use and calibration of the internal temperature...

AN1333Use and Calibration of the Internal Temperature Indicator

INTRODUCTIONMany PIC16 family devices include an internaltemperature indicator. These devices include thePIC16F72X device family, PIC16F1XXX device family,and the PIC12F1XXX device family. The temperatureindicator is internally connected to the input multiplexerof the ADC (Figure 1). Refer to the specific device datasheet for more details.

FIGURE 1: TEMPERATURE INDICATOR

These devices incorporate an internal circuit whichproduces a variable output voltage with temperatureusing internal transistor junction threshold voltages.The indicator can be used to measure the devicetemperature between -40°C and +85°C. The circuitmust be calibrated by the user to provide accurateresults, as the equation coefficients will vary betweendevices.

USING THE TEMPERATURE INDICATORThe control bits for enabling the temperature indicatorand selecting its mode of operation should be detailedin the device’s data sheet in the temperature indicatorchapter.

The indicator uses the temperature coefficient of atransistor junction threshold voltage (Vt) to produce avoltage which is temperature dependent. TheHigh-Range mode increases the number of junctionswhich gives a greater response to temperaturechanges. The Low-Range mode uses fewer junctions,which allows use of the temperature indicating circuitover a wider device operating voltage range (seeFigure 3).

The variation in Vt with temperature, measured on asingle sample device, was found to be:

EQUATION 1: Vt VOLTAGE VS. TEMPERATURE

FIGURE 2: EXAMPLE DATA ON DIODE FORWARD VOLTAGE VS. TEMPERATURE OBSERVED WITH A SINGLE SAMPLE PIC16F1937 DEVICE

Author: Jonathan DillonMicrochip Technology Inc.

VDDVDD

ADCTemperatureIndicator

Enable

Mode

Vt 0.659 Temperature C 40+ * (0.00132)–=

2010-2013 Microchip Technology Inc. DS00001333B-page 1

AN1333
FIGURE 3:
The output equations for the two modes of operation:

• Low rangeVtemp = VDD – 2*Vt

• High rangeVtemp = VDD – 4*Vt

Where:

Vtemp is the analog voltage output by the indicator

VDD is the positive voltage supplied to the device

Vt is the threshold voltage for the transistors which isdependent on the device fabrication process

Using Equation 1 with the operational modes of theindicator we have Equation 3.

The voltage, Vtemp, is measured using the internalAnalog-to-Digital Converter (ADC) and is internallyconnected to the analog channel select MUX. Refer tothe ADC chapter of the device data sheet to determinethe input channel.

The mode selection and temperature indicator enableare documented in the temperature indicator chapter ofthe data sheet.

When selecting the temperature indicator of thechannel select MUX, sufficient time must be allowed forthe ADC to acquire the voltage before conversion isstarted.

The ADC’s transfer function can be found inEquation 2. The conversion result is dependent on thesupply voltage to the Analog-to-Digital Converter’svoltage reference and, for this document, the positivereference is the supply voltage, while the negativereference is the ground.

EQUATION 2:

During operation, the supply voltage can bedetermined by performing an Analog-to-Digitalconversion of the Fixed Voltage Reference. However, ifVDD is regulated or an external reference is connectedto the ADC, the calculations can be simplified, since itcan be assumed to be constant.

VDD

VDD

ADCn

VSS

High Mode

Vtemp = VDD - 4Vt

Vt

Vt

Vt

Vt

Vtemp

VDD

VDD

ADCn

VSS

Low Mode

Vtemp = VDD - 2Vt

Vt

Vt

Vtemp

Operation above 3.6V Operation below 3.6V

Note: Care needs to be taken in selecting amode, since Vt may be as high as 0.75Vat low temperatures, while the minimumVDD of some devices can be as low as1.8V. For low-voltage operation, the lowrange is necessary, as Vtemp can only bea positive voltage. High mode is thepreferred mode of operation when thesupply voltage allows its use due to itsgreater temperature response increasingthe temperature resolution.

