NXP PCF8574T PCB footprint, checks, and sourcing guide

The NXP PCF8574T is a 8-bit quasi-bidirectional I/O expander from NXP Semiconductors. Its package or board interface is 16-pin SOIC, and its relevant electrical envelope is 2.5–6.0 V. It communicates or connects through I²C, addresses 0x20–0x27; active-low INT. Those fields belong together: substituting a familiar family name while changing package, voltage, sensing port, mount style, current class, or interface behavior can leave a PCB that passes ordinary net checks and still cannot be assembled or function safely.

PCF8574 provides eight quasi-bidirectional pins whose weak-high behavior differs from normal push-pull GPIO.

Common uses include buttons and simple low-current outputs and character-LCD backpack interfaces. Start with the manufacturer drawing and recommended application, then record the exact ordering suffix alongside the KiCad symbol and footprint. This makes the library evidence reviewable when the part is re-sourced months later.

• Match package width and pitch and place local decoupling at VDD. Put address straps, reset, and interrupt pull-ups close enough to avoid floating during power-up.
• Group repeated channels clearly and keep high-current PWM or connector returns from sharing narrow ground paths with the IC.

Strap A0–A2, pull INT up, and check whether loads can work with strong low but weak high current; use drivers for LEDs or relays.

• Set every address pin explicitly and document the resulting address. Check I/O voltage, power-up state, pull-up capability, interrupt polarity, per-pin current, total package current, and whether outputs are push-pull or quasi-bidirectional.
• Add external drivers for relays, motors, and high-current LEDs. An expander controls logic; its total package current and clamp diodes are not a substitute for power stages or connector protection.

Put the support components where their current, thermal, optical, RF, or measurement loops are actually short—not merely where ratsnest lines look tidy. Confirm pin one from the package view used in the datasheet, distinguish top view from mating face or bottom view, and check mask, paste, drill, courtyard, enclosure, and rework access independently. A correct copper pad pattern can still be a bad production footprint when the sensing opening, connector latch, exposed pad, thermal path, or cable volume is wrong.

MakeIRL’s release gate should not stop at “the symbol has the right number of pins.” For this part, a useful gate review combines ERC/DRC with the following package- and function-specific evidence:

1. Check supply, decoupling, I²C pull-ups, address straps, reset, interrupt, channel numbering, default state, per-pin and package current, and connector ESD.
2. Check that firmware address and bit order match the populated straps and schematic symbols, especially across A/B banks.
3. Check exact suffix, package and I/O architecture because similarly named expanders differ in reset, pull-ups, interrupt, and output drive.
4. For NXP PCF8574T, check A0–A2, 0x20 base family, INT pull-up, quasi-bidirectional default-high behavior, per-pin/package current, I²C pulls, and decoupling.

Then run ERC and DRC, refill zones, and inspect the fabrication and assembly outputs. Cross-probe the exact pads named by any finding, compare the BOM MPN with the footprint and electrical limits above, and verify that a real cable, enclosure, antenna, sensor stimulus, load, or thermal path can be tested on the assembled unit. An exclusion is evidence that someone dismissed a marker; it is not evidence that the underlying condition was resolved.

The most expensive errors are usually plausible: a footprint from a sibling package, a breakout-board voltage copied to the bare IC, a headline current used without thermal analysis, or a connector family selected by pitch alone. For NXP PCF8574T, review these failure modes explicitly:

• Writing a high does not drive a strong high like a push-pull output, so sourcing LED current or fast edges can fail.
• Leaving address pins open and getting a board-dependent I²C address.
• Driving LEDs or relays beyond total package current even though each individual pin appears below its limit.

Sourcing note. Use PCF8574T exactly; PCF8574A uses a different base address and can break firmware or collide with another device. The approved vendor list should preserve manufacturer, full suffix, package, voltage/range/accuracy grade, lifecycle, and mating or external components. An alternate is real only after its datasheet, land pattern, electrical behavior, firmware assumptions, and assembly process have all been compared—not because a distributor search places it in the same parametric row.