Expand the I/Os of your controller with this port expander

As a designer the number of available ports on a microcontroller are always less while designing an embedded circuit. To get more out of a single controller chip we need to have an I/O expender, so that we can use maximum of pins in our projects. There are several ways to expand the ports like decoder, Demux or the I2C port expander.

The I/O expender IC we will talking today uses only 2 pins and provides you the access to 8 pins. The PCF8574 device provides general-purpose remote I/O expansion for most microcontroller families by I 2C interface [serial clock (SCL), serial data (SDA)]. The device features an 8-bit quasi-bidirectional I/O port (P0–P7), including latched outputs with high current drive capability for directly driving LEDs. Each quasi-bidirectional I/O can be used as an input or output without the use of a data-direction control signal. At power on, I/Os are high. In this mode, only a current source to VCC is active. The SDA, SCL and the INT pins are have open drain output so they require a pull up resistor for proper functioning.

Now let us look at the details of this and how to use this. Some of the main features of this are

• Low Standby-Current Consumption of 10 μA Max

• I 2C to Parallel-Port Expander

• Open-Drain Interrupt Output

• Compatible With Most Microcontrollers

• Latched Outputs With High-Current Drive Capability for Directly Driving LEDs

• Latch-Up Performance Exceeds 100 mA Per JESD 78, Class II 2

And this can be used in

• Telecom Shelters: Filter Units

• Servers

• Routers (Telecom Switching Equipment)

• Personal Computers

• Personal Electronics

• Industrial Automation

• Products with GPIO-Limited Processors

The typical applications block diagram is sown below.

This IC is available in many packages and with varied pin numbers. Below are few popular packages this IC is available in.

Having look at the pin functions makes us more familiar with this IC.

Working

We now take a look at the working of this IC along with the application circuit. This IC communicates with the microcontroller with the popular I2C protocol. And recognises the chip with its address. The address can be set by the three address setting pins available. This configuration allows us to connect maximum of 8 ICs to the same lines.

An important note to take is that, this is available with no suffix and with a suffix of ‘A’. the starting address changes in both the cases. The part with no suffix starts its address from 0X20 and goes to 0X27 but the part with suffix ‘A’ starts its address form 0X38 and goes to 0X3F. So take care of this point whiling writing the code for this.

Further details can be found at this link. And the datasheet is available at NXP