ADCResult VtempVDD-------------- * (2n 1 –=

Where:n = number of bits of ADC resolution (8 or 10 bits)

DS00001333B-page 2 2010-2013 Microchip Technology Inc.

AN1333
EQUATION 3: VTEMP VOLTAGE FROM SERIES OF SAMPLED DIODES AS GIVEN IN Equation 1
Combining Equation 2 and Equation 3 to relate theADC conversion of the temperature indicator circuit’soutput voltage to the temperature:

EQUATION 4: RE-ARRANGING TO CALCULATE ADC RESULT USING EXAMPLE COEFFICIENTS:

EQUATION 5: RE-ARRANGING EQUATION 4 TO CALCULATE TEMPERATURE

As the temperature varies, the ADC result ofconversion of the temperature indicator channel willchange linearly as seen in Figure 4, provided thesupply voltage does not change.

Depending on the application, the Analog-to-DigitalConverter result can be either compared directlyagainst specific trip points, or used to determine theactual temperature by calculation, a look-up table or acombination of both.

Vtemp V DD mode * [0.659– Temperature C 40+ * 0.00132 –=

Where:High-Range mode = 4Low-Range mode = 2

ADCResult V DD mode * [0.659– Temperature C 40+ * 0.00132 –

VDD------------------------------------------------------------------------------------------------------------------------------------------------------------- * (2n 1 –=

Temperature C

0.659 VDDmode-------------- 1

ADCResult

(2n 1 –---------------------------–

–

0.00132---------------------------------------------------------------------------- 40–=

Note: Equation 5 uses example coefficientsdetermined from a sample device. SeeCalibration section for how to calculatethese coefficients for your device.


AN1333
FIGURE 4: EXAMPLE OF ADC RESULT (DECIMAL) VS. TEMPERATURE (REGULATED
SUPPLY VOLTAGE) FOR A CALIBRATED DEVICE

(°C)


AN1333

CALIBRATION The temperature indicator requires calibration toachieve greater accuracy due to variations in offset andin slope between devices. The indicator is dependenton the device’s transistor voltage threshold, Vt, whichwill vary within production allowances.

Calibration of the temperature indicator can beperformed during production of the target applicationby two methods:

SINGLE-POINT CALIBRATIONCalibration is performed at a single temperature andthe variation of slope is assumed to be relatively stablebetween devices. This method calibrates purely for theoffset, which typically has greater variation betweendevices.

TWO-POINT CALIBRATIONCalibration is performed at two temperatures fromwhich we can determine the offset and slope. As aresult, this method is more accurate, but requires twodistinctively different temperatures.

For both of the above methods, the temperatures canbe either forced (held to a specific value) or measuredat calibration time via an external measurement.Forced temperatures simplify the calculations requiredduring calibration, but are more difficult from aproduction view point and time may be required to

allow the device to reach temperature. Errors in theforced temperature or measured temperature will resultin reduced temperature accuracy at all temperatures.

The degree of calibration required is dependent on theapplication, where some applications do not requireprecise temperature, thus single-point calibration issuitable and faster to perform. It also avoids requiringequipment to vary temperature. For more accuratetemperature measurements, the two-point calibrationmethod is recommended.

FIGURE 5: TEMPERATURE DATA FROM 12 SAMPLE DEVICES

Note: The voltage from the temperature indica-tor is dependent on the supply voltage tothe device, which makes calibration easi-est when the voltage is regulated. Forunregulated supplies the voltage mustalso be calculated from an A/D conversionof the internal Fixed Voltage Reference.The techniques of using a Fixed VoltageReference to determine VDD can be foundin application note AN1072, “MeasuringVDD Using the 0.6V Reference.”

Temperature

AD

C re

sult


AN1333

SINGLE-POINT CALIBRATIONTesting of a limited number of sample devices as seenin Figure 5 shows a relatively constant response inVtemp with changes in temperature, however, there is agreater variation in offsets between devices.Single-point calibration corrects for this variation inoffset, but does not allow for the variation intemperature response slope between devices.

