Part

74HC595 shift register

Eight outputs from three pins. Shift a byte in and latch it.

16 pins
In 1 project
In 4 lessons

Drawn live by the editor's own code, at the size you see it.

At a glance

The 74HC595 shift register in brief

What it is, and where it sits on the bench.

The 74HC595 turns three signal pins into eight outputs, and the trick to understanding it is that there are genuinely two separate eight-bit registers inside. The shift register moves on every rising edge of SHCP, dropping whatever is on DS into the first stage and pushing everything else along by one; do that eight times and it holds a full byte. The storage register, the latch, is the one Q0 through Q7 actually show, and it only changes on a rising edge of STCP, which copies the shift register into it in one step. That is why the outputs sit completely still while a byte is clocked in and jump together only at the latch edge. MR clears the shift register alone, asynchronously, leaving the latch untouched; OE three-states all eight outputs without touching either register. Every edge carries the datasheet’s typical 15 nanosecond propagation delay before the pins actually move.

Family: Logic
Pins: 16

Pins

16 pins

Named as the editor names them, so a wire lands where you expect it.

Output9

Input or output2

Input5

MR
SHCP
STCP
OE
DS

How to use it

How to use a 74HC595 shift register with an Arduino

The 74HC595 turns three Arduino pins into eight outputs. Data goes into DS one bit at a time, each rising edge on the shift clock SHCP moves it along, and a rising edge on the latch clock STCP copies all eight bits to Q0 to Q7 at once, so the outputs never flicker through the bits on their way in. shiftOut() does the shifting in one call.

Two pins catch everybody: MR (master reset, pin 10) must be tied to 5 V or the register keeps clearing itself, and OE (output enable, pin 13) must be tied to GND or the outputs float. In the circuit here they are, and eight LEDs hang off Q0 to Q7 through 220 ohm resistors.

Part pin

Board pin

DS (pin 14)

Pin 4

SHCP (pin 11)

Pin 5

STCP (pin 12)

Pin 6

MR (pin 10) and VCC (pin 16)

5 V

OE (pin 13) and GND (pin 8)

GND

Q0 to Q7

Eight LEDs, each through 220 ohms

Chain a second 74HC595 from Q7S (pin 9) into its DS and shift sixteen bits for sixteen outputs, still on three pins.

Arduino Uno: LED bar chaserlive0.000 s 0.00x

Click to open it in the editor

This is the simulator itself, running here. Click anything to open it in the editor.

Open this project in the editor Read the full tutorial

Eight bits out, then the latch

void show(uint8_t bits) {
  shiftOut(4, 5, MSBFIRST, bits);   // DS, SHCP
  digitalWrite(6, HIGH);            // STCP: copy to Q0..Q7
  digitalWrite(6, LOW);
}

How it is modeled

What is true about the 74HC595 shift register, here

In its help article’s own words: what the simulation gets right, and where it stops.

74HC595: 8-bit shift register with output latch

The chip everyone reaches for when they run out of pins. Two registers inside it: an eight-stage shift register clocked by SHCP, which moves everything along and drops DS into stage 0 on a rising edge; and a storage register (the latch), which is what Q0 to Q7 actually show and which only changes on a rising edge of STCP. MR (active low) clears the shift register without touching the latch; OE (active low) three-states the outputs without touching either register, so a disabled chip still shifts and its last byte is unharmed underneath. Both registers power up cleared. Propagation delay: 15 ns typical, STCP to Qn. See it in: LED bar chaser.

Not modeled

One typical delay, and nothing around it. Each chip carries a single propagation delay taken from its datasheet's typical column at VCC = 5 V, CL = 15 pF and 25 °C, and uses it for every path through the package. There is no minimum or maximum, no spread between the gates in one chip, no rise or fall time, no output slew and no dependence on supply voltage, load capacitance or temperature, all of which a real 74HC part has, and all of which a datasheet gives ranges for. On the 74HC595 the one figure is the STCP-to-Qn number used for SHCP and MR to Q7S as well, which the datasheet lists a nanosecond or two apart.

No setup, hold or pulse-width checks. The 74HC74 and the 74HC595 take a clock edge whenever they see one; nothing here refuses data that changed too close to the edge or a clock pulse that was too narrow, and nothing warns about it. A real part would metastable or simply miss.

No supply current, no output current limit and no bus contention damage. An output driving into another output is resolved as two drivers on one net; nothing gets hot.

Sequential parts power up cleared rather than random, because a toggle divider fed Unknown never escapes it. See how faithfully a part behaves.

From The 74HC logic family, in full.

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