The Shift Register: Explained [74HC595]

electronica
29 Aug 201306:04
EducationalLearning
32 Likes 10 Comments

TLDRThis video introduces the 74HC595 shift register, a versatile component for controlling multiple outputs with limited microcontroller GPIO pins. The presenter explains how shift registers can manage an extensive number of LEDs or other devices by shifting data bit by bit. The 74HC595, with its 16 pins including output, control, power, and an extension pin, is used as an example. The video demonstrates the functionality of the shift register using push buttons, illustrating the process of latching data, shifting bits, and controlling outputs. It also covers chaining multiple shift registers for extended control and the use of pull-up and pull-down resistors to prevent false triggering. The explanation is aimed at helping viewers understand the practical application of shift registers in projects like LED arrays.

Takeaways
  • 😀 The 74HC595 is a commonly used shift register for controlling multiple outputs with limited GPIO pins.
  • 🔄 A shift register allows for the control of an extended number of devices by cascading multiple units together.
  • 🚀 Shift registers are particularly useful for microcontrollers like Arduino that have a limited number of GPIO ports.
  • 💡 The 74HC595 can control 100 LEDs with only three pins from an Arduino by using 13 shift registers.
  • 📦 The 74HC595 has 16 pins including 8 output pins, 5 control pins, 2 power pins, and an extension pin.
  • 🔧 The output enable pin on the 74HC595 is used to turn the outputs on or off and is active low.
  • 🔄 The SR clear pin is used to clear the internal memory of the shift register and is also active low.
  • 🕒 The three main control pins for the 74HC595 are the serial data input (DS), the shift register clock (SH_CP), and the storage register clock (SR_CP).
  • 🔀 The shift register latches data bit by bit and can copy the memory content to the output without clearing the memory.
  • 🔆 The H' (output H prime) pin is used for cascading multiple shift registers and shifts data into the next shift register's memory.
  • 🔌 Pull-up and pull-down resistors are used to prevent false triggering of inputs and maintain an inactive state when not activated.
Q & A

  • What is a shift register and why is it useful?

    -A shift register is a digital circuit that moves or 'shifts' a group of binary digits (bits) step by step. It's useful because it allows you to control a large number of devices, such as LEDs, with a limited number of input/output pins on a microcontroller, like an Arduino.

  • How does the 74HC595 shift register differ from other shift registers?

    -The 74HC595 is representative of almost all shift registers on the market, but it specifically has 16 pins, including eight output pins, five control pins, two power pins, and one extension pin. It is one of the most generic and commonly used shift registers.

  • What are the three most important control pins on the 74HC595 shift register?

    -The three most important control pins on the 74HC595 are the Serial Clock (SCLK), the R Clock (RCLK), and the Serial Data (SER) pin. These pins are crucial for shifting data into memory and controlling the output.

  • How can you control more than eight LEDs with a single 74HC595 shift register?

    -You can't control more than eight LEDs with a single 74HC595 shift register alone. To control more LEDs, you would need to daisy chain multiple shift registers together, using the output of one to control the input of the next.

  • What is the purpose of the Output Enable (OE) pin on the 74HC595 shift register?

    -The Output Enable (OE) pin on the 74HC595 shift register is used to turn the outputs on or off. If the OE pin is driven low (connected to ground), the outputs are enabled (turned on). If it's driven high (connected to the 5V rail), the outputs are disabled (turned off).

  • How does the SR Clear pin work on the 74HC595 shift register?

    -The SR Clear pin on the 74HC595 is used to clear the internal memory of the shift register. It is active low, meaning that driving the pin low will clear the memory, resetting it to zero.

  • What is the role of the H' (Output Strobe) pin in cascading shift registers?

    -The H' (Output Strobe) pin plays a crucial role in cascading shift registers. It goes high when a '1' is shifted into bit 7 and goes low when that bit is shifted into bit 8. This pin can be connected to the data line of the next shift register in the chain, allowing the cascading of multiple registers.

  • Why are pull-up and pull-down resistors used in the demonstration with push buttons?

    -Pull-up and pull-down resistors are used to ensure that the pins are in an inactive state when not being activated by a button press. They provide a default state for the pins to prevent false triggering and to ensure a stable logic level.

  • Can you explain the concept of 'bouncing' mentioned in the script in relation to push buttons?

    -Bouncing refers to the mechanical chatter of a switch, like a push button, that can cause multiple changes in the signal even though the button is pressed only once. This can result in a shift register interpreting multiple shifts when only one was intended.

  • What is the significance of the Rising Edge of a clock signal in the context of a shift register?

    -The Rising Edge of a clock signal is the transition from a low to a high state. In the context of a shift register, the memory samples the data line on the Rising Edge of the input clock, effectively shifting the data into memory.

  • How can shift registers produce a scrolling effect with LEDs?

    -Shift registers can produce a scrolling effect with LEDs by continuously shifting in new bits and updating the output. Since the memory is not cleared when copied to the output, this allows for the creation of dynamic patterns and effects, such as scrolling text or images.

Outlines
00:00
😀 Introduction to Shift Registers and the 74HC595

This paragraph introduces the concept of shift registers, highlighting their utility in controlling multiple outputs with limited input pins on microcontrollers like Arduino. The 74HC595 shift register is presented as a generic and useful example. It explains how shift registers can manage numerous LEDs or other components using only a few pins by latching and shifting data bit by bit. The 74HC595's pin configuration is detailed, including its 8 output pins, 5 control pins, 2 power pins, and an extension pin for output enable. The functions of the control pins—data line (SER), output clock (RCLK), and input clock (SCLK)—are clarified, along with the active low nature of the output enable and SR clear pins. The paragraph concludes with a demonstration of how to use a shift register with push buttons, emphasizing the importance of pull-up and pull-down resistors to prevent false triggering due to button bounce.

