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PIC Microcontroller
The PIC18F252
by Microchip was chosen
for its high versatility and low pin count. It has full hardware
support for I2C in both slave and master modes. The USBI2CIO board
only implements I2C master mode, and so the PIC must act as a
slave to the USB board. But, the RTC and the temperature sensor
both implement only I2C slave mode, and so the PIC must act as
a master to those devices. Therefore, separate I2C buses had to
be created for the USB board, and the peripheral chips. Then,
the internal hardware handled the slave mode transactions, and
software routines emulated an I2C master with other I/O pins to
communicate with the peripheral chips.
The PIC microcontroller stores all of the temperature change
entries as a table in its EEPROM memory, and keeps track of which
is the current entry based upon the time it reads from the RTC.
Every five seconds, the RTC is accessed. Then, the current temperature
change entry is calculated. And finally, the temperature sensor
is updated with the new alarm temperature.
All of the PIC code was written in C, and compiled
with Hi-Tech Software's PICC18
compiler.
Real-Time Clock
Dallas Semiconductor's
DS1307
Real-Time Clock, in combination with the DS32KHZ
Temperature-Compensated Crystal Oscillator, is accurate to
±1 Min/Yr. This allows the system to maintain a very accurate
sense of time throughout operation. The devices also have internal
circuitry which will automatically switch to battery-powered operation
when Vcc drops below a threshold voltage. As such, the current
time will still increment even while the USB Thermostat is unplugged.
Temperature Sensor
Texas Instruments' TMP101
is the temperature sensor chosen for the USB Thermostat. It has
a resolution of up to 12-bits (.1125° F), and provides an
ALERT pin which can be used to drive the relay directly. The PIC
simply tells the sensor what the current temperature should be,
and the TMP101 will automatically turn the relay on and off to
maintain such a temperature.
PCB Design
Due to the use of mains voltage in this project, it was decided
early on that a fully-enclosed solution was required to provide
adequate safety. As such, a PCB board was designed to minimize
size. Bob House of Georgia Tech milled the board in-house. A 6-pin
expansion header is located to the right of the PIC18F252, and
an I2C header is at the bottom-left of the PIC.
Final Design
The 12.6VCT transformer appearing in the top-left corner of the
picture provides a stepped-down AC voltage to the custom PCB,
which is then rectified and regulated to +5VDC. The solid-state
relay is located at the top-right corner, and is rated at 230VAC
at 10A. Our spaceheater runs off of 120VAC, but is also rated
at 10A. Because of the lowered voltage, there should be no danger
of burning out the relay, but using a higher current is not recommended.
The thermostat PCB is located at the bottom, with the USB board
located underneath, attached via stand-offs.
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