AN78920 PSoC 1 Temperature Measurement Using Diode.pdf

AN78920
PSoC® 1 Temperature Measurement Using Diode
Author: Dineshbabu M/Prem Sai V
Associated Project: Yes
Associated Part Family: CY8C28xxx Only
®
Software Version: PSoC Designer™ 5.4
Related Application Notes: AN60590
®
AN78920 explains the diode-based temperature measurement using PSoC
1 – CY8C28xxx family.
The temperature is measured based on the principle of a diode’s forward bias current dependence on temperature.
Contents
1
2
3
4
5
6
1
Introduction ...............................................................1
PSoC Resources ......................................................2
2.1
PSoC Designer ................................................2
2.2
Code Examples ...............................................3
2.3
Technical Support ............................................4
The Diode Equation ..................................................5
Measuring the Temperature .....................................5
Measuring Diode Temperature Using CY8C28xxx ...6
Block Diagram ..........................................................7
6.1
Functionality.....................................................7
6.2
Interconnect View ............................................8
6.3
Flow Chart .......................................................9
7
Hardware Connection ............................................. 10
7.1
Transistor Selection ....................................... 11
8
Selection of the IDAC Calibration Resistor ............. 11
9
Error Budget Analysis ............................................. 12
9.1
Ideality Factor of the Transistor Diode ........... 12
9.2
IDAC Current Ratio ........................................ 12
9.3
ADC Error ...................................................... 13
9.4
Summary of Error Sources ............................ 13
10 Test Results ........................................................... 13
11 CY8CKIT-036 for Diode Temperature
Measurement ......................................................... 15
12 Summary ................................................................ 15
Worldwide Sales and Design Support ............................. 17
Introduction
PSoC 1 – CY8C28xxx family has on-chip 8-bit IDAC, and a 14-bit Delta Sigma ADC, which enable accurate and highresolution temperature measurements using an external diode-connected transistor. The example projects attached
with this application note work with CY8CKIT-036 – PSoC Thermal management EBK.
There are various sensors available for measuring temperature such as Thermistor, Thermocouple, resistance
temperature detectors (RTD). Choosing a sensor or method to employ for measuring the temperature depends on
factors such as the accuracy requirement, the temperature range to be measured, and the cost of the temperature
sensor. The diode based temperature measurement is an easy, accurate, and also relatively low-cost method for
measuring the temperature. With on-chip current DACs and a 14-bit Delta Sigma ADC, PSoC 1 – CY8C28xxx family
enables simple and accurate temperature measurement using just an external diode and a calibration resistor. With
14-bit Delta Sigma ADC, it is possible to achieve resolution of 1 °C, and an accuracy of ±2 °C in temperature
measurement.
Diode based temperature measurement is typically used in one of the following two ways:
1.
Most CPU processors and some Application Specific Integrated Circuits (ASIC’s) provide access to thermal
diode in their architecture to measure the temperature of the processor core. This temperature measurement is
used for thermal management functions such as fan control to cool the processor core. PSoC 1 – CY8C28xxx
family can be used to interface with those thermal diodes to measure core temperature, and perform system
management functions such as fan speed control.
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PSoC® 1 Temperature Measurement Using Diode
2.
2
It is also possible to use a general purpose transistor (such as 2N3904) for temperature measurement. The
reason to opt for a general purpose transistor in this case would be the extremely cheap transistor cost along
with less stringent accuracy requirements (±3 °C). To perform temperature measurement, the transistor must be
configured as a diode by shorting the collector and base terminals of the transistor. Note that a general purpose
diode does not give as accurate results as a general purpose transistor (configured as a diode) while measuring
the temperature. This is due to the fact that the variations due to manufacturing processes are lesser in
transistors compared to diodes.
PSoC Resources
Cypress provides a wealth of data at www.cypress.com to help you to select the right PSoC device for your design,
and quickly and effectively integrate the device into your design. In this document, PSoC refers to the PSoC 1 family
of devices. To learn more about PSoC 1, refer to the application note AN75320 - Getting Started with PSoC 1.
The following is an abbreviated list for PSoC 1:


Overview: PSoC Portfolio, PSoC Roadmap

Datasheets: Describe and provide electrical
specifications for the PSoC 1 device family.

