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TI / NXP · Integrated circuit

PCF8574 I2C I/O Expander

Eight more pins over two wires, and the chip on every I2C LCD backpack. Quasi-bidirectional outputs, which is the catch.

Cheat sheet (SVG)
Logic level
2.5-6 V
Package
PDIP-16 / SOIC-16 / TSSOP-16

1.Pinout diagram

Highlight a signalhover to preview, click to keep

Pins 1-8

PCF8574

Pins 9-16

Pin detail

Hover a pin for its mappings and gotchas. Click to keep it here.

Logic level
2.5-6 V
Package
PDIP-16 / SOIC-16 / TSSOP-16

Fig. 1. PCF8574 I2C I/O Expander, PDIP-16. Hover or tap a signal to light up every pin that carries it.

2.Watch out

  • There are no registers. You write one byte and you read one byte, which makes it trivial to drive and impossible to configure.
  • Outputs are quasi-bidirectional: a strong pull-down but only a weak 100 uA pull-up. They sink 25 mA happily and source almost nothing, so wire LEDs to the positive rail and sink through the pin.
  • To use a pin as an input, write a 1 to it first. That releases it to the weak pull-up, and an external switch to ground then reads as 0. Writing a 0 and then reading gives you 0 forever.
  • A0, A1 and A2 set the low three address bits: 0x20 to 0x27 on the PCF8574, and 0x38 to 0x3F on the PCF8574A. Sellers mix the two parts freely, so scan the bus before assuming.
  • INT (pin 13) is open drain and pulses on any input change, which saves polling. It needs its own pull-up.
  • If you need real configurable pull-ups, push-pull outputs or per-pin direction, the MCP23017 does all three for a little more money and a register map.

3.Minimum circuit

What has to go around the chip before it will run.

VCCGNDPCF85744.7 kOhm between SDA (15) and VCC4.7 kOhm4.7 kOhm between SCL (14) and VCC4.7 kOhmWire link between A0 (1) and GNDWire linkWire link between A1 (2) and GNDWire linkWire link between A2 (3) and GNDWire link100 nF between VDD (16) and GND100 nF10 kOhm between INT (13) and VCC10 kOhmVSS (8) to GNDVDD (16) to VCCA0 1A1 2A2 3VSS 8INT 13SCL 14SDA 15VDD 16VDD 16
Fig. 2. Minimum circuit, drawn from Table 1. Dashed parts are optional. Crossing wires are not connected.
Table 1. External parts, and why each one is there.
PartBetween
4.7 kOhmSDA (15) to VCC
4.7 kOhmSCL (14) to VCC
Wire linkA0 (1) to GND
Wire linkA1 (2) to GND
Wire linkA2 (3) to GND
100 nFVDD (16) to GND
10 kOhmoptionalINT (13) to VCC

The outputs are quasi-bidirectional: they can only sink 25 mA and source about 100 uA. Drive LEDs to ground, not from the pin, and write a 1 to a pin before using it as an input.

4.Register configuration

Turning a pin on the diagram into a configured pin.

  • Read and write the port over I2C

    Linux i2c-dev

    #include <fcntl.h>
#include <unistd.h>
#include <linux/i2c-dev.h>
#include <sys/ioctl.h>

int fd = open("/dev/i2c-1", O_RDWR);
ioctl(fd, I2C_SLAVE, 0x20);        /* A2..A0 all grounded */

uint8_t out = 0x0F;                /* low nibble low, high nibble released */
write(fd, &out, 1);

/* To read a pin you must first write a 1 to it, which releases it
   to the weak pull-up. A pin you last wrote 0 reads 0 forever. */
uint8_t mask = 0xF0;
write(fd, &mask, 1);
uint8_t in;
read(fd, &in, 1);

    There are no registers and no direction bits. Writing 1 releases a pin to a weak 100 uA pull-up so it can be read; writing 0 pulls it hard low. Outputs sink 25 mA but barely source, so wire LEDs to the positive rail.

5.Full pin reference

Every pin, alias and mapping. This is the table that prints.

Table 2. Pin assignments.

Complete pin reference for PCF8574 I2C I/O Expander
PinNameFunctions

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Primary sources

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