The experimental set-up
To demonstrate the fnctionality of the I²C bus, a digital temperature sensor of type LM75 will be connected to a JControl/SmartDisplay. To do this, merely the I²C signals SDA
and SCL
have to be connected, as well as VCC
and GND
for supplying the temperature sensor with power. The exact pin assignment of the JControl/SmartDisplay may be seen in the according data sheet.
Figure 2: General schematic diagram
Hint: The circuit of the pins 5, 6 and 7 of the LM75 influence its I²C bus slave address. Further information in this matter can be found in the LM75 data sheet. In the discussed example those pins are tied to GND
, which results in the slave address 0x90
. Furthermore, a back-up capacitor (commonly 100nF) close to the I²C component is recommended in case of a long distance between the power supply and this component.
Attention: When external periphery is connected to the I²C bus, the signals SDA
and SCL
have to be tied to VCC
via a pull-up resistor (commonly 27k Ohm). Thus it is ensured that a defined voltage level is present on the signal wires, even if no bus component is driving/accessing them (see figure 2).
The evaluation boards (e.g. of the JControl/SmartDisplay) already provide those pull-up resistors, so that the periphery (here the LM75) may be directly connected to the lead through I²C signals. Further information are found in the evaluation boards' datasheets.