74HC595 shift register: pinout, timing, eight LEDs from three pins and daisy-chaining

TI sells it as SN74HC595: SN74HC595N is the plastic DIP, SN74HC595D the SOIC and SN74HC595PW the TSSOP, all with the same pin numbers. Diodes Incorporated drops the prefix (74HC595S16 for SO-16, 74HC595T16 for TSSOP-16) and has no DIP. The SN54HC595 is TI's military-temperature version in ceramic packages.

The two makers label the pins differently, and Arduino tutorials mix both sets. TI's SER, SRCLK and RCLK are Diodes' DS, SHCP and STCP; TI counts the outputs QA to QH, Diodes Q0 to Q7. The numbering is the same, so a circuit drawn with one set of names works with the other chip.

A 74HCT595 is the TTL-input version with the same pins; it runs from 4.5 to 5.5 V and is not covered here. Watch the first output: QA is on pin 15, across the chip from QB on pin 1, so a row of LEDs wired in pin order comes out with the first LED at the wrong end.

Inside are two 8-bit registers. Each rising edge on SRCLK moves every bit of the shift register one place along and takes a new bit from SER into the first place. The outputs do not move while this happens. A rising edge on RCLK then copies all eight bits into the storage register at once, and that register drives the pins. So the outputs jump from the old byte to the new one in a single step, with no flicker while the bits travel.

SRCLR low empties the shift register but leaves the outputs alone until the next latch. OE high switches all eight outputs off (high impedance) without losing the byte, which is also how a PWM signal on OE dims every LED at once. QH' always shows the last bit of the shift register, latched or not, so it can feed the next chip.

The two makers agree on the supply: 7 V absolute maximum, 2 to 6 V recommended. Each of QA to QH may carry ±35 mA, and the VCC or ground pin 70 mA for the whole chip. Diodes rates its serial output Q7S lower, at ±25 mA. TI rates the SN74HC595 from −40 to 85 °C; the Diodes part is rated to 125 °C and also states a 500 mW total power limit, which TI's sheet does not give.

The 70 mA pin limit is the one that bites. Eight outputs at 35 mA each would be 280 mA, four times what the VCC pin may carry, so the per-output figure is never available on all eight at once. Size the outputs so the sum stays well under 70 mA, as the worked example does.

At 4.5 V an input below 1.35 V counts as low and above 3.15 V as high, the same pair on both sheets. Diodes' feature list claims Schmitt-trigger action on every input; TI's sheet makes no such claim, so keep clock edges clean on long wires.

The output rows are written at 6 mA. At that current a high output from 4.5 V stays above 3.84 V over TI's full temperature range (4.3 V typical), and a low output stays below 0.33 V. The datasheets give no row at 20 mA, so anything past 6 mA per pin is outside what either maker guarantees.

Speed is rarely the limit. TI allows a 25 MHz clock up to 85 °C at 4.5 V (31 MHz at 25 °C), needs SER steady 25 ns before each SRCLK edge and nothing after it, and the outputs settle 37 ns at most after RCLK. A microcontroller toggling pins in software runs far slower than that.

The two sheets disagree on the power dissipation capacitance: TI prints 400 pF, Diodes 43 pF at 5 V and 1 MHz. It only matters when the clock runs at megahertz rates for hours; for an LED driver it is lost in the LED current.

• Eight more outputs from three microcontroller pins, and eight more again for each chip on the chain.
• LED bars, seven-segment displays and LED matrices. TI's own typical application drives eight LEDs, one per output, each through 560 Ω.
• Driving relay or motor boards through transistors, where each output only feeds a base or gate.
• Setting many control lines at once with no glitches between states, thanks to the latch.

It only outputs. To read eight switches with three pins, the matching part is a parallel-in shift register such as the 74HC165. When one line out of eight has to be picked, a 74HC138 decoder needs no clocking at all. The logic families lesson compares HC with the other CMOS and TTL families.

A 5 V microcontroller drives SER, SRCLK and RCLK. SRCLR is tied to 5 V and OE to ground. Each output drives a red LED to ground through its own resistor, so an LED lights when its bit is 1. Neither maker prints a 5 V row, so the numbers use the 4.5 V limits, the nearest supply both sheets specify. At 5 V the outputs drive a little harder, so the 4.5 V figures leave margin.

Wire QH' (pin 9) of the first chip to SER (pin 14) of the second, and run the same SRCLK and RCLK lines to both. Each clock now moves the data through sixteen places, so you send two bytes, the one for the far chip first, and then latch once. Every extra chip adds eight outputs and no microcontroller pins.

At TI's 25 MHz the chip could take two chips' worth of bits in about 0.66 µs and four chips in 1.3 µs, so the speed at which the microcontroller toggles its pins sets the pace. Each chip in the chain still has its own 70 mA supply-pin limit and needs its own 100 nF capacitor. On long chains, run the clock as a single line from chip to chip, not a star, and keep it short.

• Fit 100 nF from pin 16 to pin 8, right at the chip, on every chip in a chain.
• Never leave SRCLR or OE open. Tie SRCLR to VCC and OE to ground, or to a microcontroller pin if you want to clear or blank the outputs.
• Pull OE high with 10 kΩ at power-up if the loads must not twitch. SRCLR clears only the shift register, so the outputs can show any byte until the first latch.
• Add up the current of every output that can be on at once and keep it under about 50 mA per chip, not the 70 mA limit.
• For loads above a few milliamps, let the output drive a transistor or a 2N7000 gate rather than the load itself.

The IC power and decoupling lesson covers the capacitor, and logic levels the input thresholds when the microcontroller runs on 3.3 V.

The TI and Diodes parts swap freely: same pins, same function, same input limits at 4.5 V and the same ±35 mA output rating. Only the pin names on the drawing change.

1. Power it from 5 V with SRCLR to VCC, OE to ground and SER, SRCLK and RCLK to ground, and measure the supply current with no LEDs fitted. It should be a few microamps; TI allows 80 µA at 6 V up to 85 °C. Milliamps means a floating input or a damaged chip.
2. Set SER high, give SRCLK one pulse and RCLK one pulse. QA (pin 15) should go high and the other outputs stay as they were.
3. Give seven more SRCLK pulses with SER low, then one RCLK pulse. The 1 should now be on QH (pin 7), and QH' (pin 9) high too.
4. Pull OE high. Every output should float; with LEDs fitted, all go dark. Pull it low and the same byte comes back.

If the outputs show the byte one step late, RCLK and SRCLK are tied together: TI notes that the shift register is then one clock ahead of the outputs. Give RCLK its own pin.

Use a 74HC595 whenever a project needs more outputs than the microcontroller has: LED bars, seven-segment digits, relay boards through transistors. Give it three pins (data, clock and latch), tie SRCLR high and OE low (or OE to a PWM pin for brightness), and fit 100 nF at every chip. Size LED resistors for the whole chip, not one output: 470 Ω for red LEDs on 5 V keeps all eight under 50 mA. Buy the DIP from TI for the breadboard and either maker's SO-16 for a board.

The DIP is stocked by most Indian component sellers, usually as TI's SN74HC595N and sometimes with no maker named. Check the full number on the body: a listing for the 74HC595 may turn out to be a 74HCT595, with the same pins but TTL input levels and a 4.5 V minimum supply. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.