Issue 152 March 2003
Using Rotary Encoders as Input Devices

THE PCF8574

My regular suppliers don’t carry Philips Semiconductors devices; however, this device is now second-sourced by Texas Instruments. I accidentally discovered this as I was surfing the ’Net. Several days later, a few samples of these devices landed on my doorstep courtesy of the Texas Instruments sample program. As an extra bonus, they’re available in DIP16 packages (amongst others), which I find easier to handle.

The PCF8574 is an 8-bit quasi-bidirectional port expander (i.e., when a particular line is programmed as a zero, it can sink up to 50 mA of current). Thus, it can be used as an output port to drive LEDs. However, when it’s programmed as a one, this output stage shuts off and a weak pull-up to V_CC remains. In this state, the line can be used as in input. For example, if the line is pulled to ground by a switch, the internal input latch in the device will read a zero. Otherwise, it will register a one.

The PCF8574 communicates with the host via the I²C bus, which is also referred to as the two-wire serial interface (TWSI). The I²C bus is similar to the older SPI bus, except that it also sends device addresses along its data line and does away with the need for device-enable lines. I won’t try to explain the I²C bus protocol in this article. The TI PCF8574 application note cited at the end of this article provides a good explanation.

By choosing an I²C device to handle the front panel encoders and switches, you only tie up a few of your microcontroller’s I/O lines. You can hook up many other useful I²C peripherals to this bus, too (e.g., flash EEPROM, real-time clock chips, ADC, and DACs).

I found the PCF8574’s real selling point to be its interrupt on port change function, which I mentioned earlier in reference to PIC microcontrollers. The PCF8574 includes an exclusive OR block that senses whenever any of its eight port lines have changed from the state at which they were last read by the host via the I²C bus. The change will assert a separate *INT pin on the device.

This feature is important: the PCF8574 is a slave device on the I²C bus and, normally, would not be able to notify the host of a port pin change in a timely manner (i.e., the host, as the master device, would have to be constantly polling the PCF8574 to determine whether or not any changes had occurred). Often, this isn’t practical. By including a dedicated *INT line, which can be connected to the microcontroller’s *INT input, the PCF8574 essentially provides the interrupt on port change function that’s available on some microcontrollers but absent on AVR devices.