Issue
113 December 1999
Being
Cool is Easy
A
Temperature-Sensing Control Device
POWER
LINE INTERFACE
The
TW-523 interface is used to send and receive X-10 codes
via the AC power line. A technical note is available
online from X-10. For those of you who are long-time
subscribers to Circuit Cellar, the X-10 protocol
and theTW-523 module were described by Ken Davidson
in issues 3 and 5.
An
X-10 command consists of a start code, a House Code,
and a Key or Function Code. Transmission of each command
requires 11 power line cycles: two for the Start Code,
four for the House Code, and five for the Key or Function
Code.
Each
command is transmitted twice with at least three power
line cycles between pairs. The four House Code bits
and the five Key or Function Code bits are transmitted
in true and complement form on alternating half cycles
of the power line.
Here’s
an example. To turn on unit 5 of House Code G requires
the following X-10 commands:
Start
Code (1110), House Code G (0101), Key Code 5 (00010)
Start
Code (1110), House Code G (0101), ON Function Code
(00101)
Figure
5 illustrates the transmission of these two X-10 commands.
The House Code, Key and Function Code encoding is defined
in the X-10 technical note.
|
(Click
here to enlarge)
|
Figure
5—Transmission of each X-10 command requires
11 power line cycles. Each command is transmitted
twice with three or more cycles between pairs. |
ZERO
CROSSING DETECT
The
X-10 transmission must be synchronized with zero crossing
on each half cycle of the power line. This is the point
when the AC voltage goes from positive to negative or
from negative to positive. The TW-523 provides a zero-crossing
output. This output is a 60-Hz square wave and is connected
to the 16C73A RB0/INT pin.
The
X-10 sensor software configures the RB0/INT pin to generate
an interrupt. RB0/INT can be configured to select either
the rising edge or the falling edge. An interrupt on
both edges is desirable and is achieved by toggling
the edge select (INTEDG bit in the ’16C73A OPTION
register) just after each zero-crossing interrupt.
The
software uses the ’16C73A Timer0 in combination
with the zero-crossing interrupt to provide the precise
timing necessary for sampling of incoming and gating
of outgoing X-10 commands. This timing sequence is illustrated
in Figure 6 and described in more detail in the following
sections.
The
zero-crossing interrupt is also used to blink the heartbeat
LED (LED2) at a 1-Hz rate, which provides a warm fuzzy
indication that the temperature sensor and TW-523 module
are alive and well.
RECEIVING
REQUESTS
Each
X-10 command is transmitted twice. The TW-523 only provides
the second of the two transmissions via its Rx output.
The Rx output is valid between 500 and 700 µs after
zero crossing. The software sets up Timer0 to generate
six interrupts after each zero crossing. The first Timer0
interrupt occurs at 600 µs after zero crossing and the
Rx output is sampled at this time.
Remember
that it takes 11 power line cycles or 22 zero-crossing
events for a complete X-10 transmission. Four samples
are required for the Start Code, eight samples for the
House Code, and 10 for the Key or Function Code.
The
temperature sensor looks for a consistent X-10 transmission
which consists of:
•
valid start code: 1 1 1 0
•
four House Code bits in true and complement form
•
five Key or Function Code bits in true or complement
form
The
incoming X-10 command and any earlier received commands
are discarded if any inconsistencies are detected and
the software starts over looking for a valid start code.
