PCF8574AT/3,518 - 8-bit I2C I/O Expander | NXP SOIC-16
MPN: PCF8574AT/3,518 ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $1.78 | $1.78 |
| 10 | $1.45 | $14.50 |
| 100 | $1.18 | $118.00 |
| 500 | $0.96 | $480.00 |
| 1,000 | $0.82 | $820.00 |
| 2,500 | $0.74 | $1,850.00 |
PCF8574AT/3,518 Overview
An I/O expander is a class of digital peripheral IC that extends the limited GPIO count of a host microcontroller through a serial bus, allowing additional inputs, outputs, keypads, LED indicators, and signal-conditioning lines without increasing pin count on the MCU. Within the hierarchy of interface ICs (also called bus expanders or GPIO extenders), the PCF8574AT belongs to the I2C-bus parallel-port family, positioned between simple shift-register expanders (which lack interrupt capability) and full SPI/I2C port-mapped devices. This category sits under digital logic and interface ICs in broader semiconductor taxonomy, supporting both consumer and industrial designs where MCU pin count is constrained.
Key features include quasi-bidirectional I/O that can directly drive LEDs without external pull-ups (sinking capability of up to 25 mA per pin, max 100 mA total), an active-low interrupt output (INT) that signals the host when any input state changes, and 400 kHz Fast-mode I2C capability on the PCF8574A/PCF8574AT family variants. The device integrates a noise filter on the SCL/SDA lines and supports clock stretching as a slave device.
The PCF8574AT uses a CMOS silicon-gate architecture with latched output registers and input latches on every I/O pin. The quasi-bidirectional design uses a weak pull-up (typically 100 uA current source) to drive outputs high; pulling low requires the pin to actively sink current. This architecture eliminates the need for external pull-up resistors on outputs while maintaining low standby current.
Typical applications include driving LED arrays, LCD segment interfaces, keypad matrix scanning, fan-speed monitoring, relay control boards, and general-purpose I2C slave peripheral expansion in industrial control, consumer electronics, and embedded systems. Engineers commonly pair the PCF8574AT with low-power MCUs (such as the LPC1100 or PIC16F families) to add 8-bit digital I/O over only two signal wires.
When designing with this part, note that the quasi-bidirectional I/O pins have an asymmetric drive capability - they source weakly (100 uA) but sink strongly (25 mA per pin). This makes them ideal for LED-driving (sink mode) but less suitable as push-pull signal drivers. Always respect the absolute maximum of 100 mA total current through the VDD/GND pins to avoid latch-up.
This page synthesizes distributor pricing, drop-in same-package alternatives, and practical design notes not found in the manufacturer datasheet alone, including cross-references for the very similar PCF8574T/3 (different I2C address range).
Drop-in alternatives for PCF8574AT/3,518 — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Variants in this series
Same-series models that are drop-in compatible with PCF8574AT/3,518 (same form factor and footprint) — differing in Interrupt Output, Package, RoHS Status, Function, I2C Bus Speed.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
PCF8574AT/3
✅ Drop-In📋 Reference alternative (not in catalog)
PCF8574ATS
✅ Drop-In📋 Reference alternative (not in catalog)
PCF8574AP
✅ Drop-In📋 Reference alternative (not in catalog)
PCF8574T/3,518
✅ Drop-In📋 Reference alternative (not in catalog)
PCF8574AN
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
PCF8574AT/3,518 Specifications
| Part Number | PCF8574AT/3,518 |
| Function | 8-bit I/O Expander with I2C-bus interface |
| Number of I/O | 8 |
| Interface Type | I2C (SCL/SDA), 2-wire serial |
| Maximum I2C Clock Frequency | 100 kHz |
| Supply Voltage Range | 2.5 V to 6 V |
| I2C Address Range (base) | 0x38-0x3F (PCF8574A family: 0x38-0x3F) |
| Output Type | Quasi-bidirectional (weak pull-up current source) |
| Sink Current per Pin (max) | 25 mA |
| Total Sink Current (max) | 100 mA |
| Source Current (typical) | 100 uA (weak pull-up) |
| Interrupt Output | Yes, active-low INT pin (open-drain) |
| Technology | CMOS silicon-gate |
| Operating Temperature Range | -40 C to +85 C |
| Package Type | SOIC-16 (MS-013 / SOT-162-1), 7.50 mm body, lead-free |
| Mounting Type | Surface Mount |
| RoHS Status | Compliant (lead-free) |
PCF8574AT/3,518 Pin Configuration
| Pin 1 | A0 — Hardware address input 0 (sets I2C address LSB) |
| Pin 2 | A1 — Hardware address input 1 |
| Pin 3 | A2 — Hardware address input 2 (sets I2C address MSB) |
| Pin 4 | P0 — Quasi-bidirectional I/O port 0 |
| Pin 5 | P1 — Quasi-bidirectional I/O port 1 |
| Pin 6 | P2 — Quasi-bidirectional I/O port 2 |
| Pin 7 | P3 — Quasi-bidirectional I/O port 3 |
| Pin 8 | VSS — Ground (0 V) |
| Pin 9 | P4 — Quasi-bidirectional I/O port 4 |
| Pin 10 | P5 — Quasi-bidirectional I/O port 5 |
| Pin 11 | P6 — Quasi-bidirectional I/O port 6 |
| Pin 12 | P7 — Quasi-bidirectional I/O port 7 |
| Pin 13 | INT — Interrupt output (active-low, open-drain) |
| Pin 14 | SCL — I2C serial clock input |
| Pin 15 | SDA — I2C serial data (bidirectional, open-drain) |
| Pin 16 | VDD — Supply voltage (2.5 V to 6 V) |
Typical Applications
PCF8574AT/3,518 is suitable for 6 applications: MCU GPIO Expansion, LED Array and Display Control, Keypad Matrix Scanning, Industrial Control and Relay Driving, LCD Segment Driver Interface, Sensor Hub and Fan Monitoring.
