SIPO and PISO shift registers with Arduino and ESP (ESP8266 / ESP32)

When working with Arduino, ESP8266 or ESP32, you quickly hit a simple limit: not enough GPIO pins.
Instead of stacking boards or using complex expanders, there is a very clean and proven solution: shift registers.

The two main types are:

• SIPO – Serial In, Parallel Out
• PISO – Parallel In, Serial Out

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Why use shift registers?

• Save GPIO pins
• Clean wiring
• Easy to scale
• Very cheap
• Used everywhere in real hardware (industry, vending, panels)

With only 3 pins, you can manage 8, 16, 32+ inputs or outputs.

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SIPO – Serial In, Parallel Out

What it does

You send data bit by bit (serial), and the chip exposes them all at once on parallel outputs.

Common chip

74HC595

• 8 digital outputs
• Can be daisy-chained
• Very reliable
• Fast enough for almost any project

Important pins

Pin	Role
DS	Serial data
SH_CP	Clock
ST_CP	Latch
Q0–Q7	Outputs
OE	Output enable (optional)
MR	Reset (optional)

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Arduino example – driving 8 LEDs

int dataPin = 11;
int clockPin = 13;
int latchPin = 10;

void setup() {
  pinMode(dataPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(latchPin, OUTPUT);
}

void loop() {
  digitalWrite(latchPin, LOW);
  shiftOut(dataPin, clockPin, MSBFIRST, B10101010);
  digitalWrite(latchPin, HIGH);
  delay(500);
}

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Typical SIPO use cases

• LEDs
• Relays
• MOSFET boards
• 7-segment displays
• Control panels
• Power boards

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PISO – Parallel In, Serial Out

What it does

Reads many inputs at once, then sends their state serially to the microcontroller.

Common chip

74HC165

• 8 digital inputs
• Can be chained
• Very simple timing

Important pins

Pin	Role
PL	Parallel load
CP	Clock
Q7	Serial output
D0–D7	Inputs

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Arduino example – reading 8 buttons

int loadPin = 8;
int clockPin = 12;
int dataPin = 11;

void setup() {
  pinMode(loadPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, INPUT);
}

byte readInputs() {
  digitalWrite(loadPin, LOW);
  delayMicroseconds(5);
  digitalWrite(loadPin, HIGH);
  return shiftIn(dataPin, clockPin, MSBFIRST);
}

void loop() {
  byte buttons = readInputs();
}

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SIPO vs PISO

Feature	SIPO (74HC595)	PISO (74HC165)
Direction	Output	Input
GPIO needed	3	3
Chainable	Yes	Yes
Speed	High	High
Typical use	LEDs, relays	Buttons, switches

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Arduino vs ESP considerations

Arduino (5V)

• Works perfectly with HC / HCT chips
• Very forgiving

ESP8266 / ESP32 (3.3V)

• Prefer 74HCT or power the chip at 3.3V
• Avoid floating OE / MR pins
• Logic levels matter more

In practice, 74HC often works, but it’s not guaranteed in all cases.

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Daisy-chaining

You can chain registers easily:

• Q7’ → DS for SIPO
• Q7 → DS for PISO
• Same clock and latch

Examples:

• 4 × 74HC595 → 32 outputs
• 2 × 74HC165 → 16 inputs

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Alternatives

Chip	Type	Why use it
MCP23017	I²C GPIO	Interrupts
PCF8574	I²C GPIO	Very simple
TLC5940	PWM LED	Brightness control
ULN2003	Driver	Current handling

Still, SIPO / PISO are hard to beat for simplicity and reliability.

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When SIPO / PISO are the right choice

Good choice if you want:

• Simple hardware
• Deterministic timing
• No complex bus
• Easy scaling

Not ideal if you need:

• Analog inputs
• Per-pin interrupts
• Complex feedback

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Conclusion

SIPO and PISO shift registers are basic but extremely powerful tools.
They are cheap, fast, reliable, and perfectly suited for Arduino and ESP projects.

If you are building:

• a custom board
• a control panel
• a vending machine
• home automation hardware

This should be one of your first tools.