### AN1333

```AN1333
Use and Calibration of the Internal Temperature Indicator
Author:
USING THE TEMPERATURE
INDICATOR
Jonathan Dillon
Microchip Technology Inc.
INTRODUCTION
Many PIC16 family devices include an internal
temperature indicator. These devices include the
PIC16F72X device family, PIC16F1XXX device family,
and the PIC12F1XXX device family. The temperature
indicator is internally connected to the input multiplexer
of the ADC (Figure 1). Refer to the specific device data
sheet for more details.
FIGURE 1:
TEMPERATURE INDICATOR
VDD
Enable
Mode
Temperature
Indicator
VDD
These devices incorporate an internal circuit which
produces a variable output voltage with temperature
using internal transistor junction threshold voltages.
The indicator can be used to measure the device
temperature between -40°C and +85°C. The circuit
must be calibrated by the user to provide accurate
results, as the equation coefficients will vary between
devices.
FIGURE 2:
The control bits for enabling the temperature indicator
and selecting its mode of operation should be detailed
in the device’s data sheet in the temperature indicator
chapter.
The indicator uses the temperature coefficient of a
transistor junction threshold voltage (Vt) to produce a
voltage which is temperature dependent. The
High-Range mode increases the number of junctions
which gives a greater response to temperature
changes. The Low-Range mode uses fewer junctions,
which allows use of the temperature indicating circuit
over a wider device operating voltage range (see
Figure 3).
The variation in Vt with temperature, measured on a
single sample device, was found to be:
EQUATION 1:
Vt VOLTAGE VS.
TEMPERATURE
V t = 0.659 –  Temperature C + 40  * (0.00132)
EXAMPLE DATA ON DIODE FORWARD VOLTAGE VS. TEMPERATURE
OBSERVED WITH A SINGLE SAMPLE PIC16F1937 DEVICE
 2010-2013 Microchip Technology Inc.
DS00001333B-page 1
AN1333
FIGURE 3:
VDD
VDD
Vt
Vt
Vt
Vt
Vt
VDD
VDD
Vt
n
Vtemp
Vtemp
Operation above 3.6V
Operation below 3.6V
The output equations for the two modes of operation:
• Low range
Vtemp = VDD – 2*Vt
• High range
Vtemp = VDD – 4*Vt
Where:
Vtemp is the analog voltage output by the indicator
The ADC’s transfer function can be found in
Equation 2. The conversion result is dependent on the
supply voltage to the Analog-to-Digital Converter’s
voltage reference and, for this document, the positive
reference is the supply voltage, while the negative
reference is the ground.
EQUATION 2:
V temp
n
ADC Result = -------------- * (2 – 1 
V DD
VDD is the positive voltage supplied to the device
Vt is the threshold voltage for the transistors which is
dependent on the device fabrication process
Using Equation 1 with the operational modes of the
indicator we have Equation 3.
Care needs to be taken in selecting a
mode, since Vt may be as high as 0.75V
at low temperatures, while the minimum
VDD of some devices can be as low as
1.8V. For low-voltage operation, the low
range is necessary, as Vtemp can only be
a positive voltage. High mode is the
preferred mode of operation when the
supply voltage allows its use due to its
greater temperature response increasing
the temperature resolution.
Low Mode
Vtemp = VDD - 2Vt
Vtemp = VDD - 4Vt
Note:
VSS
VSS
High Mode
n
Where:
n = number of bits of ADC resolution (8 or 10 bits)
During operation, the supply voltage can be
determined by performing an Analog-to-Digital
conversion of the Fixed Voltage Reference. However, if
VDD is regulated or an external reference is connected
to the ADC, the calculations can be simplified, since it
can be assumed to be constant.
The voltage, Vtemp, is measured using the internal
Analog-to-Digital Converter (ADC) and is internally
connected to the analog channel select MUX. Refer to
the ADC chapter of the device data sheet to determine
the input channel.
The mode selection and temperature indicator enable
are documented in the temperature indicator chapter of
the data sheet.