Application Notes and Code Examples:
Cover a broad range of topics, from basic to
advanced level. Many of the application
notes include code examples.

2.1

Product Selectors: PSoC 1, PSoC 3,
PSoC 4, or PSoC 5LP. In addition, PSoC
Designer includes a device selection tool.
Technical Reference Manuals (TRM):
Provide detailed descriptions of the internal
architecture of the PSoC 1 devices.

Development Kits:

CY3215A-DK
In-Circuit
Emulation
Lite
Development Kit includes an in-circuit emulator
(ICE). While the ICE-Cube is primarily used to
debug PSoC 1 devices, it can also program PSoC
1 devices using ISSP.

CY3210-PSOCEVAL1 Kit enables you to evaluate
and experiment Cypress's PSoC 1 programmable
system-on-chip
design
methodology
and
architecture.

CY8CKIT-001 is a common development platform
for all PSoC family devices.
The MiniProg1 and MiniProg3 devices provide an
interface for flash programming.
PSoC Designer
PSoC Designer is a free Windows-based Integrated Design Environment (IDE). Develop your applications using a
library of pre-characterized analog and digital peripherals in a drag-and-drop design environment. Then, customize
your design leveraging the dynamically generated API libraries of code. Figure 1 shows PSoC Designer windows.
Note: This is not the default view.
1.
Global Resources – all device hardware settings.
2.
Parameters – the parameters of the currently selected User Modules.
3.
Pinout – information related to device pins.
4.
Chip-Level Editor – a diagram of the resources available on the selected chip.
5.
Datasheet – the datasheet for the currently selected UM
6.
User Modules – all available User Modules for the selected device.
7.
Device Resource Meter – device resource usage for the current project configuration.
8.
Workspace – a tree level diagram of files associated with the project.
9.
Output – output from project build and debug operations.
®
Note: For detailed information on PSoC Designer, go to PSoC
Designer Specific Documents > IDE User Guide.
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2
PSoC® 1 Temperature Measurement Using Diode
Figure 1. PSoC Designer Layout
2.2
Code Examples
The following webpage lists the PSoC Designer based Code Examples. These Code Examples can speed up your
design process by starting you off with a complete design, instead of a blank page and also show how PSoC
Designer User modules can be used for various applications.
http://www.cypress.com/go/PSoC1Code Examples
To access the Code Examples integrated with PSoC Designer, follow the path Start Page > Design Catalog >
Launch Example Browser as shown in Figure 2.
Figure 2. Code Examples in PSoC Designer
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PSoC® 1 Temperature Measurement Using Diode
In the Example Projects Browser shown in Figure 3, you have the following options.




Keyword search to filter the projects.

Create a new project (and a new workspace if needed) based on the selection. This can speed up your design
process by starting you off with a complete, basic design. You can then adapt that design to your application.
Listing the projects based on Category.
Review the datasheet for the selection (on the Description tab).
Review the code example for the selection. You can copy and paste code from this window to your project, which
can help speed up code development, or
Figure 3. Code Example Projects, with Sample Codes
2.3
Technical Support
If you have any questions, our technical support team is happy to assist you. You can create a support request on the
Cypress Technical Support page.
You can also use the following support resources if you need quick assistance.


