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Interactive Audio Lesson
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Introduction to Open Collector and Open Drain Outputs
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Today, we will discuss gates with open collector and open drain outputs. Can anyone tell me what these terms might mean?
Are those the gates that connect to an external resistor?
Exactly! Open collector gates, typically found in TTL, and open drain gates in MOS logic families both require an external pull-up resistor to operate effectively. This resistor is pulled up to the DC power supply. What happens when the gate is not active?
It connects to the power supply through the resistor, right?
Correct! When the gate output is inactive, the resistor pulls the voltage high. Let's remember this with the mnemonic 'COLLECT UP to HIGH!' for open collector outputs. Now, do you see any advantages in this configuration?
It allows for multiple connections to work together?
Yes! This configuration enables the wire-AND operation, where multiple open collector outputs can be tied together through a common pull-up resistor. Everyone clear on this?
I think so! It sounds useful!
Great! Remember, this is a unique feature of open collector/drain gates.
Configuration and Operation
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Let's examine how we connect these gates. What do we need to connect multiple open collector gates?
We need to connect them all to a single pull-up resistor?
Yes, and that creates a common voltage level. Just to clarify, what happens when one of the gates pulls the output low?
The others can't pull it high anymore, so the output reflects an AND operation.
Exactly! You all are really getting it! We can summarize that multiple connections yield 'one low makes all low' principle. Can anyone tell me what might be a disadvantage of this configuration?
It might not be as fast as regular gates?
Right! They are slower and noisier than traditional gates, so they aren't ideal for high-speed applications. Any questions before we move on?
Practical Applications and Considerations
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In what scenarios do you think we would want to use open collector outputs instead of standard logic gates?
Maybe in simple circuits where only a few gates are used?
That's a start, but they are often used when we want to control a bus line, where devices need to share a single connection. Can anyone give me an example?
How about I2C bus communication? Those can use open drain outputs!
Excellent example! In I2C, multiple devices can pull the bus low while a pull-up resistor keeps it high. This is one of the core advantages of using open collectors. Keep in mind the trade-offs tooβwhat are they again?
Slower speeds and more noise!
Exactly! Letβs keep these considerations at the forefront when deciding your circuit designs.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
Open collector and drain outputs are unique gate configurations used primarily in TTL and MOS logic families. They utilize an external pull-up resistor to yield ANDing operations through a common connection, referred to as a wire-AND configuration, and are slower and noisier than standard gates.
Detailed
Gates with Open Collector/Drain Outputs
Open collector or open drain logic gates are special types of digital logic gates that feature non-standard outputs. Specifically, these gates require a pull-up resistor connected between the output and a positive DC power supply (V+). This configuration allows the gate output to either connect to ground (active low) or be pulled high by the resistor when the gate is inactive.
These gates are commonly found in different logic families; in TTL (Transistor-Transistor Logic), they are termed 'open collector,' while in MOS configurations, they are designated as 'open drain.' One of the remarkable advantages of open collector/drain outputs is their ability to function in an ANDing configuration. When multiple open collector gates are connected together to a common pull-up resistor, they can perform the logic AND operation without requiring a dedicated AND gate. This connection is referred to as a wire-AND connection, as represented in various examples throughout the text.
However, the performance of open collector/drain gates comes with trade-offs, primarily slower response times and greater susceptibility to noise compared to standard gates. Due to these disadvantages, they are not recommended in applications where processing speed is critical.
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Definition of Open Collector/Open Drain Gates
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These are gates where we need to connect an external resistor, called the pull-up resistor, between the output and the DC power supply to make the logic gate perform the intended logic function. Depending on the logic family used to construct the logic gate, they are referred to as gates with open collector output (in the case of the TTL logic family) or open drain output (in the case of the MOS logic family). Logic families are discussed in detail in Chapter 5.
Detailed Explanation
Open collector and open drain gates operate differently compared to regular logic gates. They require a pull-up resistor connected to a power source. This setup allows the gates to pull the output to a LOW state when activated, while the pull-up resistor brings the output HIGH when not actively being driven LOW. This functionality is essential in certain circuit designs, especially when multiple outputs need to be combined.
