Issue
163 February 2004
The
Growth of the Atmel AVR Family
THERMISTOR
A
negative temperature coefficient (NTC) thermistor is
used as the lower leg of a voltage divider with a 10-kW
resistor to VREF. Because the thermistor has a 10-kW
resistance at 25°C, the voltage at the junction of the
two devices (and the input to an A/D channel) will be
0.5 VREF at room temperature. The thermistor’s equation
for temperature is:
where
b is 3450, VREF is 1.263
V, TZERO is 273°K, TAMB is 298°K (273° + 25°), and VADC
is the analog-to-digital conversion voltage.
To
allow this project to proceed without some serious calculation
routines, I used my trusty Radio Shack scientific calculator
to solve for the VADC values for all the Fahrenheit
temperatures between 40° and 99°F. The VADC values were
then converted to the 8-bit conversion equivalents and
stored in a look-up table.
The
ADC is initialized for continuous single ended conversions
of channel 0 using a division factor of 32 on the system
clock. The ADC actually does a 10-bit measurement conversion
and stores the value either left or right justified
into a 16-bit extended I/O register. By using left justification,
the high byte of the conversion is the high 8 bits of
a 10-bit conversion (or an 8-bit conversion value).
Eight of these conversions are added together, and the
total is shifted right 3 bits to divide the total by
eight and end up with an average. A low conversion value
of 99 corresponds (coincidentally) to a high temperature
of 99°F. A high conversion value of 180 corresponds
to a low temperature of 40°F.
To
find the actual temperature, the conversion average
is compared to the first value in the TEMP_TABLE (with
the first entry 180 = 40°F). If the table entry is greater
than the conversion average, then the temperature variable,
which is initialized to 40, is incremented and the next
table value is grabbed and the comparison repeats until
the TEMP_TABLE entry value is less than or equal to
the conversion average. The temperature variable has
now incremented to the temperature in degrees Fahrenheit
that corresponds to the appropriate average A/D conversion
value. To stay compatible with the thermostat conversion
protocol that I discussed last month (“Global XPortation,”
Circuit Cellar 162), the temperature is finally
converted into two ASCII digits as TEMPH and TEMPL.
You’ll notice that all variables are stored as ASCII
values to remain compatible with the protocol.
BUTTONS
Five
user inputs provide local mode changes to the thermostat.
The first button, Mode, increments the MODE variable
through four possibilities: H, C, A, and O. Heat places
the thermostat in Heat mode and asks the furnace for
heat if the temperature falls below the heat set point
by setting the HVACSTATUS to “H” (heating). HVACSTATUS
returns to “I” (idle) when the temperature rises to
heat set point + 4°F (an arbitrary delta I added here,
but is not present in the virtual thermostat).
Cool
places the thermostat in to Cool mode and asks the air
conditioner for cool if the temperature rises above
the cool set point by setting the HVACSTATUS to “C”
(cooling). HVACSTATUS returns to “I” when the temperature
falls to cool set point – 4°F (an arbitrary delta).
Auto
places the thermostat into both Heat and Cool modes
(i.e., it can call for heating or cooling as necessary).
Obviously, if the heat set point and cool set point
are too close, the systems will fight. Although this
wasn’t implemented in the virtual thermostat, it is
prevented here in the Change_Plus
and Change_Minus
routines by comparing the heat and cool set points.
They are automatically adjusted to keep a two times
delta distance between them. Off, or disable mode, shuts
down the call for heat/cool operation by setting HVACSTATUS
to “O.”
The
second button, Fan, sets the FAN variable to either
a one (on) or a zero (auto). The one value forces the
furnace/air conditioner fan on; the zero value allows
the furnace/air conditioner to determine the state of
the fan based on its own criteria.
The
third button, Set point, allows you to change the heat
and cool set points. This button sets the display mode
to one of three values: 0x00, 0x01, and 0x02. A display
mode value of 0x00 indicates the LCD should display
the actual temperature. A value of 0x01 indicates the
LCD should display the heat set point. A value of 0x02
displays the cool set point.
After
the heat set point or cool set point has been selected,
the fourth button increments the set point by one. The
fifth button decrements the set point by one.