MCU GPIO Expansion
The PCF8574AT/3,518 extends the limited pin count of low-cost microcontrollers by adding 8 quasi-bidirectional I/O lines over a 2-wire I2C bus, consuming only SCL and SDA on the host MCU. Its 2.5-6 V supply matches common MCU rails (3.3 V, 5 V), while the 100 kHz I2C clock is sufficient for digital control signals at low update rates. With three hardware address pins, up to 8 PCF8574AT devices can share one bus, delivering 64 extra I/O from a 2-pin MCU interface. Unlike SPI shift-register expanders, the integrated INT pin interrupts the host only when input states change, reducing polling overhead.
Recommended
LED Array and Display Control
The PCF8574AT's 25 mA sink capability per pin (100 mA total) directly drives standard 5 mm or 3 mm LEDs with an external current-limiting resistor on the LED anode side, eliminating the need for external transistor drivers. The quasi-bidirectional output architecture means LEDs can be wired active-low (LED anode to VDD, cathode to PCF8574AT pin) for simple software control with low standby current. Its 100 kHz I2C interface refreshes up to 8 LEDs faster than 60 Hz, eliminating visible flicker. The PCF8574AT is widely used in industrial 7-segment displays, status indicator panels, and decorative LED matrices.
Recommended
Keypad Matrix Scanning
In a 4x4 or 8x8 keypad matrix, the PCF8574AT serves as both row driver and column scanner over I2C, eliminating dedicated MCU pins for keypad interfacing. The quasi-bidirectional I/O can source weak current (100 uA) for row scanning and sink strong current (25 mA) when a key is pressed - the natural pull-up/pull-down asymmetry suits keypad scanning without external resistors. The INT pin wakes the host MCU only when a key event occurs, conserving power. With debounce handled in firmware, the PCF8574AT supports up to 64 keys when stacked with a second PCF8574T on the same bus.
Recommended
Industrial Control and Relay Driving
In industrial control boards, the PCF8574AT interfaces low-power relay drivers, optocouplers, and 24 V logic translation circuits to a host MCU across long I2C cable runs (up to several meters with proper buffering). Its -40 C to +85 C industrial operating temperature range meets IEC 60068 environmental standards for factory automation. The 2.5-6 V supply tolerance handles noisy industrial rails. With the INT pin, the PCF8574AT supports event-driven control loops in PLCs, sensor hubs, and distributed I/O modules where polling overhead is unacceptable.
Recommended
LCD Segment Driver Interface
Character LCD modules (HD44780-compatible) with I2C backpack boards typically use the PCF8574AT or PCF8574T as the I2C-to-parallel bridge, reducing MCU pin usage from 6-10 pins down to 2. The PCF8574AT's 100 kHz I2C clock is sufficient for character LCD refresh rates (typically <1 kHz data updates). With 8 I/O pins, the PCF8574AT supports 4-bit data mode (D4-D7) plus RS, RW, EN control lines. Combined with the LCD backlight control pin, this fits a 16x2 or 20x4 character display with a single I2C bus transaction. Common in 3D printers, point-of-sale terminals, and embedded HMIs.
Recommended
Sensor Hub and Fan Monitoring
In server, networking, and PC peripheral designs, the PCF8574AT aggregates discrete signals from multiple sources - fan tachometer inputs, chassis detection switches, button inputs, and indicator outputs - into a single I2C addressable port. The INT pin alerts the host BMC or MCU when any input changes state (e.g., a fan stall signal or chassis intrusion), enabling event-driven firmware without constant polling. Its 2.5-6 V supply integrates directly with 3.3 V or 5 V system rails. The PCF8574AT is found in IPMI management cards, NAS systems, and router control boards where GPIO count is limited.
