The 8-bit I/O chip “Type PCF8574 is very popular: it is used, among others, on the AD-DA 1/0 module, the Opto relay card, and the LC display – all published within the past 12 months (see References). Since it is often desirable to connect a number of 8574 modules to the 12C bus, it is possible to give each IC a different address via inputs AO-A2. As therefore only eight addresses can be set, it might be expected that this is also the maximum number of ICs that can be connected to the bus. It is, however, possible to connect 16 PCF8574s, because there are two variants of the IC, each with a different address. The standard PCF8574 has, as is well known, the basic address 40HEx• The variant, the PCF8574A, has the basic address70HEx• Otherwise the two types are identical and, therefore, completely interchangeable. Thus, eight standard and eight variant types of the IC can be connected to one bus.
References:
1. 12C interface for PCs. Elektor Electronics
February 1992.
2. ADC/DAC and I/O for 12C bus. Elektor
Electronics March 1992
3. I2C LED display. Elektor Electronics
June 1992.
4. 12C opto/relay card. Elektor Electronics
February 1993.
The PCF8574 is a silicon CMOS circuit. It provides general-purpose remote I/O expansion for most microcontroller families via the two-line bidirectional bus (I2C)
The devices consist of eight quasi-bidirectional ports, a 100 kHz I2C-bus interface, three hardware address inputs and interrupt output operating between 2.5 V and 6 V. The quasi-bidirectional port can be independently assigned as an input to monitor interrupt status or keypads, or as an output to activate indicator devices such as LEDs. System master can read from the input port or write to the output port through a single register.
Features
- Low Standby-Current Consumption of 10 μA Max
- 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 Ex
Applications
- Telecom Shelters: Filter Units
- Servers
- Routers (Telecom Switching Equipment)
- Personal Computers
- Personal Electronics
- Industrial Automation
- Products with GPIO-Limited Processors
