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Return to BlogPIC-Controlled USB Thermostat
Posted Jan 25, 2012 at 10:56 am
Figure 1
The USB thermostat utilizes PIC18F252 to store all of the temperature change entries as a table in its EEPROM memory.
Based upon the time it reads from the Real-Time Clock (RTC), the microcontroller keeps track of the current entry as the RTC is accessed every 5 seconds. The current temperature change entry will be calculated while the temperature sensor is updated with the new alarm temperature.
Due to low pin count and high versatility, the PIC18F252 was chosen as it supports full hardware for I2C in both master and slave modes. The I2C master mode is implemented by the USBI2CIO board while the PIC functions as slave to the USB board. Only I2C slave mode is implemented by the RTC and temperature sensor where the PIC functions as master. This makes I2C to have separate buses for the USB board and the peripheral chips.
A very accurate sense of time throughout the operation is maintained by the use of RTC since it contains internal circuitry which will automatically switch the battery-powered operation when the voltage drops below the threshold value. Even while the USB is unplugged, this will cause the current time to increased value.
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2 years ago: What could be used to set the threshold value of the voltage? Should there be a potentiometer or will it be software based?