Recommended
Recommended Products Summary
Engineering reference data for PCF8574AT/3,518 — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | PCF8574AT/3 | PCF8574ATS | PCF8574AP | PCF8574T/3,518 | PCF8574AN |
|---|---|---|---|---|---|---|
| Brand | NXP Semiconductors | NXP Semiconductors | NXP Semiconductors | NXP Semiconductors | NXP Semiconductors | NXP Semiconductors |
| Package | SOIC-16 (SOT-162-1) | SOIC-16 (SOT-162-1) - same | SOIC-16 (SOT-162-1) - same | SOIC-16 (SOT-162-1) - same | SOIC-16 (SOT-162-1) - same | SOIC-16 (SOT-162-1) - same |
| I2C Base Address | 0x3E (range 0x38-0x3F) | 0x3E (range 0x38-0x3F) - same | 0x3E (range 0x38-0x3F) - same | 0x3E (range 0x38-0x3F) - same | 0x20 (range 0x20-0x27) - different | 0x3E (range 0x38-0x3F) - same |
| Number of I/O | 8 | 8 | 8 | 8 | 8 | 8 |
| Supply Voltage Range | 2.5 V to 6 V | 2.5 V to 6 V - same | 2.5 V to 6 V - same | 2.5 V to 6 V - same | 2.5 V to 6 V - same | 2.5 V to 6 V - same |
| I2C Max Clock Frequency | 100 kHz | 100 kHz - same | 100 kHz - same | 100 kHz - same | 100 kHz - same | 100 kHz - same |
| Sink Current per Pin (max) | 25 mA | 25 mA - same | 25 mA - same | 25 mA - same | 25 mA - same | 25 mA - same |
| Interrupt Output | Yes (active-low INT, open-drain) | Yes - same | Yes - same | Yes - same | Yes - same | Yes - same |
| Operating Temperature Range | -40 C to +85 C | -40 C to +85 C - same | -40 C to +85 C - same | -40 C to +85 C - same | -40 C to +85 C - same | -40 C to +85 C - same |
Key Differentiators
- Different I2C address base (0x3E) for bus multiplexing (vs PCF8574T/3,518)
- Integrated interrupt output with change-detection (vs MCP23008 (Microchip))
- Quasi-bidirectional I/O eliminates external pull-ups on outputs (vs Standard push-pull GPIO expanders (e.g., PCA9554))
- Wide supply voltage range (2.5 V to 6 V) covers 3.3 V and 5 V rails (vs MCP23008 (1.8 V to 5.5 V))
Design Notes
The PCF8574AT supply voltage (2.5 V to 6 V) directly feeds the VDD pin and the internal I/O pull-up current sources. For 3.3 V or 5 V systems, connect VDD to the same rail as the host MCU. Add a 100 nF decoupling capacitor as close to VDD (pin 16) as possible, with a 10 uF bulk capacitor on the supply rail if multiple I2C devices share the bus. The device draws approximately 40 uA standby plus I/O source current (100 uA per output high). Total worst-case supply current at full I2 load is around 200 uA - negligible compared to LED sink current.
Place the PCF8574AT within 50 mm of the host MCU's I2C pins to minimize bus capacitance. Route SCL and SDA as a matched differential pair with 4-8 mil traces, keeping them away from switching signals and noisy power traces. Add 4.7 kohm pull-up resistors on both SCL and SDA to VDD (closer to the PCF7514A than to the MCU for multi-device buses). The exposed SO-16 thermal pad (none on this package) does not require special heatsinking - the device dissipates under 100 mW. Ground plane under the SO-16 improves EMI immunity.
Critical: never exceed 100 mA total sink current across all 8 I/O pins simultaneously - this stresses the VDD/VSS bond wires and can cause latch-up. Do not drive inductive loads (relays, solenoids) directly from the quasi-bidirectional pins without external flyback diodes and drivers. Note that the quasi-bidirectional I/O cannot actively source 25 mA - the 100 uA weak pull-up only drives logic high; if a load requires more source current, use an external buffer (e.g., 74HC244). The INT pin is open-drain and requires an external pull-up resistor (typically 10 kohm) to VDD.
The PCF8574AT integrates a 50 ns spike filter on SCL/SDA lines, suppressing noise-induced glitches from capacitive coupling on long bus traces. However, for cable runs exceeding 1 meter, add a 100 ohm series termination at each end to dampen ringing. The INT pin output should be treated as asynchronous - debounce in firmware by waiting at least 1 ms after INT goes low before reading the port state. When multiple PCF8574AT devices share a bus with different A0-A2 address pins, ensure no two devices have identical address combinations to avoid bus contention.
Compliance Information
RoHS compliant and lead-free per NXP product page (lead-free SO-16 MS-013 SOT-162-1 package). AEC-Q100 automotive qualification not available for PCF8574AT - for automotive applications use PCF8574AT-Q1 (separate qualified variant).
