74HC165 shift register: pinout, read order, eight buttons on three pins and chaining

The chip has two modes, picked by SH/LD. While SH/LD is low the eight flip-flops follow the inputs A to H, and the clock is ignored. When SH/LD goes high they hold what they had, and from then on each rising edge on CLK shifts everything one place toward QH, with SER filling the empty place at the A end.

The first bit needs no clock. Read QH right after the load, then clock and read seven more times. Code that clocks first and reads afterwards loses H and picks up a bit from SER at the end. Arduino's shiftIn() raises the clock before each read, so set CLK high before the load pulse. Its first raise then makes no edge, and with MSBFIRST H lands in bit 7 and A in bit 0.

It runs from 2 V to 6 V, so a 5 V Arduino board or a 3.3 V ESP32 board can power it directly. Power it from the same supply as the controller, so its QH output swings to the controller's own rail.

The input thresholds track the supply: TI gives 3.15 V and 1.35 V at 4.5 V, and 4.2 V and 1.8 V at 6 V. Both pairs are 0.7 and 0.3 of VCC, so on 5 V a high must be at least 3.5 V and a low at most 1.5 V.

Wire QH of the second chip to SER of the first, give both chips the same SH/LD and CLK, and read QH of the first chip as before. After eight clocks the first chip's bits are out and the second chip's H is on QH, so sixteen reads give both bytes. Tie SER on the last chip in the chain to ground, so the bits that follow read as 0.

Each extra chip adds eight inputs, so four chips read 32 on the same three pins. TI's application section draws the same cascade, QH to SER, with 0.1 µF from VCC to ground at each chip.

Each input needs a defined level when its button is open. The usual way is a pull-up resistor to 5 V on every input and the button to ground, so a pressed button reads 0. With 10 kΩ:

Debounce in software: a contact can bounce for a while after it closes, and the chip reads whatever the pin shows at the load pulse. Reading every few milliseconds and acting on two equal reads in a row is enough for buttons.

A read is one load pulse of at least 20 ns, 20 ns of set-up before the first clock, then eight clock periods per chip. At the chip's 25 MHz limit (SN74, up to 85 °C, 4.5 V) one chip reads in 360 ns. Software on a small microcontroller usually clocks nearer 1 MHz, and then one chip takes 8.04 µs and four take 32 µs.

Hardware SPI can do the shifting: CLK on the SPI clock, QH on MISO, and SH/LD on any spare pin pulsed low before each transfer. Keep the clock under 25 MHz and the chip keeps up.

• Read QH before the first clock. That bit is input H, and it is the one people lose. With Arduino's shiftIn(), set CLK high before the load pulse.
• Tie CLK INH to ground and SER on the last chip to ground. Neither may float.
• Fit 100 nF from VCC to ground at every chip; TI's sheet asks for 0.1 µF at each VCC pin.
• Pin numbers do not follow the letters: A is pin 11 and H is pin 6. Check the pinout before you wire the buttons in order.
• If the bits never change when you press a button, check the SH/LD line first. If it never goes low, the chip never loads and only shifts out what SER feeds in.

Any 74HC165 from another maker should drop into the same socket with the same pins; we could only read TI's sheet for this page, so check another maker's timing table before running near the clock limit.

1. Wire VCC, ground and 100 nF; tie CLK INH and SER to ground; a 10 kΩ pull-up on each of A to H.
2. Connect SH/LD and CLK to two output pins and QH to an input pin.
3. Pulse SH/LD low, then read QH with no button pressed. It should read 1 (input H).
4. Press the button on H and repeat. The first bit should now read 0, and the other seven should stay 1.
5. Press the button on A. Only the eighth bit should change.

The logic levels lesson explains the 0.7 VCC and 0.3 VCC input thresholds, and the 74HC595 is the output twin of this chip.

Use the SN74HC165N for breadboard input expansion: 10 kΩ pull-ups, buttons to ground, CLK INH and the last SER to ground, 100 nF at each chip. Read QH before the first clock (with shiftIn() and MSBFIRST, hold CLK high through the load) and debounce in software.

Shops list it as 74HC165 or SN74HC165N in DIP-16, and as 74HC165D for boards. Buy a branded part for anything that runs near the clock limit. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.