For this calibration, we need to have an ideal ADCresult value for either our forced temperature orotherwise at the measured temperature. The change inVt by temperature varies between devices and, as aresult, single-point calibration may only be accurate atthe calibration temperature, and error will increase as itmoves further from the calibration temperature (seeFigure 6). The bow tie shape of the plotted ADC resultsdue to the possible variation in temperature response.

If the temperature is measured, the calculation requiredto get the ideal ADC result value is given in Equation 3,otherwise, for forced temperatures, the result can becompared to a constant ideal result for thattemperature. Ideally, the temperature is in the middle ofthe operating range seen by the application, as thiscenters the bow tie and minimizes temperature errorover the applications operating range. For applicationswhich only need to know a certain temperature, suchas a temperature limit, the best accuracy results can beachieved by calibrating at that temperature.

The ADC conversion results may have a dynamicrange approaching 8 bits for some combinations ofmode and voltage and, as a result, it is recommendedto maintain the two-byte ADC result data type. Forhigher voltage operation, the dynamic range of theADC result between -40°C to +85°C is small enoughthat it could be scaled down to an 8-bit number.

With a sample PIC16F1937 device under the followingconditions:

• powered at 5V• high-range 4Vt operation• 25°C forced temperature

The Analog-to-Digital conversion gives a result of 561decimal.

Typical A/D conversion result at 25°C is calculated as554 decimal using Equation 3.

For single-point calibration, the difference between theconversion result and the ideal A/D conversion resultvalue is the calibration value.

Thus:

EQUATION 6:

Consequently, for this device the calibration valuewould be 7. Store this in the nonvolatile program or dataEEPROM memory within the device for use whentaking temperature measurements.

Single-point calibration assumes that all devices havea similar slope, however, as the temperature movesfurther from the calibration temperature, the greater thepotential error as seen in Figure 6.

When taking measurements, the ADC result ismodified by the calibration value to adjust for the offset.

EQUATION 7:

EQUATION 8:

Ideal – measured = calibration value

554 – 561 = 7

Calibrated result = ADC result – calibration value

Temperature = (ADC result – calibration value)K


AN1333
FIGURE 6: SINGLE TEMPERATURE CALIBRATION
TWO-POINT CALIBRATIONTwo-point calibration measures the temperatureresponsivity of that device, as well as the offset. As aresult, it offers increased temperature accuracy byovercoming the assumption of single-point calibration,that all devices have the same temperature response.

Two-point calibration requires two distinctively differenttemperatures across the applications temperaturerange. As with single-point calibration, thesetemperatures can either be forced or measured, thoughforced temperatures again simplify the requiredcalculations.

FIGURE 7: TWO-POINT CALIBRATION

For unregulated supply voltages, designers mustcalculate the temperature responsivity of the diode,which requires additional steps.

EQUATION 9:

Calibration is required to determine A and B, whichmodifies the ADC result for the variation in diode Vt andtemperature response. The ideal ADC result for eachcalibration temperature can be stored as a constant ifthe temperature is forced to known levels, otherwisethe ideal must be calculated if it is measured externallyduring calibration. The calibrated result can then beused in Equation 5 to calculate the temperature.

Typical

Max Slope

Min Slope

Calibration Temperature

(°C)

ADC Result calibrated = A + (B * ADC Result)


AN1333
EQUATION 10:
This two-point calibration significantly reduces theeffect of variations in temperature response of thediodes, but is dependent on being able to accuratelycalculate the responsivity.

SINGLE-POINT CALIBRATION FOR UNREGULATED VOLTAGESFor regulated voltages, the calibration can be simplifieddown to an adjustment to the ADC result.

For unregulated supplies, the calibration is also afunction of VDD causing a change in the ADC result,and the Vt temperature offset must be calculated. This

requires that VDD be known along with the calibrationtemperature and ADC result. From Equation 3,substituting for the Vt offset:

The Vt offset can be calculated by performing a singleADC conversion at a known temperature and voltage.For unregulated applications, the supply voltage can bedetermined from a conversion of the internal FixedVoltage Reference or by supplying a known voltageduring calibration.