When selecting the temperature indicator of the
channel select MUX, sufficient time must be allowed for
the ADC to acquire the voltage before conversion is
started.
DS00001333B-page 2
 2010-2013 Microchip Technology Inc.
AN1333
EQUATION 3:
VTEMP VOLTAGE FROM SERIES OF SAMPLED DIODES AS GIVEN IN Equation 1
V temp = V DD – mode * [0.659 –   Temperature C + 40  * 0.00132  
Where:
High-Range mode = 4
Low-Range mode = 2
Combining Equation 2 and Equation 3 to relate the
ADC conversion of the temperature indicator circuit’s
output voltage to the temperature:
EQUATION 4:
RE-ARRANGING TO CALCULATE ADC RESULT USING EXAMPLE
COEFFICIENTS:
V DD – mode * [0.659 –   Temperature C + 40  * 0.00132  
n
ADC Result = ------------------------------------------------------------------------------------------------------------------------------------------------------------- * (2 – 1 
V DD
EQUATION 5:
RE-ARRANGING EQUATION 4 TO CALCULATE TEMPERATURE
V DD 
0.659 – --------------  1 – --------------------------
n
mode 
(2 – 1  
Temperature C = ---------------------------------------------------------------------------- – 40
0.00132
Note:
Equation 5 uses example coefficients
determined from a sample device. See
Calibration section for how to calculate
As the temperature varies, the ADC result of
conversion of the temperature indicator channel will
change linearly as seen in Figure 4, provided the
supply voltage does not change.
Depending on the application, the Analog-to-Digital
Converter result can be either compared directly
against specific trip points, or used to determine the
actual temperature by calculation, a look-up table or a
combination of both.
 2010-2013 Microchip Technology Inc.
DS00001333B-page 3
AN1333
FIGURE 4:
EXAMPLE OF ADC RESULT (DECIMAL) VS. TEMPERATURE (REGULATED
SUPPLY VOLTAGE) FOR A CALIBRATED DEVICE
(°C)
DS00001333B-page 4
 2010-2013 Microchip Technology Inc.
AN1333
CALIBRATION
The temperature indicator requires calibration to
achieve greater accuracy due to variations in offset and
in slope between devices. The indicator is dependent
on the device’s transistor voltage threshold, Vt, which
will vary within production allowances.
Calibration of the temperature indicator can be
performed during production of the target application
by two methods:
allow the device to reach temperature. Errors in the
forced temperature or measured temperature will result
in reduced temperature accuracy at all temperatures.
The degree of calibration required is dependent on the
application, where some applications do not require
precise temperature, thus single-point calibration is
suitable and faster to perform. It also avoids requiring
equipment to vary temperature. For more accurate
temperature measurements, the two-point calibration
method is recommended.
SINGLE-POINT CALIBRATION
Calibration is performed at a single temperature and
the variation of slope is assumed to be relatively stable
between devices. This method calibrates purely for the
offset, which typically has greater variation between
devices.
TWO-POINT CALIBRATION
Calibration is performed at two temperatures from
which we can determine the offset and slope. As a
result, this method is more accurate, but requires two
distinctively different temperatures.
Note:
The voltage from the temperature indicator is dependent on the supply voltage to
the device, which makes calibration easiest when the voltage is regulated. For
unregulated supplies the voltage must
also be calculated from an A/D conversion
of the internal Fixed Voltage Reference.
The techniques of using a Fixed Voltage
Reference to determine VDD can be found
in application note AN1072, “Measuring
VDD Using the 0.6V Reference.”
For both of the above methods, the temperatures can
be either forced (held to a specific value) or measured
at calibration time via an external measurement.
Forced temperatures simplify the calculations required
during calibration, but are more difficult from a
production view point and time may be required to
TEMPERATURE DATA FROM 12 SAMPLE DEVICES
FIGURE 5:
Temperature
 2010-2013 Microchip Technology Inc.
DS00001333B-page 5
AN1333
SINGLE-POINT CALIBRATION
Testing of a limited number of sample devices as seen
in Figure 5 shows a relatively constant response in
Vtemp with changes in temperature, however, there is a
greater variation in offsets between devices.
