Datasheet

Jon Waddington
27
3.5 Temperature
The “Temperature” item displays the inside and outside temperatures in Celsius on the
screen until the user wishes to return to the main menu. Two temperature sensors are
used in the Information System. An internal temperature sensor measures the
temperature inside the home and is wired directly to the Propeller chip. An external
temperature sensor measures the temperature outside the home and remotely transmits
the data to the Propeller chip.
3.5.1 Internal Temperature
Figure 3.5.1 shows the circuit diagram for the internal temperature sensor. The DS18B20
chip [35] was used as it communicates digitally with 1 wire to the Propeller chip. A
device with an analogue output was not used due to the Propeller chip not having
analogue to digital conversion capabilities. The DS18B20 is capable of reading
temperatures from -55°C to 125°C which is easily enough range for internal
temperatures.
3.5.2 1-Wire Communications
To communicate with the Propeller, the DS18B20 uses a 1-Wire protocol. There are six
signal types defined by this protocol. These are the Reset pulse, Presence pulse, write 1,
write 0, read 1 and read 0. These signals are each initiated by the master devic e, in this
case, the Propeller, with the exception of the Presence pulse.
To begin communicating, the Propeller pulls the 1-Wire bus down for a minimum of
Figure 3.5.1. Circuit Diagram showing the Internal Temperature
Sensor