Internal temperature sensor in CC2540

John,

  From your question, I am assuming you are wanting an accurate temperature measurement, so please note that the internal TEMP SENSOR in the CC2540 is not for this purpose, it will only give an estimated temperature.  For more accurate measurements, there is an input to the on-chip ADC that can be used with an external temp sensor.

For either implementation, please read the CC2540 User Guide, starting with Section 13.2: http://www.ti.com/litv/pdf/swru191d .  From section 13.2:

13.2 Using the Battery Monitor for Temperature Monitoring

The battery monitor can also be used to do some simple temperature monitoring. When the battery monitor is connected to the internal temperature sensor instead of the supply voltage AVDD5 (see the description of MONMUX in Section 13.3), it can indicate whether the temperature is above or below a certain level. This is done by comparing the voltage coming from the temperature sensor to the voltage trigger point of the battery monitor. The controls for this measurement are the same as for the normal use of the battery monitor (see the description of BATTMON in Section 13.3). It is important to understand that due to the nature of the battery monitor (optimized for voltages arround 2V) and the output voltage range of the temperature sensor, there are only about 8 temperature trigger values in the temperature range of –40°C to 125°C (see Table 13-1). As a result, the battery monitor gives only a rough indication of the temperature range, but this is useful for doing temperature compensation on analog components in a system. See the device's data sheet (Appendix C) for performance characteristics details.

Table 13-1. Values Showing How Different Temperatures Relate to BATTMON_VOLTAGE for a Typical Device

Temperature     BATTMON_VOLTAGE

-40°C                       22

-26°C                       21

-11°C                       20

7°C                          19

25°C                        18

47°C                        17

70°C                        16

97°C                        15

128°C                      14 

The temperature sensor is inversely proportional to BATTMON_VOLTAGE. The temperature (in °C) corresponding to a given BATTMON_VOLTAGE is given by:

TEMP = [ (A) / BATTMON_VOLTAGE<4:0>] – B

-Leonard

Analog Applications - ACAT Dallas 

Leonard,

The first line of Chapter 13 states: "The battery monitor (in the CC2533 only) enables simple voltage monitoring in the devices that do not include an ADC."  So I don't think this applies to the CC2540.

John

Thanks for that highlight, John, and I'm verifying if the remainder of Section 13 applies with that differentiation.  In the meantime, here is an E2E post that addresses the CAL process for the CC2540: http://e2e.ti.com/support/low_power_rf/f/538/t/256580.aspx .

The internal temp sensor is connected to the ADC input internally.

Internal Reference = 1.24V  Temperature coefficient: 4.5bits /1°C at 12-bit resolution. (as shown in the datasheet )

So if the internal reference is connected to the ADC, the ADC readout should be

= (2^{(n bit – 1)} – 1)/1.24

= (2¹¹ - 1)/1.24

= (2048 – 1)/1.24

= 2047/1.24

= 1650.81

Now, when the temp sensor is connected, the temperature coefficient can then be calculated as

= 4.5bits /1°C

= 4.5 (internal reference voltage/Max ADC readout for 12bit resolution)

= 4.5 (1.24/2047)

= 2.7259 mV/1°C

Now, from the number above, you’d see that 10°C requires 27.26mV. In the internal structure, one can have offsets of 5mV or so, which might further reduce the accuracy of the internal temp sensor. Also remember, the internal chip temperature may differ from the ambient temperature, that is if the chip is actively chugging away.

To do the single point, use the following app note as a reference. The SoC ADC is similar, so the steps should be to

http://www.ti.com/litv/pdf/swra101a

Essentially,

1. Set room temperature to 25°C
2. Select the temp sensor as an ADC input
3. Measure the ADC readout (The ADC value always resides in MSB section of ADCH:ADCL)

Chatto,

The CC2540 datasheet says

Voltage coefficient: 1 / 0.1 V is misleading.

Now, the temperature coefficient = 2.7259 mV/1°C that solves my question.

Best Regards,

CK Lui

CK,

The voltage coefficient is the amount the temp sensor value changes with changing supply voltage to it. It has nothing to do with the temp coefficient.

The temp coefficient is 4.5bits per degree C which translates to 2.7259mV per degree C