Single-point calibration corrects for this variation in
offset, but does not allow for the variation in
temperature response slope between devices.
For this calibration, we need to have an ideal ADC
result value for either our forced temperature or
otherwise at the measured temperature. The change in
Vt by temperature varies between devices and, as a
result, single-point calibration may only be accurate at
the calibration temperature, and error will increase as it
moves further from the calibration temperature (see
Figure 6). The bow tie shape of the plotted ADC results
due to the possible variation in temperature response.
If the temperature is measured, the calculation required
to get the ideal ADC result value is given in Equation 3,
otherwise, for forced temperatures, the result can be
compared to a constant ideal result for that
temperature. Ideally, the temperature is in the middle of
the operating range seen by the application, as this
centers the bow tie and minimizes temperature error
over the applications operating range. For applications
which only need to know a certain temperature, such
as a temperature limit, the best accuracy results can be
achieved by calibrating at that temperature.
Consequently, for this device the calibration value
would be 7. Store this in the nonvolatile program or data
EEPROM memory within the device for use when
taking temperature measurements.
Single-point calibration assumes that all devices have
a similar slope, however, as the temperature moves
further from the calibration temperature, the greater the
potential error as seen in Figure 6.
When taking measurements, the ADC result is
modified by the calibration value to adjust for the offset.
EQUATION 7:
Calibrated result = ADC result – calibration value
EQUATION 8:
Temperature = (ADC result – calibration value)K
The ADC conversion results may have a dynamic
range approaching 8 bits for some combinations of
mode and voltage and, as a result, it is recommended
to maintain the two-byte ADC result data type. For
higher voltage operation, the dynamic range of the
ADC result between -40°C to +85°C is small enough
that it could be scaled down to an 8-bit number.
With a sample PIC16F1937 device under the following
conditions:
• powered at 5V
• high-range 4Vt operation
• 25°C forced temperature
The Analog-to-Digital conversion gives a result of 561
decimal.
Typical A/D conversion result at 25°C is calculated as
554 decimal using Equation 3.
For single-point calibration, the difference between the
conversion result and the ideal A/D conversion result
value is the calibration value.
Thus:
EQUATION 6:
Ideal – measured = calibration value
554 – 561 = 7
DS00001333B-page 6
 2010-2013 Microchip Technology Inc.
AN1333
FIGURE 6:
SINGLE TEMPERATURE CALIBRATION
Typical
Max Slope
Min Slope
Calibration Temperature
TWO-POINT CALIBRATION
Two-point calibration measures the temperature
responsivity of that device, as well as the offset. As a
result, it offers increased temperature accuracy by
overcoming the assumption of single-point calibration,
that all devices have the same temperature response.
FIGURE 7:
Two-point calibration requires two distinctively different
temperatures across the applications temperature
range. As with single-point calibration, these
temperatures can either be forced or measured, though
forced temperatures again simplify the required
calculations.
TWO-POINT CALIBRATION
(°C)
For unregulated supply voltages, designers must
calculate the temperature responsivity of the diode,
EQUATION 9:
 2010-2013 Microchip Technology Inc.
Calibration is required to determine A and B, which
modifies the ADC result for the variation in diode Vt and
temperature response. The ideal ADC result for each
calibration temperature can be stored as a constant if
the temperature is forced to known levels, otherwise
the ideal must be calculated if it is measured externally
during calibration. The calibrated result can then be
used in Equation 5 to calculate the temperature.
DS00001333B-page 7
AN1333
EQUATION 10:
A = (Ideal @ T1 – Ideal @ T2)/(Actual @ T1 – Actual @ T2)
B = Actual @ T1 - (A * Ideal @ T1)
Where:
T1
calibration temperature 1
T2
calibration temperature 2
This two-point calibration significantly reduces the
effect of variations in temperature response of the
diodes, but is dependent on being able to accurately
calculate the responsivity.
