pin high. When the module is available, it will set the Ready
The first piece of information transmitted is the current
temperature. Remember that the current temperature
control in the VCP was first; hence, it has an index value of
0. When we address controls in the VCP, the address used
by I2CIN and I2COUT will be the control index (0 – n).
This keeps us from having to know the location of data
within the module.
Storing the temperature may take a moment, so we’ll
monitor the clock (SCL) line. The FlexiPanel module uses
“clock stretching” (pulls the clock line low) to indicate
that it’s busy, so waiting for the clock line to go high
before moving to the next control is a good idea after
The next control is the current system status text. For
me, the cleanest implementation was the use of the
SELECT-CASE structure. To be honest, I’m not a big fan
of this structure for embedded control (it’s a code hog),
but, in this case, it just makes good sense. Note that each
string is terminated with a 0 — this is a requirement of the
Next, we read in the setpoint, mode, and fan controls
from the VCP.
At this point, all the information has been exchanged,
so we can release the module and apply the air conditioning
The processing logic is very simple. We start by
checking the mode, and if it’s in an active mode (cool
or heat), check the current temperature against the
setpoint. Here’s a case where you might be inclined to use
SELECT-CASE, but there is so much code involved that it
can become a bit unwieldy. No problem — BRANCH
works really well here.
BRANCH sysMode, [System_Off, Cool_On, Heat_On]
IF (fanCtrl = FanAuto) THEN
status = StatOff
Fan = IsOff
status = StatFan
Fan = IsOn
Cool = IsOff
Heat = IsOff
As you can see, BRANCH selects the code segment
that corresponds with the operating mode. Off is
the easiest to deal with; turn off the control outputs —
unless the user has placed the fan in manual mode for
The logic for cooling and heating is identical, so we’ll
just look at cooling. Really, this code is so easy that I
didn’t even put comments in my original listing.
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Circle #131 on the Reader Service Card.