05:01
😀 Expanding Shift Register Capabilities with Serial Connection

The second paragraph delves into connecting multiple shift registers to control more than eight LEDs. It explains the role of the H' (output enable) pin in cascading shift registers, which goes high when a '1' is shifted into bit 7 and low when shifted into bit 8, and vice versa for a '0'. The paragraph illustrates how to link the H' pin of the first shift register to the data line of the second, allowing the output clocks and input clocks of the first to synchronize with those of the second. This setup effectively extends the number of controllable outputs by an additional eight, as the two shift registers function as one unit. The paragraph encourages further exploration and experimentation with shift registers for a deeper understanding of their capabilities.

Mindmap
Keywords
💡Shift Register
A shift register is a type of digital memory storage device that stores data in a series of flip-flops. In the context of the video, it is used to control multiple LEDs or other devices with limited GPIO (General Purpose Input Output) pins on a microcontroller, such as an Arduino. The video explains that shift registers can effectively control an 'infinite' number of devices by cascading them, as long as there are enough shift registers available.
💡74HC595
The 74HC595 is a specific type of shift register that the video focuses on. It is an 8-bit serial-in, parallel-out shift register with a storage register and output latch. The video uses the 74HC595 as an example to illustrate how shift registers work, highlighting its 16 pins, including 8 output pins, 5 control pins, and 2 power pins, which are typical for many shift registers in the market.
💡GPIO Ports
GPIO stands for General Purpose Input Output, which are the pins on a microcontroller that can be programmed to read or write data. The video mentions that microcontrollers like Arduino have a limited number of GPIO ports, and shift registers can be used to expand these ports to control more devices, such as LEDs, than the microcontroller's GPIO can handle on its own.
💡Latch
In the context of shift registers, a latch is a circuit that stores data. The video explains that a shift register latches data bit by bit and can hold eight bits in the case of the 74HC595. The data is stored in the shift register's memory and can be shifted out to the output pins without losing the data stored in memory.
💡Output Enable Pin
The output enable pin is a control pin on the shift register that determines whether the outputs are active or not. As described in the video, it is active low, meaning that when the pin is driven low (connected to ground), the outputs are turned on. Tying it to the 5V rail turns the outputs off, and the states in memory are preserved until the output enable pin is driven low again.
💡SR Clear
The SR clear, or Shift Register clear, is a control pin used to clear the internal memory of the shift register. The video mentions that it is active low, so driving this pin low will clear the memory. It is important to tie this pin to the 5V rail when it is not in use to prevent accidental clearing of the memory.
💡Clock Pins
Clock pins are essential for the operation of shift registers. The video distinguishes between the Serial Clock (SCLK) and the Register Clock (RCLK), which are renamed in the script to the data line (input clock) and the output clock (output clock) for clarity. The input clock shifts data into memory, while the output clock triggers the copying of data from memory to the output pins.
💡Data Line
The data line, also referred to as the Serial Data Input (DS) in the context of the 74HC595, is the pin where data is input into the shift register. The video demonstrates how data is latched into the shift register's memory on the rising edge of the input clock when the data line button is pressed.
💡Output Clock
The output clock, or Register Clock (RCLK) in the context of the 74HC595, is the control pin that triggers the transfer of data from the shift register's memory to the output pins. The video shows that when the output clock button is pressed, the LEDs light up according to the data stored in memory, demonstrating the output clock's function.
💡H' Prime
The H' Prime, or the output of the storage register, is a feature of the 74HC595 that allows cascading multiple shift registers. The video explains that it goes high on the rising edge of the input clock when a '1' is shifted into bit 7 and goes low when shifted into bit 8. This feature is crucial for controlling more than eight LEDs by connecting multiple shift registers in a chain.
💡Pull-Up and Pull-Down Resistors
Pull-up and pull-down resistors are used to maintain a default state on a pin to prevent it from floating, which can cause erratic behavior. In the video, pull-down resistors are used to connect buttons to ground, and pull-up resistors are used to connect them to the positive voltage. These resistors ensure that the buttons can override the inactive state set by the resistors and are essential for stable operation of the shift register interface.
Highlights

Introduction to the 74HC595 shift register, a fundamental component for controlling multiple outputs with limited microcontroller GPIO ports.

Explanation of how a shift register can control an infinite number of devices with a finite number of shift registers.

The 74HC595 shift register can control 100 LEDs with only three pins on an Arduino.

Shift registers latch data bit by bit and can output eight bits at a time.

The 74HC595 has 16 pins including eight output pins, five control pins, and two power pins.

The output enable pin controls the activation of the shift register's outputs.

The SR clear pin is used to clear the internal memory of the shift register.

The SCK, RCK, and SR clocks are the three most important control pins for the shift register.

Demonstration of how to write to the shift register's memory and control outputs with push buttons.

Explanation of the bouncing effect of push buttons and the use of pull-up and pull-down resistors.

How to shift data into memory and copy it to the output without clearing the memory.

Creating a scrolling effect by continuously shifting in new bits.

Connecting multiple shift registers to control more than eight LEDs.

The role of the H' (output H Prime) pin in cascading shift registers.

How the H' pin acts as a delayed push button for cascading registers.

Encouragement to experiment with shift registers for a deeper understanding.

Call to action for viewers to comment, subscribe, and engage with the content.

Transcripts

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