27-02-2012, 04:59 PM
Home Automation Using the PIC16F877A
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INTRODUCTION
X-10 is a communication protocol designed for sending
signals over 120 VAC wiring. X-10 uses 120 kHz bursts
timed with the power line zero-crossings to represent
digital information. Plug-in modules available from various
vendors enable users to create home automation
systems by using the AC wiring already installed within
a home. Readers who would like an overview of the
X-10 signal format may refer to Appendix A.
PICmicro® microcontrollers can easily be used in
conjunction with X-10 technology to create home
automation applications. The specific PICmicro
microcontroller (MCU) used should be selected based
on RAM, ROM, operating frequency, peripheral, and
cost requirements of the particular application. The
PIC16F877A was selected for this application because
of its versatility as a general purpose microcontroller,
its FLASH program memory (for ease of development),
data EEPROM, and ample I/O.
HARDWARE OVERVIEW
The home controller application described in this application
note allows the user to program on and off times
for up to sixteen devices, using a 2 x 16 liquid crystal
display and five push buttons. A built-in light sensor can
be used to turn on lights at dusk, and turn them off at
dawn.
The home controller is designed to facilitate experimentation
with home automation using the
PIC16F877A. In addition to the PIC16F877A, the board
will accept any other PICmicro MCU that shares the
same pinout, such as the PIC18F452. Therefore,
experimenters may expand on the application using the
higher performance of the PIC18 family of parts without
changing the hardware.
Zero-Crossing Detector
In X-10, information is timed with the zero-crossings of
the AC power. A zero-crossing detector is easily created
by using the external interrupt on the RB0 pin and
just one external component, a resistor, to limit the
current into the PICmicro MCU (see Figure 3).
In the United States, Vrms = 117 VAC, and the peak
line voltage is 165V. If we select a resistor of 5 MΩ,
Ipeak = 165V/5 MΩ = 33 μA, which is well within the
current capacity of a PICmicro MCU I/O pin.
Input protection diodes (designed into the PICmicro
MCU I/O pins) clamp any voltage higher than VDD or
lower than VSS. Therefore, when the AC voltage is in
the negative half of its cycle, the RB0 pin will be
clamped to VSS - 0.6V. This will be interpreted as a
logic zero. When the AC voltage rises above the input
threshold, the logical value will become a ‘1’.
