CD74HC238E Pinout, Specifications & Common Mistakes

CD74HC238E Pin Diagram

Key Specifications

Why do these numbers matter? The ~14ns address-to-output delay means a chip-select can decode in one logic step, and the 4µA quiescent current keeps the decoder invisible on the power budget — the 2–6V range covers 3.3V and 5V rails without level translation.

How the Enables Work

But how do the enables gate the outputs? All three must be satisfied before any output goes HIGH. E1 and E2 are active-LOW, E3 is active-HIGH: the decoder only operates when E1 = LOW, E2 = LOW, E3 = HIGH.

If any enable is in the wrong state, every output is forced LOW.

The double active-LOW plus single active-HIGH enable is the cascading feature — tie them in different patterns and you can stack decoders, or use one enable as a data input for demultiplexing.

Typical Application Circuits

Eight-device chip-select: three address lines from the MCU into A0–A2, the eight outputs into the enable pins of eight peripherals that need an active-HIGH select. One 238 replaces eight discrete address-decode gates, with the ~14ns delay sitting inside one bus cycle.

1-of-32 expansion: four 238s plus one inverter decode five address lines into 32 selects — the extra address bit picks which decoder is enabled through the enable inputs. The Arduino-forum example goes further: a 238 feeding the active-HIGH E3 enables of eight 138s gives 1-of-64 selection from six wires.

Common Mistakes When Using CD74HC238E

1. Floating enable pins — the top field failure. An E1 or E2 left unconnected floats into the wrong state and every output sits LOW.

The fix is boring and mandatory: tie unused enables to their active level (E1/E2 to GND, E3 to VCC) at layout time. Forum threads on "decoder does nothing" consistently resolve to this.

2. Confusing 238 and 138 output polarity. The two parts are pin-identical and the datasheet says so — only the outputs are non-inverting versus inverting. A board designed for the 138's active-LOW selects will have every LED, chip-enable, and load wired backwards with a 238.

An Arduino forum build hit exactly this: LEDs connected to ground the wrong way because the output polarity changed.

3. Assuming the outputs drive power loads. They're standard HC drive (4mA-class at 5V). A relay coil, buzzer, or LED directly at that current works; a MOSFET gate or a long wire at speed needs a buffer.

The decoder decodes; it doesn't drive power.

4. Forgetting the gap in the output numbering. Pin 7 is Y7 and pins 9–15 are Y6–Y0 — pin 8 is GND between them. Board layouts that route "outputs 0–7 in a row" across pin 7 to pin 9 silently mis-wire Y7.

5. Cascading with overlapping enables. When you stack decoders, only one output anywhere in the tree must be active. If two enable paths overlap, two loads get selected at once — the forum 1-of-64 build warns explicitly that the single-output-at-a-time rule is what the enable math is for.

Frequently Asked Questions

Q1: What's the difference between CD74HC238 and 74HC138?

A: Output polarity only — everything else is identical. The datasheet states it directly: the 238 is identical to the 138 but has non-inverting (active-HIGH) outputs. Same DIP-16 pinout, same address and enable logic, same speed. The 138's selected output goes LOW; the 238's goes HIGH. Pick by what your load expects.

Q2: Why are all my outputs LOW no matter what?

A: One of the three enables is in the wrong state — usually floating. The decoder only operates with E1 = LOW, E2 = LOW, E3 = HIGH. Any other combination forces all outputs LOW. Tie E1 and E2 to GND, E3 to VCC, and drive them when you need strobe control — floating enables are the number one cause of "decoder does nothing."

Q3: What do I do with unused enable pins?

A: Tie them to their active level — never leave them floating. Unused E1/E2 go to GND, unused E3 goes to VCC. CMOS inputs float into undefined states, and a floating enable makes the whole decoder behave randomly — it's a layout-time decision, not a field-time one.

Q4: Can I decode more than 8 outputs?

A: Yes — cascade through the enables. Four 238s plus one inverter give 1-of-32 from five lines. The forum-tested pattern goes further: a 238's active-HIGH outputs drive the E3 enables of eight 138s for 1-of-64 from six wires. The enable structure is exactly what makes this expansion possible.

Q5: How do I wire LEDs to the outputs?

A: Output to LED to ground — because the selected output is HIGH. With the 238's active-HIGH outputs, the LED turns on when its output goes HIGH, so the LED sits between the output pin and GND through a current-limit resistor. With a 138 it's the mirror image (LED from VCC to the output). Getting this backwards is the classic polarity mistake between the two parts.

Q6: What supply voltage does it work at?

A: 2V to 6V — 3.3V and 5V rails both work directly. The HC family uses CMOS logic levels, so a 3.3V MCU can drive it without translation. If your system is mixed TTL/CMOS, the 74HCT variant accepts TTL input levels directly.

Q7: Can I use it as a demultiplexer?

A: Yes — feed data into one enable input. With the address pins selecting the destination, drive an active-LOW enable (E1 or E2) with the data signal and keep the other enables active. The selected output echoes the data; the others stay LOW. Unused enables still need their fixed tie.

Q8: Where can I buy CD74HC238E?

A: From ICMASS — DIP-16 or the SO/TSSOP variants on request. We stock the 238/138 decoder family with batch-verified part marking. For a decode tree, the same order can carry the 138s and the pull resistors — one shipment from Shenzhen covers the whole logic section.