to only read or load the least significant byte (LSB) of the count.
- 10 means you'll deal only with the most significant byte (MSB).
- 11 allows loading or reading both bytes sequentially, which is often needed for setting up or checking larger count values.
Examples & Analogies
Think about filling a glass of water. If you need just a quick sip, you might only pour a little in (loading LSB). If you're preparing for a party and need a full glass, you pour in more water, using both the top and bottom of the glass (loading both LSB and MSB).
Mode Selection
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M2, M1, M0 (Mode Select): Selects the operating mode of the counter.
○ 000 = Mode 0 (Interrupt on Terminal Count)
○ 001 = Mode 1 (Hardware Retriggerable One-Shot)
○ 010 = Mode 2 (Rate Generator)
○ 011 = Mode 3 (Square Wave Generator)
○ 100 = Mode 4 (Software Triggered Strobe)
○ 101 = Mode 5 (Hardware Triggered Strobe)
Detailed Explanation
The next three bits in the control word (M2, M1, M0) are used to specify the operational mode of the selected counter. Each mode has different functional characteristics:
- Mode 0: Generates an interrupt when the count reaches zero.
- Mode 1: This mode is a one-shot timer that requires a retrigger signal to start counting again.
- Mode 2: Functions as a rate generator, repeatedly generating pulses.
- Mode 3: Creates a continuous square wave.
- Mode 4: Allows the user to initiate a strobe signal.
- Mode 5: Works similarly but is triggered by external hardware.
Examples & Analogies
Selecting a mode is like choosing a feature on a smartphone. You can set it to silent (Mode 0 - no interruptions), take a photo (Mode 1 - one-shot), play music (Mode 2 - continuous), and so on. Just as each smartphone function serves a different purpose, each counter mode is designed for specific timing applications.
BCD/Binary Mode Selection
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BCD (BCD/Binary Mode):
○ 0 = Binary Counter (16-bit)
○ 1 = BCD Counter (4-decade)
Detailed Explanation
The last bit in the control word is used to select the counting mode:
- A value of '0' indicates that the counter will operate in a standard binary mode, allowing it to count all 16 bits (0 to 65535).
- A value of '1' means the counter will treat the counts in Binary-Coded Decimal format, which can count 0 to 9 for each decimal place and is limited to four-bits per decimal digit, hence four-decade counting.
Examples & Analogies
Using binary mode is like counting using fingers, where you can represent 0-15 with two hands (16 bits). On the other hand, using BCD mode is like counting using a tally system, where you only represent numbers up to 9 and need to reset after reaching that number.
Numerical Example for Control Word
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To configure Counter 0 for Mode 3 (Square Wave Generator), to load LSB then MSB, and to count in Binary:
● SC1 SC0 = 00 (Counter 0)
● RW1 RW0 = 11 (Read/Load LSB then MSB)
● M2 M1 M0 = 011 (Mode 3)
● BCD = 0 (Binary)
● Resulting Control Word (Binary): 00110110b = 36H (Hex)
Detailed Explanation
This example illustrates how to construct a control word for configuring the 8254's first counter:
- By selecting Counter 0 (00), you indicate the first counter.
- By setting RW1 RW0 to 11, you prepare to load both the least and most significant bytes.
- The group setting 011 is what selects Mode 3 for a square wave generator operation, and by choosing BCD as 0, you ensure binary counting is used.
- When all these bits are combined, they form the control word 00110110, which converts to hexadecimal as 36H.
Examples & Analogies
Constructing the control word is similar to writing a recipe. Each ingredient (or bit) you add contributes to the final dish (or functionality of the timer). Just like a specific combination of ingredients leads to a unique flavor, the right combination of bits defines the timer's operation.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Control Word Format: Defines how to program the 8254 timer.
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Counter Selection: Bits SC1 and SC0 determine which of the three counters to configure.
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Read/Write Control: Bits RW1 and RW0 specify reading or loading of count values.
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Mode Selection: M2, M1, and M0 define the operational mode of the counter.
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BCD vs Binary Counting: BCD allows for decimal count representation, while binary counts in numerical values.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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To configure Counter 0 in Mode 3, the control word would be set to 00110110, representing the necessary settings for generating a square wave signal.
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If we wanted to load a count of 100 into the timer for a square wave with the Maximum Clock Frequency (M_clk), we would set necessary bits in the control word accordingly.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To configure the Timer 8254, remember this to explore: SCs for counters, RWs for reads, modes decide the output from our needs.
📖 Fascinating Stories
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Imagine a clockmaker's shop with three clocks. The clockmaker uses different tools (the control word) to make each clock tick just right for its purpose.
🧠 Other Memory Gems
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Remember 'SCRWMD' - Select Counter, Read/Write control, Mode, and Discuss counting types to configure the 8254.
🎯 Super Acronyms
Use 'BMRC' to remember
- BCD Mode
- Read/Load Count.
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 word used to configure the 8254 timer, specifying counter selection, read/write control, operational mode, and counting mode (BCD/Binary).
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Term: Counter
Definition:
A component in the 8254 timer that counts pulses and can operate in various modes to generate different outputs.
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Term: BCD (BinaryCoded Decimal)
Definition:
A binary encoding of decimal numbers where each digit is represented by its own binary sequence.
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Term: Square Wave Generator
Definition:
A mode of the 8254 timer that produces a square wave output with a defined frequency.