SN74LS166 8-bit Shift Register : Pin Configuration & Its Applications

In Digital Electronics, a sequential logic circuit used to transfer or store the data in the binary format is known as a shift register. It is used to shift the data on the input of the circuit to its output for each clock pulse. The number of latches required to design the shift register depends on the number of input bits to be shifted. For example, the 8-bit shift register requires 8 individual data latches. The shift registers are mainly used in calculators and computers and can convert the data from either parallel to serial or serial to parallel. This article gives a brief description of an SN74LS166 8-bit shift register like pin configuration, circuit diagram, and working with applications.


What is the SN74LS166 8-Bit Shift Register?

The SN74LS166 is a parallel-in-serial-out 8-bit shift register digital integrated circuit, which can send the data parallelly or serially to convert and control the output data in serial form. It is compatible with TTL logic devices and it is known as parallel in and serial-out shift register or serial in and serial-out shift register designed with 77 equivalent gates on a single chip. This digital IC has 8 parallel input pins, serial input-1 & serial output-1. The data of the shift register is controlled by the two CLK pins – clock, clock inhibit, and shift/load pin. To clear the register and reset the stored data, a clear pin is used.

SN74LS166 IC
SN74LS166 IC

The SN74LS166 8-bit shift register operates in two modes, which are parallel in serial out mode and serial in parallel out mode based on the state of the SH/LD pin. The mode of operation is controlled by the shift/load input pin. When the SH/LD pin is high, it enables serial data shifting by coupling the 8 flip-flops with each clock pulse. When the SH/LD is low, the parallel data on the parallel inputs A to H are enabled and synchronous loading occurs with each clock pulse.

Pin Configuration of SN74LS166 8-Bit Shift Register:

The SN74LS166 pin configuration/pin diagram with inputs and outputs is illustrated below.

SN74LS166 8-Bit Shift Register Pin Diagram
SN74LS166 8-Bit Shift Register Pin Diagram
  • Pin 1 (SERIAL IN): This represents serial input pin or SER. The data on this pin is shifted serially to the first stage of the register only when this serial/parallel load input is high. It is used to give the serial i/p data to the IC.
  • Pin 2 (INPUT A): This pin represents parallel input data A, which is the first bit of the 8-bit
    Parallel data.
  • Pin 3 (INPUT B): This represents parallel input data B, which is the second bit of the 8-bit parallel data input.
  • Pin 4 (INPUT C): This represents parallel input data C, which is the third bit of the 8-bit parallel data input.
  • Pin 5 (INPUT D): This pin represents parallel input data D, which is the fourth bit of the 8-bit parallel data input of the shift register.
  • Pin 6 (CLOCK INHIBIT or CLK INH): It is an active-low clock inhibit pin, used to control the CLK signal.
  • Pin 7 (CLOCK): This input pin represents the user to change the condition of the IC according to the time.
  • Pin 8 (GND or VSS): This pin refers to the common ground connection for the receiver, sender, and power supply.
  • Pin 9 (CLEAR): This pin is used to clear all the internal stored data, o/p data, and reset the shift register.
  • Pin 10 (INPUT E): This pin represents the parallel data input E, which is the fifth bit of the 8-bit parallel data input of the shift register.
  • Pin 11 (INPUT F): This pin represents the parallel data input F, which is the sixth bit of the 8-bit parallel input data of the shift register.
  • Pin 12 (INPUT G): This pin represents the parallel data input G, which is the seventh bit of the 8-bit parallel data input of the shift register.
  • Pin 13 (SERIAL OUT or OUTPUT QH): This pin receives the 8-bit data that is converted from parallel form to serial form. It is a complementary output pin of the 74LS166 8-bit shift register.
  • Pin 14 (INPUT H): This pin represents the parallel data input H, which is the eight-bit or LSB of the 8-bit parallel data input of the shift register.
  • Pin 15 (SH/LD (shift/Load)): For every clock pulse, this pin enables the 8-bit parallel data to shift serially.
  • Pin 16 (VCC): This pin is used to give the power supply of 5V or 10V or 15V to the shift register to start working.

Technical Specifications:

The distinct characteristics or technical specifications of the SN74LS166 8-bit Shift Register are listed below.