When measuring the temperature the supply voltagemust also be calculated and the Vt offset from thecalibration used.

During calibration, is calculated and stored innonvolatile memory for use during operation. Theresults of the A/D conversion are inserted intoEquation 11 along with the supply voltage to give theoperating temperature.

EQUATION 11:

EQUATION 12:

Re-arranging:

EQUATION 13:

TWO-POINT CALIBRATION FOR UNREGULATED VOLTAGESFor unregulated supply, such as direct connection to abattery, we need to calculate VDD once or twice, if itvaries between the two calibration temperatures, suchas reduced battery voltage with temperatures.

From the operation of the temperature indicator wehave the following:

EQUATION 14:

Where, for two-point calibration with an unregulatedvoltage, we need to calculate alpha () and beta ().Re-arranging the equations and calibrating at twotemperatures (Equation 15):

Key points to consider:

• The results are most accurate between the cali-bration temperatures.

• The calibration temperatures need to be suitably far apart to allow an accurate calculation of the slope given the ADC resolution. Calibration temperatures around 20% and 80% of the operating temperature range are recommended.

• Any error in calibration temperature or voltage sig-nificantly increases the error of the readings due to the inaccurate slope and offset.

• Regulated voltage, calibration performed at 20°C and 60°C.

A = (Ideal @ T1 – Ideal @ T2)/(Actual @ T1 – Actual @ T2)

B = Actual @ T1 - (A * Ideal @ T1)

Where:T1 calibration temperature 1T2 calibration temperature 2

Temperature V DD

4----------– * 1ADCResult

1023---------------------------–

0.00132----------------------------------------------------------------- 40–=

ADCResult V DD 4 * [– Temperature C 40+ * 0.0132 –

VDD-------------------------------------------------------------------------------------------------------------------------------------- * 1023=

V DD4---------- * 1

ADCResult 1023---------------------------–

Temperature C 40+ * 0.00132 +=

Vtemp V DD 4 * Temperature C 40+ – –=

ADCResult VtempVDD-------------- * (2n 1 –=

VtempADCResult

1023--------------------------- * VDD=


AN1333
Temperature error will be minimized at the calibrationtemperatures as shown Figure 8 for a sample batch ofdevices, where the maximum temperature errorbetween the calibration temperatures is 5°C.
EQUATION 15:

EQUATION 16:

FIGURE 8: ABS TEMPERATURE ERROR

V1 * Temp2 40+ * 1

ADCResult11023-----------------------------–

V2 * Temp1 40+ * 1ADCResult2

1023-----------------------------–

–

4 * Temp2 Temp1– ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=

V1 V2–

11023------------* V2* ADCResult2 V1* ADCResult1 – +

4 * Temp2 Temp1– ----------------------------------------------------------------------------------------------------------------------------------------------=

Where:

Temp1, Temp2 calibration temperaturesV1, V2 VDD voltage at Temp1 and Temp2ADCresult1, ADCresult2 A/D Convertor result at Temp1 and Temp2

Temperature C V DD

4----------– * 1ADCResult

1023---------------------------–

----------------------------------------------------------------- 40–=

0

2

4

6

8

10

12

-40 -30 -20 -10 0 10 20 25 30 40 50 60 70 80 85Temperature

Abs temp error

Absolute Error °C

Abs

olut

e Er

ror °

C

(°C)


AN1333

CONCLUSIONThe on-board temperature indicator can be used tomeasure the device temperature, which willcorrespond to the temperature in its environment withsome delay. The indicator is measured using the ADCand can be used uncalibrated for coarse temperaturemeasurements. For more precise temperaturemeasurements, calibration is required to account fordevice parameter variation. Depending on theapplication, calibration measurements at one or twotemperatures may be required. Since the ADC resultsare dependent on its provided references, the fixedreferences need to be supplied either by using theon-board fixed references, or by using a regulatedsupply. Otherwise, the device supply voltage must becalculated using the Fixed Voltage Reference.


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