Self-help
Local Sales Office Locations
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PSoC® 1 Temperature Measurement Using Diode
3
The Diode Equation
The current
through a forward biased diode is given by the equation,
I  I s exp(V /VT )
Equation 1
Where,
– The diode forward voltage drop
Is – The reverse saturation current
 – A constant that has a value between 1 and 2, depending on the material and the physical structure of the diode.
VT – The thermal voltage given by,
VT  KT / q
Equation 2
Where,
– Boltzmann’s constant (1.38 x 10
-23
J/K)
T– The absolute temperature in Kelvin
q– The magnitude of electronic charge (1.602 x 10-19 C)
The temperature-dependent factors in Equation 1 are IS and VT. The reverse saturation current IS typically doubles
for every 5 °C rise in temperature. IS depends on the physical properties of the diode. VT is directly proportional to
temperature.
4
Measuring the Temperature
The technique for measuring the temperature is based on applying two different known currents to flow through the
diode, and measuring the diode voltage in each case.
For two different currents I1 and I2, such that I2=NI1
I1  I s exp(V1 /VT )
I 2  I s exp(V2 /VT )
I2
 N  exp((V2  V1 ) /VT )
I1
Equation 3
Taking natural logarithm on both sides,
ln( N )  (V2  V1 ) /VT
Equation 4
Using Equation 2, the temperature (T) in Kelvin is given by,
T (in Kelvin )  (V2  V1 ) 
q
ln( N )  K 
T (in Kelvin)  c  V
Equation 5
Where,
– The difference in diode forward voltage drop for two different currents.
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PSoC® 1 Temperature Measurement Using Diode
‘c’ – A constant given by,
Equation 6
For
' '  1, ' N '  20 we get ‘c’ ≈ 3875 Kelvin/Volt.
T (in C )  (V  3875)  273
Equation 7
A 1 °C or 1-K difference in temperature translates into a change in
considered values translates to around 258 µV.
For increasing values of current ratio ‘N’, there would be greater change in
the Figure 4.
by a factor (1/c), which for the above
with respect to temperature, as seen
Figure 4. Theoretical Variation of ∆VD with Temperature for Different Current Ratios (N)
Theoritical variation of ∆VD with
Temperature for different Current
Ratios(N)
120000
100000
80000
∆VD
(uV)
N=5
60000
N=10
40000
N=20
N=30
20000
0
0
10
20
30
40
50
60
70
80
Temperature(oC)
5
Measuring Diode Temperature Using CY8C28xxx
CY8C28xxx family has powerful analog architecture that enables the accurate measurement of Diode temperature.
The implementation in CY8C28xxx is based on Equation 5. Figure 5 shows the block diagram for this implementation
showing the external diode connected transistor and also the external calibration resistor. This calibration resistor is
used to accurately calculate the IDAC current ratio ‘N’ used in Equation 6.
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PSoC® 1 Temperature Measurement Using Diode
6
Block Diagram
Figure 5. Block Diagram
IDAC
R_Cal
PSoC1
CY8C28xxx
Amux
Pos
PGA_POS
ADC (14-bit)
+ Input
Output
- Input
CPU
Amux
Neg
PGA_NEG
LCD
6.1
Functionality



Current DAC (IDAC) forces two known currents I1, I2 through the transistor diode.

The AMUX8 component in PSoC Designer is used to multiplex the input pins to perform ADC Offset calculation,
Current ratio calculation and Temperature calculation.