  • It is an 8-bit serial to serial or serial to parallel shift register.
  • Its operating voltage ranges from 3volts to 18volts.
  • It requires a maximum supply voltage of 5.5volts.
  • It produces a propagation delay time of 25ms.
  • It requires a maximum clock frequency of 35MHz.
  • It is operated at temperatures ranging from 0°C to 70°C.
  • It is manufactured in 16-pin packages like GDIP, PDIP, and PDSO.
  • The clock frequency is 45MHz.
  • Pulse duration 10ns ( for clock high and clock low).

Alternatives of SN74LS166 8-bit Shift Register:

The replacement or alternatives of SN74LS166 8-bit shift registers are 4014 4-bit static shift register, CD4035 8-bit shift register, 74LS379 quad parallel shift register, 74LS323 8-bit storage shift register, 74HC595 dual 4-bit static shift register, 74LS299 8-bit storage shift register, and 74LS164 serial/parallel shift register.

Circuit Diagram/How to use it?

The circuit diagram of the SN74LS166 8-bit shift register is designed with 16-pins, including 8 parallel input pins, 2 clock pins, one serial output pin, and one serial input pin, shift/load pin, clear pin, VCC, and ground pin. The CLK INHIBIT pin is used to control the clock pulse given to the flip-flops connected internally by the CLOCK pin. When the CLK INHIBIT pin is enabled, then the CLK signal shifts towards the flip-flops.

The CLR pin is used to reset the register by clearing the internal and external data. When the CLEAR (CLR) pin is active low, then the data obtained at the output pin, control pins, and the flip-flops are cleared. The control pin, that is shift/load pin, is used to shift the input data towards the flip-flops to a clock pulse. The logical circuit diagram of the SN74LS166 8-bit shift register is shown in the figure below.

Circuit Diagram of SN74LS166 8-Bit Shift Register
Circuit Diagram of SN74LS166 8-Bit Shift Register

The SN74LS166 8-bit shift register comes with 9 data i/p pins, of which 8 of them are parallel input pins and the final one is a serial input pin. It sends or shifts the 8-bit parallel data and after that, the serial data is shifted. As the data is shifted from one latch to another latch in the sequence manner, the output data is based on the output of internal flip-flops. While the output of the serial pin is the output of the flip-flop connected at the end.

The truth table of the SN74LS166 8-bit shift register is shown below.

CLR SH/LD CLK INH CLK SER Parallel inputs

A….H

Internal output QA Internal Output QB Output   QH
L X X X X X L L L
H X L L X X QA0 QB0 QH0
H L L Pulse X a ….h a b h
H H L Pulse H X H QAn QGn
H H L Pulse L X L QAn QGn
H X H Pulse X X QA0 QB0 QH0

Where ‘H’ indicates a High level.

‘L’ indicates Low level.

‘X’ indicates Don’t care condition.

Pulse represents the transition from Low to High level.

a…..h indicates the level of inputs at A to H (steady-state inputs).

QA0, QB0, QH0 indicate the level of QA, QB, QH respectively, before the indicates steady-state conditions were established.

QAn and QHn indicate the level of QA, QG respectively, before the most recent clock pulse.

The SN74LS166 is an 8-bit serial-in or parallel in or serial-out shift register. It can send the data serially or parallelly and control the output in serial at the end. It is mainly used in various applications where the LEDs are controlled by microcontrollers. It can act as a data bit for the LCDs to interface the LCD screen.

To expand the functionalities of GPIO pins, it is extensively used with microprocessors or microcontrollers. For interfacing, the four pins of the SN74LS166, such as SER, CLK INH, CLK, and SH/LD are connected to the GPIO pins of the microcontroller. While the input pins A to H are connected to the switches or buttons.

Where to use/Applications:

The applications of an SN74LS166 8-bit Shift Register include the following.

  • Used in registers and counters
  • Used in control circuits
  • Used in the conversion of a code
  • Used to generate a sequence
  • Used in shift-right and shift-left registers
  • Used in arithmetic unit registers.
  • Used in data conversions in parallel to serial or serial to parallel.
  • Used as an interfacing device to provide communication between computers and peripherals.
  • Used to blink LEDs in different patterns.

Please refer to this link to know more about the SN74LS166 8-bit shift register datasheet.

Thus, this is all about an overview of the SN74LS166 8-bit shift register. Here is a question for you, “What are the advantages of SN74LS166 8-bit shift register?”

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