Examples & Analogies
Imagine a group of friends using a shared umbrella on a rainy day. Each friend can either use the umbrella ('pulling' the output low) or leave it closed (allowing it to stay up without using it). When no one uses the umbrella, it remains upright (pulled high). This reflects how open collector gates function in a circuit.
ANDing Operation with Open Collector/Open Drain Gates
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The advantage of using open collector/open drain gates lies in their capability of providing an ANDing operation when outputs of several gates are tied together through a common pull-up resistor, without having to use an AND gate for the purpose. This connection is also referred to as WIRE-AND connection.
Detailed Explanation
When multiple open collector gates are connected to a single pull-up resistor, they can create an AND operation. When any of the connected gates are activated, they pull the output to LOW. The output will only be HIGH if all the gates are inactive, effectively functioning as an AND gate with minimal components.
Examples & Analogies
Consider a group of students (the gates) in a classroom, and they all decide to keep quiet (pull the output LOW) to signal that a teacher is not in the room. The only time the room appears noisy (output HIGH) is when all the students are quiet. This showcases how open collector connections work to achieve an AND operation.
Wire-AND Connection Example
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Figure 4.26(a) shows such a connection for open collector NAND gates. The output in this case would be Y = AB + CD + EF.
Detailed Explanation
In this example, the output Y represents the combined result of multiple input signals A, B, C, D, E, and F. The equation indicates that Y will be High when none of the inputs trigger a LOW output. This effectively illustrates the WIRE-AND concept where various conditions contribute to the final result simultaneously.
Examples & Analogies
Think of a fire alarm system where multiple sensors (A, B, C, D, E, F) need to remain inactive (no fire detected) for the alarm to stay silent. If even one sensor detects fire, the system signals an alert (output goes LOW). In this scenario, all sensors must agree on no fire for silence, representing the WIRE-AND operation.
Limitations of Open Collector/Open Drain Gates
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The disadvantage is that they are relatively slower and noisier. Open collector/drain devices are therefore not recommended for applications where speed is an important consideration.
Detailed Explanation
Open collector and open drain gates generally incur longer response times because the pull-up resistor creates a delay in restoring the output to HIGH. Additionally, noise can affect these signals, leading to unreliable performance, especially in high-speed or high-frequency circuits.
Examples & Analogies
Imagine using an old dial-up internet connection instead of a fast fiber-optic line. The dial-up is prone to interference and takes a long time to connect, making it less suitable for fast communication needs. Similarly, the slower response times and susceptibility to noise make open collector/drain configurations less ideal for high-speed applications.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Open Collector: Refers to TTL gates needing external wiring.
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Open Drain: Refers to MOS gates needing external connections.
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Pull-Up Resistor: Used to ensure voltage levels when gates are not actively driven.
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Wire-AND: Multiple gates connected together to behave as an AND.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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An open collector output can be seen in many microcontrollers, which allows multiple devices to share a single connection.
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In I2C communication protocols, devices are connected via open drain outputs, ensuring that the bus line remains high until actively pulled low.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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If the output is low, then the gates do show / All are together, through wire-AND we go.
π Fascinating Stories
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Imagine multiple friends (gates) holding hands (pulling together). If one friend lets go (goes low), the whole line drops!
π§ Other Memory Gems
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Remember 'COLLECT UP' for open collectors to pull the output high!
π― Super Acronyms
GATE - Gates Attract Together for Emergency when Open to ground!
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Open Collector
Definition:
A type of output that connects to ground instead of a defined voltage level, requiring a pull-up resistor for high output.
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Term: Open Drain
Definition:
Similar to open collector, but used in MOS technology; it requires an external pull-up for proper voltage levels.
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Term: PullUp Resistor
Definition:
An external resistor used to pull the output of a gate to a high voltage level when not actively driven low.
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Term: WireAND
Definition:
A configuration where multiple open collector outputs are connected together through a pull-up resistor, behaving like an AND gate.
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Term: Logic Family
Definition:
A group of related integrated circuits that share the same logic function characteristics, like TTL or CMOS.
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Term: TTL
Definition:
Transistor-Transistor Logic, a type of digital circuit using bipolar junction transistors.
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Term: MOS
Definition:
Metal-Oxide-Semiconductor; a technology for constructing integrated circuits, including open drain configurations.