The ADC offset calculation is performed by connecting the two ADC inputs to AGND and measuring the ADC
output. This offset correction is used while doing IDAC calibration to measure the current ratio accurately.
The corresponding base-emitter voltages V1, V2 are measured using Delta Sigma ADC.
Equation 5 is then used to calculate the temperature in firmware. To ensure that the IDAC current ratio ‘N’ (N =
I2/I1) in Equation 5 is computed accurately, a calibration resistor is connected at the output of IDAC in series with
the transistor diode. The ratio of voltages across the calibration resistor gives the IDAC current ratio. This
calibration removes the error due to IDAC offset, non-linearity in temperature measurement. By using 14-bit
Delta Sigma ADC, the current ratio can be calculated accurately.
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PSoC® 1 Temperature Measurement Using Diode
6.2
Interconnect View
Figure 6. PSoC Designer Interconnect View
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PSoC® 1 Temperature Measurement Using Diode
6.3
Flow Chart
A
Start
MEASURING DIFFERENCE
IN VOLTAGE DROPS
INITIALIZATION
Start IDAC, PGAs, ADC,
AMUX
Using AMUX8 components, set input of PGA_Pos as
anode of diode and input of PGA_Neg as cathode of
diode
Calculate multiply_factor 'C' as:
C = (Q * Adc_Resolution) / (K*calibration_factor)
Set IDAC value as LOW(30 uA)
B
MEASURING OFFSET
Skip 3 ADC samples
Set inputs of both PGA_Pos and
PGA_Neg as Analog Ground
Take ‘ITER’ number of ADC samples and add them.
This gives V1_counts
Skip 3 ADC samples
Set IDAC value as HIGH(600 uA)
Take ‘ITER’ number of ADC samples and average them.
This gives OFFSET
Skip 3 ADC samples
MEASURING CURRENT RATIO
Take ‘ITER’ number of ADC samples and add them.
This gives V2_counts
Using AMUX8 components, set input of PGA_Pos as
top of calibration resistor and input of PGA_Neg as
bottom of calibration resistor
Calculate Vbe_diff_counts = V2_counts – V1_counts
Set IDAC value as LOW(30 uA)
Skip 3 ADC samples
Take ‘ITER’ number ADC samples and average them.
Subtract OFFSET.
This gives I1_counts
CALCULATING
AND
DISPLAYING
TEMPERATURE
Calculate Temperature T(in degree Celcius) as:
T = [ C * (Vbe_diff_counts) / (ITER * ln(N)) ] - 273
Display Temperature on LCD
Set IDAC value as HIGH(600 uA)
B
Skip 3 ADC samples
Take ‘ITER’ number ADC samples and average them.
Subtract OFFSET.
This gives I2_counts
Calculate Current Ratio N = I2_counts/I1_counts
A
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PSoC® 1 Temperature Measurement Using Diode
7
Hardware Connection
The following schematic shows the connection between the pins of PSoC (that is placed on CY8CKIT-001) to the
transistor present on CY8CKIT-036 EBK. The calibration resistor is an external resistor. The CY8CKIT-025 EBK that
has on-board calibration resistor can also be used with this application note.
Figure 7. Connection Between CY8CKIT-001 and CY8CKIT-036 EBK
Figure 8. Snapshot of Hardware Setup
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PSoC® 1 Temperature Measurement Using Diode
7.1
Transistor Selection
The selection of right transistor is very important for accurate temperature measurement. The CY8CKIT-036 uses the
MMBT3904 transistor from Fairchild Semiconductors for temperature measurement. The following factor should be
considered for choosing the transistor that gives the best results for temperature measurement.
7.1.1
Ideality Factor
Ideality factor ( ) of diode is involved in temperature measurement as shown in Equation 5 and Equation 6. The
datasheets of the general purpose transistors do not provide ideality factor value in their datasheets. But a correct
measurement of the ideality factor is required for accurate temperature measurement. The procedure to calculate the
ideality factor is given as follows:
1.
Assuming an ideality factor
CY8C28xxx.
(for example: 1), measure the diode temperature (Tmeasured in Kelvin) using
2.
Measure the ambient temperature ‘Tactual’ (in Kelvin) using an accurate temperature measurement source. In lab
testing, the MicroTherma2 temperature measurement system was used to find the ambient temperature
accurately. Since the self-heating of diode is negligible, both the diode temperature and ambient temperature will
be almost the same.
Using the assumed ideality factor
, and the two temperature parameters (Tmeasured, Tactual), the correct
ideality factor can be calculated as given by below equation.
Ideality Factor ,correct 
Tmeasured
 assumed
Tactual
Based on above equation, the ideality factor of the MMBT3904 transistor used in this application note is 1.043619.
The error in temperature measurement due to a wrong calculation of ideality factor is given by,
T  Tactual  Tmeasured
T  Tactual(inKelvin )  (1  correct / assumed )
Equation 8
Tactual is the expected temperature for correct ideality factor correct , and Tmeasured
assumed ideality factor
is the measured temperature for
. It can be inferred from Equation 8 that the error due to wrong ideality factor
increases with increasing temperature. A 1 percent error in ideality factor
assumed  1.01 correct
would cause a
measurement error of 3.7 °C or 3.7 K at temperature of 373 K (100 °C).
8
Selection of the IDAC Calibration Resistor
An external resistor in series with the diode is used for calibrating IDAC so as to calculate the IDAC current ratio
accurately. This resistor need not be highly accurate as the absolute value of the resistance does not matter when
taking ratio of voltages.
From Equation 5,
Current ratio, N = I2/I1 = (V2 * R)/(V1 * R) = V2/V1
The two limiting factors while choosing the resistor are:
1.
This minimum value of calibration resistor must be such that the voltage-drop across the calibration resistor
when passing the minimum current is greater than the ADC’s resolution.
2.
The maximum value of the calibration resistance is determined by two independent factors:
a.
The IDAC compliance voltage: The compliance voltage of the IDAC is (V DD – 1), where VDD is supply
voltage.
RA < (VDD - 2)/(Imax)
A difference of 1 V is also present along with IDAC compliance voltage of (V DDA – 1), which is to account for
