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Application Report
SCPA032 - August 2001
1
Improving System Interrupt Management Using the
PCF8574 and PCF8574A I/O Expanders for the I
2
C Bus
Emmanuel Tomdio Nana Standard Linear & Logic
ABSTRACT
In today’s complex, embedded, computing and data-communication systems, interrupts are
used extensively to service peripheral devices. However, because of the pin-number
limitation on packages, most microprocessors have only one or two interrupt lines.
Consequently, several devices usually are connected to the same interrupt line. The
drawback to this configuration is that the overhead processing time to identify the device that
requested the interrupt service may be too long (in the order of microseconds). The Texas
Instruments PCF8574 and PCF8574A remote, 8-bit I/O expander for the I
2
C bus is designed
for the specific application of monitoring a multiparallel bus. The remote I/O expander can
inform the microprocessor if there is incoming data on the port or if there is a change of port
data, without having to communicate via the I
2
C bus. This application report discusses the
logic functionality of the PCF8574 and the PCF8574A, the specifics of the I
2
C interface, the
electrical characteristics of the device, and its use in the intended applications. The
information in this application report, along with the data sheet, should enable a system
designer to successfully implement a parallel-bus monitor solution.
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