74HC138 3-to-8 decoder: pinout, truth table, an LED select circuit and 16-line expansion

TI sells it as SN74HC138: SN74HC138N is the plastic DIP, SN74HC138D the SOIC and SN74HC138PW the TSSOP, all with the same pin numbers. Diodes Incorporated drops the prefix (74HC138S16 for SO-16, 74HC138T16 for TSSOP-16) and has no DIP. The SN54HC138 is TI's military-temperature version in ceramic packages.

The two makers label the same six pins differently. TI calls the address inputs A, B and C and the enables G1, G2A and G2B; Diodes calls them A0, A1, A2 and E3, E1, E2. The numbering is the same, so a circuit drawn with one set of names works with the other chip. Watch the enables: TI's G1, the active-high one, is Diodes' E3, not E1.

A 74HCT138 is the TTL-input version with the same pins; it runs from 4.5 to 5.5 V and is not covered here. A 74LS138 is the older bipolar part, also with the same pinout but with different input currents and levels.

With G1 high and both G2 pins low, the chip reads C B A as a binary number from 0 to 7 and pulls that output low: 0 0 0 selects Y0, 1 0 1 selects Y5, 1 1 1 selects Y7. If any enable is in its wrong state (G1 low, or G2A or G2B high), all eight outputs go high and the address is ignored. Both datasheets give the same table.

The outputs are active low. TI aims the part at memory decoders, and memory chip-select inputs are active low too. To drive something that wants an active-high signal, add an inverter or a transistor stage, or wire the load from VCC to the output so it runs when the output goes low, as the worked example below does.

The two makers agree on the supply: 7 V absolute maximum, 2 to 6 V recommended. Each output may carry ±25 mA and the VCC or ground pin 50 mA for the whole chip. TI rates its SN74HC138 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.

Because the decoder only ever pulls one output low, the 50 mA ground-pin limit is hard to reach in normal use. The per-output limit is the one to watch when an output sinks an LED or a relay coil directly.

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 do not count on hysteresis when the part might come from either maker. Tie unused enables to their active level and never leave an input open.

The output figures that matter for a load: at 4 mA an output stays below 0.26 V when low at 25 °C and below 0.33 V over the full temperature range. Typical is 0.17 V on TI's sheet and 0.15 V on Diodes'.

At 4.5 V the worst address-to-output delay is 36 ns at 25 °C on TI's sheet (18 ns typical) and 45 ns up to 85 °C; Diodes gives 30 ns at 25 °C. When a microcontroller sets the address in software, that is too short to matter; on a memory bus it counts. Also, while the address bits change one after another, the chip briefly decodes the in-between values, so a different output can pulse low for a few nanoseconds. Hold an enable inactive while the address changes if those glitches would hurt.

• Chip-select decoding: three address lines pick one of eight SPI devices, memories or peripherals.
• Scanning an LED matrix or a multiplexed seven-segment display one column at a time. TI's own application section shows it as the column selector beside a 74HC595.
• Saving microcontroller pins: three pins select eight lines, four pins and two chips select sixteen.
• Demultiplexing: put a data signal on one enable and it appears, inverted or not, on the selected output only.

It is a poor choice when two outputs must be active at once, because it can only ever pull one low; a shift register such as the 74HC595 handles that. For the gates that usually sit around it, see the 74HC logic gates page. The logic families lesson compares HC with the other CMOS and TTL families.

A 5 V microcontroller drives A, B and C. G1 is tied to 5 V and G2A and G2B to ground, so the chip is always enabled. Each output gets a red LED and a resistor from the 5 V rail: the LED lights when its output goes low. Neither maker prints a 5 V row, so the numbers use the 4.5 V limits, the nearest supply both sheets specify.

To scan all eight LEDs so they look lit together, step the address through 0 to 7 a few hundred times a second. Each LED is then on for an eighth of the time, so it looks dimmer; the LED circuits lesson covers brightness and duty cycle.

The three enables are there so decoders can be stacked with no extra logic. For 16 lines, wire A0 to A2 to both chips. Send the fourth address bit, A3, to G2A of the first chip and to G1 of the second. When A3 is low the first chip is enabled and the second is disabled, so lines 0 to 7 come from the first chip; when A3 is high it is the other way round.

TI's overview says the same trick reaches 24 lines with three chips and no inverter, and 32 lines with four chips and one inverter. Diodes gives the 16-line and 32-line versions.

• 1 chip: 8 lines from 3 address bits, extra logic: none.
• 2 chips: 16 lines from 4 address bits, extra logic: none.
• 3 chips: 24 lines from 5 address bits, extra logic: none.
• 4 chips: 32 lines from 5 address bits, extra logic: one inverter.

• Fit 100 nF from pin 16 to pin 8, right at the chip.
• Never leave an enable open. A floating G2A or G2B can read high, and then every output stays high and the chip looks dead.
• When reading a Diodes or other non-TI schematic, map E1, E2 and E3 by pin number, not by the digit: E3 is the active-high one.
• Change the address with an enable held off if the load reacts to nanosecond glitches, such as a clock or a latch input.
• 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 and pull-up and pull-down lessons cover the first two points in detail.

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

1. Power it from 5 V with G1 to VCC, G2A and G2B to ground and A, B, C to ground, and measure the supply current. 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. With a meter on pin 15 (Y0), check it reads near 0 V and that pin 14 (Y1) reads near 5 V.
3. Move A to 5 V: Y0 should go high and Y1 low. Step through all eight addresses and check one output at a time goes low, following Figure 2.
4. Pull G1 to ground. Every output should go high, whatever the address. If one stays low, that output or an enable pin is damaged.

Use a 74HC138 whenever a few microcontroller pins have to pick one of many things: chip selects, LED columns, relay drivers. Power it from the same rail as the microcontroller, tie the enables you do not use to their active level, and wire loads from VCC to the outputs. Size LED resistors for 4 mA with the worst-case 0.33 V low level, which gives 680 Ω on 5 V for a red LED. 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 SN74HC138N and sometimes with no maker named. Check the full number on the body: a listing for the 74HC138 may turn out to be a 74HCT138 or a 74LS138, both with the same pins but different input levels. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.