SINGLE-POINT CALIBRATION FOR
UNREGULATED VOLTAGES
For regulated voltages, the calibration can be simplified
For unregulated supplies, the calibration is also a
function 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 calibration
temperature and ADC result. From Equation 3,
substituting  for the Vt offset:
The Vt offset can be calculated by performing a single
ADC conversion at a known temperature and voltage.
For unregulated applications, the supply voltage can be
determined from a conversion of the internal Fixed
Voltage Reference or by supplying a known voltage
during calibration.
When measuring the temperature the supply voltage
must also be calculated and the Vt offset from the
calibration used.
During calibration,  is calculated and stored in
nonvolatile memory for use during operation. The
results of the A/D conversion are inserted into
Equation 11 along with the supply voltage to give the
operating temperature.
EQUATION 11:
V DD
 – ---------- *  1 – ---------------------------

4
1023 
Temperature = ----------------------------------------------------------------- – 40
0.00132
EQUATION 12:
V DD – 4 * [  –   Temperature C + 40  * 0.0132  
V DD
Re-arranging:
EQUATION 13:
V DD
Result 
 = ---------- *  1 – --------------------------+   Temperature C + 40  * 0.00132 
4
1023 
TWO-POINT CALIBRATION FOR
UNREGULATED VOLTAGES
For unregulated supply, such as direct connection to a
battery, we need to calculate VDD once or twice, if it
varies between the two calibration temperatures, such
as reduced battery voltage with temperatures.
From the operation of the temperature indicator we
have the following:
EQUATION 14:
Vtemp = V DD – 4 *   –  Temperature C + 40   
V temp
n
ADC Result = -------------- * (2 – 1 
V DD
V temp = --------------------------- * V DD
1023
DS00001333B-page 8
Where, for two-point calibration with an unregulated
voltage, we need to calculate alpha () and beta ().
Re-arranging the equations and calibrating at two
temperatures (Equation 15):
Key points to consider:
• The results are most accurate between the calibration 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 significantly increases the error of the readings due
to the inaccurate slope and offset.
• Regulated voltage, calibration performed at 20°C
and 60°C.
 2010-2013 Microchip Technology Inc.
AN1333
Temperature error will be minimized at the calibration
temperatures as shown Figure 8 for a sample batch of
devices, where the maximum temperature error
between the calibration temperatures is 5°C.
EQUATION 15:
V 1 *  Temp 2 + 40  *  1 – ----------------------------- – V 2 *  Temp 1 + 40  *  1 – -----------------------------


1023 
1023 
 = --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------4 *  Temp2 – Temp 1 
1
V 1 – V2 + ------------ *   V 2 * ADC Result2  –  V 1 * ADC Result1  
1023
 = ---------------------------------------------------------------------------------------------------------------------------------------------4 *  Temp 2 – Temp 1 
Where:
Temp1, Temp2
calibration temperatures
V1, V2
VDD voltage at Temp1 and Temp2
A/D Convertor result at Temp1 and Temp2
EQUATION 16:
V DD
 – ---------- *  1 – ---------------------------

4
1023 
Temperature C = ----------------------------------------------------------------- – 40

FIGURE 8:
ABS TEMPERATURE ERROR
Abs temp error
12
Absolute Error °C
10
8
C
°
r
o
rr
E 6
e
t
u
l
o
s
b 4
A
2
0
-40
-30
-20
-10
0
 2010-2013 Microchip Technology Inc.
10
20
25
30
40
Temperature (°C)
50
60
70
80
85
DS00001333B-page 9
AN1333
CONCLUSION
The on-board temperature indicator can be used to
measure the device temperature, which will
correspond to the temperature in its environment with
some delay. The indicator is measured using the ADC
and can be used uncalibrated for coarse temperature
measurements. For more precise temperature
measurements, calibration is required to account for
device parameter variation. Depending on the
application, calibration measurements at one or two
temperatures may be required. Since the ADC results
are dependent on its provided references, the fixed
references need to be supplied either by using the
on-board fixed references, or by using a regulated
supply. Otherwise, the device supply voltage must be
calculated using the Fixed Voltage Reference.
DS00001333B-page 10
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ISBN: 9781620775851
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DS00001333B-page 11
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