maximum diode forward voltage drop. I max is the maximum current output of IDAC. For example, V DD =5 V,
Imax = 600 µA results in maximum resistance of ~5 kΩ.
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PSoC® 1 Temperature Measurement Using Diode
b.
The voltage drop across the calibration resistor when passing the maximum current must be lesser than the
ADC’s range in the positive side (RefHi-AGND)
RB < (RefHi-AGND)/( Imax)
The maximum value of calibration resistor is the lesser of R A and RB, i.e.,
Rmax = Min(RA, RB)
An optimized resistance value of 450 Ω, which satisfies the above conditions, is used in this application
note.
9
Error Budget Analysis
This section discusses the different factors that affect the accuracy of diode temperature measurement. This includes
the error caused by the PSoC resources (PGA, ADC, and IDAC), and the external transistor diode. The analysis in
this section is based on the following equation:
Equation 9
9.1
Ideality Factor of the Transistor Diode
The following equation gives the error due to the ideality factor:
Equation 10
is the expected temperature in Kelvin for the correct ideality factor
, and
is the measured
temperature in Kelvin for the assumed ideality factor
. It can be inferred from Equation 10 that the error due
to a wrong ideality factor increases with the increasing temperature. A 0.1 percent error in the ideality factor
would cause a measurement error of 0.36 °C at a temperature of 85 °C.
9.2
IDAC Current Ratio
The final temperature measurement accuracy is highly dependent on the excitation current ratio I2/I1. Therefore, you
need to calibrate the IDAC output current before the actual measurement. The basic theory is to measure the voltage
drop on Rcal at I1 and I2 and use the actual N = I2/I1 temperature calculation in Equation 9.
The error is mainly due to the ADC and the unity gain PGAs present in the input path.
Error due to INL:
Rcal = 450 Ω
At I1 = 30 µA, VRcal (voltage drop across the calibration resistor) is 13.5 mV.
At I2 = 600 µA, VRcal = 270 mV.
The 8-LSB INL (max. limit) of ADC at a 12-bit resolution, which means a maximum 5-mV voltage measurement error.
Then, the total equivalent gain error caused by the ADC INL in the excitation current ratio calibration is +/–38.8
percent.
Error due to the PGA gain: 0.5 percent
Therefore, the I2/I1 total error is about 39.3 percent.
The gain error of the ADC does not affect the current ratio calibration because it affects both I1 and I2 measurement
and will be canceled when you divide I1 by I2.
In this case, the final temperature error caused by the current ratio calibration is:
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PSoC® 1 Temperature Measurement Using Diode
A 39.3% error in the current ratio would cause a measurement error of 0.35 °C at a temperature of 85 °C.
9.3
ADC Error
The ADC has three sources of error: offset error, gain error, and ADC nonlinearity. The offset calibration process in
firmware takes care of the ADC offset.
9.3.1
Gain Error
Considering a 1.3% gain error, the temperature error caused by ADC in 85 °C is about 4.7 °C.
9.4
Summary of Error Sources
Table 1 summarizes the temperature error due to various error sources. In the table, all error sources except the
ideality factor are due to the PSoC signal chain. The errors due to the PSoC signal chain are for worst-case
conditions. The Test Results section provides the practically observed results of the temperature measurement.
Table 1. Temperature Measurement Error Sources
Error Source
Error at 85 °C
Comments
IDAC Current Ratio
0.35 °C
INL error calculation is the worst case; in practice, the error should be much smaller.
ADC Gain Error
4.7 °C
Use internal reference.
ADC Gain Drift
–
–
Ideality Factor
0.36 °C
For a 0.1 % error in ideality factor at 85 °C. This error is due to the transistor itself,
not to the PSoC signal chain.
(Special)
10
Test Results
Temperature measurement in the range of 0-80 °C was made using the hardware setup shown in Figure 7 and
Figure 8. Thermonics instrument was used to force different temperatures onto the transistor on the CY8CKIT-036.
The actual temperature of the transistor was measured using an instrument called MicroTherma2, which was used as
the reference while making temperature measurements. The MicroTherma2 is an accurate, thermocouple (Type-K)
based temperature measurement instrument.
A calibration resistor of 450 Ω was used and the correct ideality factor was found to be 1.043619.
Figure 9 shows the observed variation of ∆VD with respect to temperature. This variation is seen to be almost linear
and similar to the theoretical ‘Temperature versus ∆VD’ plot shown in Figure 4 for N=20.
Figure 9. Measured ∆VD Using PSoC 1
Measured ∆VD using PSoC1
100
95
90
∆VD
(uV)
85
80
75
70
4
9
14
19
24
29
34
39
44
49
54
59
64
69
74
79
o
Temperature ( C)
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PSoC® 1 Temperature Measurement Using Diode
Figure 10 shows a comparison between the actual temperature and the temperature measured by PSoC 1. An
accuracy of ±2 °C and resolution of less than 1 °C was observed over the range of 0-80 °C.
Figure 10. Actual Temperature and Measured Temperature using PSoC 1
Actual Temperature and Measured
Temperature using PSoC1
Temperature
(oC)
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
Actual Temperature
PSoC1 Temperature Reading
1 2 3 4 5 6 7 8 9 10 11 12 13
Sample Number
To verify the repeatability of the results, temperatures of 1, 40 and 80 °C were forced multiple times onto the
transistor and the PSoC1’s readings were observed. The Figure 11 shows a plot of the multiple readings at these 3
temperatures.
Figure 11. Repeatability Test Results
PSoC1
Temperature
Reading (oC)
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
-5
Temperature=1
Temperature=40
Temperature=80
1
2
3
4
5
6
Iterations
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PSoC® 1 Temperature Measurement Using Diode
11
CY8CKIT-036 for Diode Temperature Measurement
CY8CKIT-036 is intended to provide a demonstration and development platform for developing system thermal
management coprocessor solutions with compelling example projects that demonstrate a variety of modes:




