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Home > Other > 74ls90 BCD Decade Counter IC: Pinout, Application, and Datasheet

74ls90 BCD Decade Counter IC: Pinout, Application, and Datasheet

Update Time: 2023-11-29 13:46:08


The 74LS series ICs are among the top favorites in the transistor-transistor logic (TTL) category. The 74LS series is a low-power Schottky bipolar integrated circuit. The 74LS90 IC encompasses four primary-secondary flip flops with extra gating, enabling it to function as a half-divider and three-tier binary counter. This new technologies article delves into the 74LS90 BCD Counter, including pinout, features, applications, datasheet, etc.

What is a BCD Counter or Decade Counter?

A BCD counter belongs to the category of 4-bit binary counters. It begins its count from 0 and progresses up to a specified count under the influence of a clock signal. Once it achieves the set count threshold, all its flip flops are reset, and it initiates its count from 0. Constructed using 4 JK flip flops, this counter operates between 0 and 9, displaying the outcome digitally. Upon hitting a count of 9 (1001), it undergoes a reset and commences its count anew.

BCD counter.png

The 74LS90 BCD Counter

The 74LS90 IC essentially functions as a MOD-10 decade counter, delivering a BCD-coded output. Within the 74LS90, four master-slave JK flip flops are arranged to offer both a MOD-2 (up-to-2) counter and a MOD-5 (up-to-5) counter. This circuit features a standalone toggle JK flip flop powered by the CLK A input, alongside three toggle JK flip flops creating an asynchronous counter, which is influenced by the CLK B input, as depicted.

74LS90 BCD Counter

The counter outputs are labeled with the letter Q, followed by a number corresponding to the binary weight of the related bit in the BCD counter circuit's code. For instance, QA, QB, QC, and QD. The 74LS90 counting process begins when the clock signal, CLK, transitions from a logic 1 (HIGH) state to a logic 0 (LOW) state.

The extra input terminals, R1 and R2, function as the "reset" controls, whereas S1 and S2 act as "set" controls. When the Reset pins R1 and R2 are linked to logic 1, the counter reverts to its initial state, 0 (0000). Conversely, when the Set pins S1 and S2 are linked to logic 1, they initialize the counter to its peak value, 9 (1001), irrespective of the current count or position.

As mentioned, the 74LS90 counter comprises a divide-by-2 and a divide-by-5 counter in a single unit. We can utilize just the divide-by-2 frequency counter just the divide-by-5 frequency counter, or combine them to generate a divide-by-10 BCD counter as we require.

Suppose the four flip flops of the divide-by-5 counter segment are deactivated, and a clock pulse is fed to the input terminal 14 (CLKA), with the output derived from pin 12 (QA). In that case, generating a standard divide-by-2 binary counter suitable for frequency division circuits is possible.

74LS90 BCD Counter.png

74LS90 Divide-by-2 Counter

To generate a typical divide-by-5 counter, we can turn off the flip flop mentioned above and introduce the clock signal straight to pin 1 (CLKB). As illustrated, the output can be obtained from pin 11 (QD).

74LS90 Divide by 2 Counter.png

74LS90 Divide-by-5 Counter

Be aware that in this divide-by-5 counter setup, the output waveform isn't balanced; it exhibits a 4:1 mark-space ratio. This means four input clock pulses result in a LOW or logic "0" output, while the fifth input clock pulse leads to a HIGH or logic "1" output.

74LS90 Divide by 5 Counter.png

74LS90 Divide-by-10 Counter

We observe that BCD counters are a type of binary counters which cycle from 0000 to 1001, then revert to their initial state because they're equipped to reset all their flip flops post the ninth count. The counter increments each time the button is let go by attaching a pushbutton switch (SW1) to the CLKA clock input. We can visually track the progression of the binary-coded decimal count by connecting light-emitting diodes (LEDs) to the output ports, namely QA, QB, QC, and QD.

74LS90 Divide-by-10 Counter.png

74LS90 BCD Decade Counter

Repeated pressing of the push-button switch, SW1 will increment the count until it reaches nine, represented as 1001. On the tenth press, the outputs ABCD will revert to zero, initiating a new counting cycle. With this MOD-10 count, the decade counter can power a digital display.

The BCD output must first be decoded accordingly to showcase the counting sequence on a seven-segment display. A digital mechanism capable of interpreting the four outputs from the 74LS90 BCD counter and illuminating the necessary segments on display is termed a Decoder.

74LS90 BCD Decade Counter_light_gray.png

74LS90 Pin Configuration

74LS90 pinout diagram

74LS90 pinout diagram

The pin configuration for the 7490-decade counter is displayed here.

Pin No.Pin NameDescription
1CLKBClock Input 2
2R1Reset 1
3R2Reset 2
4NCNot Connected
5VccPositive Supply Input
6R3Reset 3
7R4Reset 4
8QcOutput Pin 3
9QbOutput Pin 2
QdOutput Pin 4
12QaOutput Pin 1
13NCNot Connected
14CLKAClock Input 1

74LS90 Circuit

A basic circuit for a Mod 10 Counter utilizing the 74LS90 IC is presented.

74LS90 Circuit.png

74LS90 Features

  • It serves as a basic counter ranging from 0 to 9.

  • The chip can autonomously initiate at 0 and conclude at 9.

  • The chip is compatible with any TTL-based apparatus and microcontrollers due to its TTL output.

  • The IC boasts energy-efficient operation.

  • It is available in various packaging types, including PDSO, PDIP, and GDIP.

  • The IC is safeguarded internally against clamp voltages.

Note: Comprehensive technical details can be available in the 74LS90 datasheet provided at the end of this page.

