In this project the TMP102 temperature sensor module is used to read and display the ambient temperature on the PC screen. This sensor is I²C bus compatible.

11.3.2 Aim

The aim of this project is to show how an I²C bus compatible device can be connected to the Nucleo-F411RE development board and also how it can be programmed using Mbed.

11.3.3 Block Diagram

The block diagram of the project is shown in Fig. 11.2.

11.3.4 Circuit Diagram

TMP102 module (see Fig. 11.3) has the following pins:

• GND: Power supply ground
• Vcc: Power supply (1.4–3.6 V)
• SDA: I²C data line
• SCL: I²C clock line
• ALT: Alert
• ADD0: I²C address select

The ADD0 pin is used to select the device address. By default the device address is 0x48. Four different addresses can be selected depending on where the ADD0 pin is connected to:

Fig. 11.4 shows the circuit diagram of the project. Here, the default device address of 0x90 is selected by leaving the ADD0 pin unconnected. The alert pin is used to generate a signal when the temperature is above or below a limit and this pin is not used in this project. The SDA and SCL pins of the module are connected to I²C1 pins (PB_7, I²C1 SDA, and PB_6, I²C1 SCL) of the development board. Pins Vcc and GND of the module are connected to pins + 3.3 V (pin 16, connector CN7) and GND (pin 8, connector CN7) of the development board, respectively. The sensor module has built-in 4.7 K pull up resistors on the SDA and SCL lines as required by the I²C specifications.

11.3.5 The Construction

Fig. 11.5 shows the circuit built on a breadboard.

11.3.6 The PDL

Fig. 11.6 shows the program PDL.

11.3.7 Program Listing

TMP102 is a very low-current (10 μA) temperature sensor chip that can be used to measure ambient temperature in the range − 40°C to + 125°C with a resolution of 0.0625°C and an accuracy of ± 0.5°C. The device has a 12-bit resolution and is I²C, Two-Wire, and SMBus compatible. TMP102 can have four different device addresses and hence up to four such devices can be connected to the bus. The programming model of the device is shown in Fig. 11.7. The Pointer Register is used to select the register to be configured and it selects the other registers as follows:

The Configuration Register is a 16-bit register and after power up or reset it is necessary to configure this register. The register is configured by sending two consecutive bytes to it. The following options can be configured (see the TMP102 data sheet for details: http://www.ti.com/lit/ds/symlink/tmp102.pdf):

• • shutdown mode
• • thermostat mode
• • polarity
• • fault queue
• • conversion resolution
• • one-shot mode
• • extended conversion mode
• • alert bit
• • conversion rate

In this example we will configure this register with the normal operating values by sending 0x60 followed by 0xA0. These are also the default register values after power up or reset. 0x60 sets the conversion resolution to 12 bits, continuous conversion mode. 0xA0 sets nonextended mode of operation and 4 Hz conversion rate.

The Temperature Register returns the temperature in two bytes as shown in Fig. 11.8, where 4 bits of the lower byte are cleared to 0.

The temperature can be extracted from these two bytes by the following steps:

For positive temperatures:

• • Shift the HIGH byte by 4 bits to the left into a 16-bit word called say, M.
• • Shift the LOW byte by 4 bits to the right into a byte called say, L.
• • Add M and L to find the 16-bit result.
• • Multiply the result with 0.0625 (resolution) to find the temperature.

For negative temperatures:

• • Shift the HIGH byte by 4 bits to the left into a 16-bit word called say, M.
• • Shift the LOW byte by 4 bits to the right into a byte called say, L.
• • Add M and L to find the 16-bit result.
• • Generate 2's complement of the result (complement and add 1).
• • Multiply the result with 0.0625 (resolution) to find the temperature.

For example, if the returned two bytes are: 0011 0010 1000 0000

• M = 0011 0010 0000
• L = 1000
• M + L = 0011 0010 1000 which is equivalent to 808 in decimal. Therefore, 801 × 0.0625 = 50.5°C.

Similarly, for a negative temperature, if the returned bytes are: 1110 0111 1000 0000

• M = 1110 0111 0000
• L = 1000
• M + L = 1110 0111 1000

Taking the complement and adding 1 (2's complement) we get: 0001 1000 0111 + 1 = 0001 1000 1000 which is equivalent to 392. Therefore 392 × 0.0625 = 24.5°C, or − 24.5°C.

Fig. 11.9 shows the program listing (program: TMP102). At the beginning of the program GPIO pins PB_7 and PB_6 are assigned to I²C1 with the name TMP102, the device address is set to 0x90, and the registers used in the program are defined. Inside the main program function ConfigureTMP102 is called to configure the sensor to normal mode of operation. The remainder of the program runs in an endless loop. Inside this loop the temperature is read as a 12-bit data and is converted into a 16-bit data in variable M. The actual temperature in °C is then calculated and displayed on the PC screen in the format: nn.nn. Notice that only positive temperatures are displayed by this program. The loop is repeated every second.

A typical display of the temperature is shown in Fig. 11.10.

11.3.8 Suggestions for Additional Work

Modify the program given in Fig. 11.9 so that the negative temperatures can also be displayed.