Temperature monitoring
Open-loop and closed-loop fan control
Thermal zone management: the relationship between temperatures and cooling functions
Algorithms to detect thermal and cooling failures or warnings
This kit comes with two MMBT3904 (SOT-23 package type) transistor diodes connected in anti-parallel fashion. This
kit has been used for the example project of this application note. More details about the kit can be found in the Kit
User Guide.
Figure 12. CY8CKIT-036 PSoC Thermal Management Expansion Board Kit
12
Summary
This application note explains how the analog features in CY8C28xxx family of devices enable accurate temperature
measurement using general purpose transistor or thermal diodes.
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PSoC® 1 Temperature Measurement Using Diode
Document History
®
Document Title: AN78920 – PSoC 1 Temperature Measurement Using Diode
Document Number: 001-78920
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
3632571
DIMA
05/31/2012
New application note
*A
3691380
PRKU
07/24/2012
Document revised to improve clarity in images.
*B
4404906
SREH
06/11/2014
Sunset Review. Updated PSoC Designer version to 5.4.
*C
4762918
DIMA
05/12/2015
Added Error Budget Analysis.
Updated project to PSoC Designer 5.4 SP1.
Sunset Review.
Updated template
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Document No. 001-78920 Rev. *C
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PSoC® 1 Temperature Measurement Using Diode
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use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a
malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress’ product in a life-support systems
application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges.
Use may be limited by and subject to the applicable Cypress software license agreement.
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Document No. 001-78920 Rev. *C
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