74LS90 Specifications

  • The IC accepts a power input between 4.75 and 5.25 volts.

  • The IC's functional temperature span is from 0 to 70 degrees.

  • When the IC is in the HIGH state, the input voltage starts at a minimum of 2.0; in the LOW state, it doesn't exceed 0.7.

  • In the HIGH state, the IC's output current consumption is -0.4mA, while in the LOW state, it's 8.0mA.

  • The protection range of the integrated clamp diode is -1.5V.

74LS90 Alternative Equivalents

74LS90 Datasheet

74LS190 Datasheet (PDF)  - Texas Instruments

How to Use Decade Counter IC?

The following schematic represents a BCD decade counter. By applying alternating HIGH and LOW logic to the CLKA pin, the IC initiates counting from zero to 8 with each HIGH input. To visualize the number on a 7-segment display, a seven-segment display driver IC is necessary.

BCD decade counter circuit diagram.png

How does 7490 Counter Work? 

In this section, we will delve into the functionality of the 7490 counter. This integrated circuit (IC) comprises an internal arrangement of four flip-flops. The initial flip-flop functions as a MOD 2 counter, while the remaining three serve as MOD 5 counters. Two clock pins are pivotal, determining state transitions. The reset pins are controlled via an AND gate.

74LS90 internal diagram.png

The IC is equipped with four reset pins, two clock pins, and four output pins. To effectively utilize this IC, it's crucial to comprehend the role of the reset pins. These four reset pins offer a total of 16 combinations, but only specific combinations yield fixed outputs, as illustrated in the following table.

7490 Truth Table

7490 Truth Table.png

The second clock pin (Pin 1) links to the Least Significant Bit (LSB) of the IC, ensuring a sequential progression between MOD 2 and MOD 5. The first clock pin (Pin 14) is responsible for supplying the IC with clock input signals. The output is influenced on each transition from a HIGH to LOW state. However, it is imperative to always bear in mind the behavior of the reset pins; otherwise, the IC may yield unpredictable results or no output at all.

Here is the complete functional circuit. When a pulse is applied, the IC generates output in binary form, with each binary configuration representing a decimal number, as depicted in the accompanying table.

74ls90 truth table for every binary number.png

Counter Example 74LS90

The IC 74LS90 functions as a basic counter, though its output is always binary. In this instance, we'll employ the IC to count from 0-9 and display it on the 7-segment. To operate with the 7-segment, the IC must be paired with another called 74LS47. While the 74LS90 produces binary data, the 74LS47 translates this data into a decimal format for display on the 7-segment. Below is the circuit and its corresponding OUTPUT.

Counter Example 74LS90.png

The IC 74LS47 is specifically designed to work with a common anode 7-segment. Hence we pair the common anode with IC74LS47. Alternatively, 74LS48 can be employed. We've grounded all reset pins in this setup because our sole objective is counting; we don't aim for the IC to execute additional tasks. While a single IC can tally up to 9, employing several ICs allows for extended counting. For multi-decimal counts, other IC types and techniques can be adopted. The 74LS90 IC lacks dedicated pins for serial decimal connections or interfacing with the 7-segment. Custom solutions incorporating other gates are necessary to utilize the IC for counts beyond 9 using the 74LS90.

Proteus Simulation

Applications 7490 BCD Counter

  • The IC is suitable for basic binary counting ranging from 0 to 9.

  • This IC is often found in digital devices paired with a 7-segment display.

  • The 74LS90 IC is also employed in servers, networking, and various digital systems.

2D-model and Dimensions

74LS90-2D-model and Dimensions.png


In this guide, we've learned that a BCD Counter is a device cycling through ten stages upon being clocked, resetting to 0 post the ninth count. In our basic illustration, we utilized input clock pulses from a push-button switch, but counters can tally numerous real-life occurrences, like counting objects in motion.

The 74LS90 BCD Counter boasts a versatile counting mechanism. It can function as a frequency divider or be tailored to divide any full number between 2 and 9 by channeling the relevant outputs back into the IC's Reset and Set inputs.

Read More

Previous: CR1220 Battery Equivalent, Specification, Application

Next: The Ultimate Guide to Ring Counter: Working, Types & Applications


  • What does 74LS90 do?
  • The 74LS90 functions as a MOD-10 decade counter, producing a BCD output code. It's composed of four master-slave JK flip flops. Internally, these are configured to facilitate a MOD-2 (counting up to 2) and MOD-5 counter.

  • What form does IC 7490 give output in?
  • The 7490 IC serves as a BCD (binary coded decimal) counter. It's designed to hold decimal numbers in a 4-bit binary format.

  • How many flip-flops are needed for a decade counter?
  • A decade counter requires a minimum of 4 flip flops in its sequence since it tallies up to ten and resets to zero after surpassing 9. For counting up to the binary representation of nine, 4 flip flops are essential.

  • What is the difference between IC 7490 and 7493?
  • The distinction between the 7490 and the 7493 is that the former is a MOD-10, while the latter is a MOD-16 counter. This means the 7490 cycles through 10 states, while the 7493 goes through 16 states.

  • Is BCD counter a decade counter?
  • A decade counter is also referred to as a BCD Counter.

  • Which IC is a decade counter?
  • The CD4017 IC functions as a decade counter that counts to ten.

  • Is IC 7490 asynchronous or synchronous counter?
  • The 74LS90 operates as a 4-bit asynchronous decade counter, activated on the negative edge, and equipped with asynchronous clear and present inputs for customizable counter tasks. It exclusively counts in an upward order. The IC comprises two distinct counters, adaptable for three varied operational modes.

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