Overview

The 74HCT245 (commonly SN74HCT245N in a 20-pin DIP package) is an 8-bit non-inverting bidirectional 3-state bus transceiver IC. As a member of the HCT logic family, it operates on a 5 V5\text{ V} supply while accepting TTL-compatible input thresholds (VIH≥2.0 VV_{IH} \ge 2.0\text{ V}).

Because its high-level input voltage threshold (VIH=2.0 VV_{IH} = 2.0\text{ V}) is fully compatible with 3.3 V3.3\text{ V} CMOS signals, the 74HCT245 is widely used as a 3.3V-to-5V level shifter and bus buffer between 3.3V microcontrollers (ESP32, STM32, Raspberry Pi) and 5V peripherals (WS2812B LEDs, 5V LCD displays, legacy buses).

Quick reference

Logic FamilyHigh-Speed CMOS with TTL Inputs (HCT)
Package20-Pin DIP (Through-Hole) / SOIC-20 / TSSOP-20
Supply Voltage Range (VCCV_{CC})4.5 V4.5\text{ V} to 5.5 V5.5\text{ V} DC (5 V5\text{ V} nominal)
Bus Width8 Bits (Bidirectional A1…A8↔B1…B8A_1 \dots A_8 \leftrightarrow B_1 \dots B_8)
TTL Input Threshold (VIHV_{IH})≥2.0 V\ge 2.0\text{ V} (Compatible with 3.3V MCU outputs)
Propagation Delay (tpdt_{pd})15 ns15\text{ ns} typical at VCC=5VV_{CC} = 5\text{V}
Output Sink/Source CurrentUp to 35 mA35\text{ mA} high-drive 3-state output

Pinout (20-Pin DIP Package)

text
        ┌──────────────┐
    DIR ─│ 1         20 │─ VCC
     A1 ─│ 2         19 │─ OE
     A2 ─│ 3         18 │─ B1
     A3 ─│ 4         17 │─ B2
     A4 ─│ 5         16 │─ B3
     A5 ─│ 6         15 │─ B4
     A6 ─│ 7         14 │─ B5
     A7 ─│ 8         13 │─ B6
     A8 ─│ 9         12 │─ B7
    GND ─│ 10        11 │─ B8
        └──────────────┘
PinNameDescription
1DIRDirection control pin (High = A to B, Low = B to A)
2–9A1–A8Side A bidirectional bus pins
10GNDGround reference (0 V)
11–18B8–B1Side B bidirectional bus pins
19~OE~Active-LOW Output Enable (Low = Enabled, High = High-Z disconnect)
20VCCPower supply input (+4.5V to +5.5V DC)

Function Table

Output Enable (~OE~)Direction (DIR)Operation
Low (0)High (1)AA data to BB bus (A→BA \to B)
Low (0)Low (0)BB data to AA bus (B→AB \to A)
High (1)X (Don't care)Isolation / High-Impedance (Z)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply VoltageVCCV_{CC}4.55.05.5VDC operational range
High-Level Input VoltageVIHV_{IH}2.0——VTTL compatible (VCC=4.5V…5.5VV_{CC} = 4.5\text{V} \dots 5.5\text{V})
Low-Level Input VoltageVILV_{IL}——0.8VTTL compatible
High-Level Output VoltsVOHV_{OH}3.844.4—VVCC=4.5V,IOH=−6mAV_{CC} = 4.5\text{V}, I_{OH} = -6\text{mA}
Propagation DelaytPLH/tPHLt_{PLH} / t_{PHL}—1525nsVCC=4.5V,CL=50pFV_{CC} = 4.5\text{V}, C_L = 50\text{pF}
Quiescent Supply CurrentICCI_{CC}—8.080μA\mu\text{A}VIN=VCCV_{IN} = V_{CC} or GND\text{GND}

Common mistakes

  • Using 74HC245 instead of 74HCT245 for 3.3V-to-5V translation: Standard 74HC245 powered at 5V requires VIH≥3.15 VV_{IH} \ge 3.15\text{ V}, so 3.3V signals from ESP32/Pi pins are unreliable. 74HCT245 only requires VIH≥2.0 VV_{IH} \ge 2.0\text{ V}, making it the correct part for 3.3V to 5V conversion.
  • Floating OE or DIR pins: Always pull ~OE~ to GND for continuous buffer operation or pull to VCCV_{CC} with a resistor if 3-state control is needed.

Notes

  • 74HC vs 74HCT: "HCT" specifies TTL-level input thresholds at 5V supply, allowing direct control from 3.3V logic outputs without active boost circuits.

Assets & downloads