7400 Series Guide: 74HC90/74LS90 (Decade Counter)

The 74×90 (ex 74LS90) is a binary decade counter chip. It’s used to count from 0 to 9 in binary format and then reset to 0. You can use it to build digital clocks, frequency counters, and for other digital counting needs.

In this guide, you’ll learn the things you need to know about this chip in order to use decade counters in your own projects.

What does the 74HC90 / 74LS90 do?

The 74×90 gives you a decade counter that counts from 0 to 9 and gives you the result in a 4-bit binary format. Then it resets to back to 0. It is specifically designed to count ten occurrences of an event.

The chip consists of a divide-by-2 and a divide-by-5 counter which lets you create a divide-by-10 counter.

How To Use This Chip

The 74HC90 comes in a 14-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC90 supports 2V to 6V, while the 74LS90 only supports 5V.

The maximum current you can pull out of one output pin of the 74LS90 is 0.8 mA when the pin is high (sourcing) or 16 mA when the pin is low (sinking). For 74HC chips, it’s usually 4 mA both sourcing and sinking.

This can differ between models, so check the datasheet of your model to verify.

The chip actually contains two counters, so to count up to 10, you need to connect the CLK1 input to the Q0 output. Then use CLK0 as the input for your counting pulses.

By using CLK0 as your clock input and connecting Q0 to CLK1, you’re setting up the IC to count to ten (1010 in binary). The CLK0 input drives the divide-by-2 counter, and the connection of Q0 to CP1 enables the divide-by-5 counter. Together, these configure the IC to divide the input clock by 10, thereby counting from 0 to 9 before resetting to 0.

74×90 Pinout

The 74×90 has 14 pins and contains a 4-bit asynchronous decade counter with its pins laid out as shown in the pinout diagram below:

Pin Name	Pin #	Type	Description
CLK1	1	Input	Clock input for divide by 2 counter (active low).
–	2	NC	Not connected.
RST2	3	Input	Reset for divide by 5 counter (when high it resets the counter).
–	4	NC	Not connected.
VCC	5	Power	Positive power supply. Connect to +5V power.
SET1	6	Input	Set input for divide by 5 counter (used to preset the counter).
SET2	7	Input	Set input for divide by 5 counter (used to preset the counter).
Q2	8	Output	Q2 output of the divide by 10 counter.
Q1	9	Output	Q1 output of the divide by 10 counter.
GND	10	Power	Connect to ground (GND).
Q3	11	Output	Q3 output of the divide by 10 counter.
Q0	12	Output	Q0 output of the divide by 10 counter, least significant bit.
–	13	NC	Not connected.
CLK0	14	Input	Clock input for divide by 5 counter (active low).

Alternatives and Equivalents for 74HC90 / 74LS90

There are many versions of the 74×90 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

• 74HC90 (High-speed CMOS)
• 74HCT90 (High-speed CMOS, TTL compatible)
• 74LS90 (High-speed TTL)
• 74LVC90 (Low Voltage TTL)
• 74AC90 (Advanced CMOS)
• 74ALS90 (Advanced Low-Power Schottky TTL)
• 74F90 (Very High Speed)
• 74C90 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74LS90 by Texas Instruments.

Can’t find the 74×90 anywhere? Then try one of the following IC alternatives:

• 74×142 – Decade counter for Nixie tubes.
• 74×143 – Decade counter/latch/decoder/7-segment driver (constant current).
• 74×144 – Decade counter/latch/decoder/7-segment driver (open collector).
• 74×160 – Synchronous presettable 4-bit decade counter.
• 74×162 – Synchronous presettable 4-bit decade counter.
• 74×168 – Synchronous presettable 4-bit up/down decade counter.
• 74×190 – Synchronous presettable 4-bit up/down decade counter.
• 74×192 – Synchronous presettable 4-bit up/down decade counter.
• 74×196 – Presettable 4-bit decade counter/latch.
• CD4017 – Decade counter with 10-output decoder.
• CD4026 – Decade counter with decoded 7-segment display outputs.
• CD4029 – Presettable up/down counter, binary or BCD-decade.
• CD4033 – Decade counter with decoded 7-segment display outputs.

If you can’t find the 74×90 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS90 and 74HC90 chips

Download the PDF datasheet for your version of the 74×90 here:

• SN74LS90 (Texas Instruments)
• 74LS90 (Futurlec)
• MM74C90 (Fairchild)