AN2278
Application note
Temperature sensor application
using ST LM135
Introduction
This application note describes how to develop a temperature sensor application using the
ST LM135 temperature sensor. Application development was done on a Raisonance REva
board. The Raisonance REva board is a universal board with several useful features for
application development. It has been designed for quick and easy evaluation of a wide range
of microcontrollers. This application uses the ST72325 microcontroller, which is interfaced to
REva using a daughter board.
The main components required for the development of this application are:
■
■
■
■
ST LM135 temperature sensor
ST72325 microcontroller
ST24C02 I
2
C EEPROM
ST3232 communication interface
Digital and analog I/O evaluation features including on-board LEDs, buttons, switches,
external analog connector, temperature sensor and potentiometer
On-board I
2
C EEPROM and bus extension connector
On-board RS232 driver and DB9 connector
SPI, CAN and USB connections (depending on the target device)
Embedded RLink for in-circuit debugging and in-circuit programming V
DD
settings for
1.8 V, 3.3 V, 5 V microcontrollers
USB powered, no external power required
Some of the useful features of the REva board are:
■
■
■
■
■
■
Section 1
highlights the important features of the LM135 sensor and explains the
temperature calculation used in the application.
Section 2
explains the temperature sensor
management and
Section 3
focusses on explaining the application flow.
Section 4
gives an
overview of the hardware setup required to implement the application.
September 2007
Rev 3
1/9
www.st.com
Contents
AN2278
Contents
1
2
3
4
5
6
7
LM135 temperature sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Temperature sensor application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Software configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Hardware configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2/9
AN2278
LM135 temperature sensor
1
LM135 temperature sensor
LM135 is a precision temperature sensor and can be easily calibrated. It operates as a 2-
terminal zener and the break-down voltage is directly proportional to the absolute
temperature at 10 mV/K. The temperature measurement range of the LM135 is -55 °C to
+150 °C.
Temperature calculation in Celsius:
The ST72325 microcontroller has a 10-bit ADC, so the maximum digital value it is capable of
holding is 0x3FF. The maximum voltage available at the V+ terminal of the LM135 (refer to
Figure 4)
is approximately 4800 mV (after taking into account a voltage drop of 200 mV).
Lets say x is the ADC value corresponding to an unknown temperature value.
Therefore the temperature value in kelvin is k= [(x * 4800) / (0x3FF)]/10; as zener
breakdown voltage for ST LM135 changes by 10 mV for each degree Kelvin change. The
temperature value in Celsius is C=(k - 273) / 100.
2
Temperature sensor application
This temperature monitoring application senses and logs the temperature value into an
EEPROM. If the sensed temperature is beyond the threshold limits, specified by the user, an
alarm signal is indicated by toggling the LED.
The application flow is as follows:
1.
2.
On starting the application, it enters configuration mode and the user is prompted to set
the lower and upper bounds for the normal temperature values.
Once the threshold values are set, the application enters normal mode. An average of
16 sample values, taken over a period of 1 second, is displayed by the Windows
Hyperterminal application via the on-chip SCI interface.
Note:
It is advisable that number of samples to be recorded should be of order 2^n, where n is an
integer greater than 0. This will help in calculating the average temperature value using a
‘right shift(>>)’ operator.
3.
4.
When the temperature crosses the threshold values, the application shows a ‘Critical
value’ message in the hyperterminal window and the LEDs on the board toggle.
The application keeps account of the minimum and maximum temperature values over
a span of 1 hour. At the end of an hour these minimum and maximum temperature
values are stored in the on-board I
2
C EEPROM.
You can re-enter configuration mode and change the threshold values by pressing
Button5 on the board. Since Button5 is not connected to one of the microcontroller
interrupt pins, you have to keep the button pressed until the firmware polls it.
To view the log of temperature values stored in the EEPROM, you have to press
Button6 on the board. The temperature values (in Celsius) are displayed in the
hyperterminal window. Since Button6 is not connected to one of the microcontroller
interrupt pins, you have to keep the button pressed until the firmware polls it.
5.
6.
3/9
Software configuration
AN2278
3
Software configuration
All the source files are in ‘C’ language and the application uses ST7 software library
functions.
The application works in two modes:
1.
2.
Configuration mode (refer to the flowchart in
Figure 1)
Normal mode (refer to the flowcharts in
Figure 2
and
Figure 3)
Since Button5 and Button6 are not connected to the microcontroller interrupt I/Os, polling
mode is used to read the request status resulting from pressing these buttons.
The main function is organized as follows:
while(1)
{
------------------------------
------------------------------
code=1;
switch (code)
{
case 1:
//Configuration Mode
break;
case 2:
//Read temperature log from EEPROM and display on hyperterminal
break;
case 3:
// Sense temperature in 16 samples (refer to the flowchart in
Figure
3.)
// Display average temperature every 1 sec on hyperterminal
// if temperature is out of bounds(set in configuration mode)
// then toggle LED
// else continue
// Record max and min temperature over 1 hr
// Write max and min temperature to EEPROM
break;
}
-----------------------------
-----------------------------
if(!(IO_Read(IO_PORT_C) & 0x20)) //Poll for Button 5
code=1;
else if(!(IO_Read(IO_PORT_C) & 0x80)) //Poll for Button 6
code=2;
------------------------------
------------------------------
}
4/9
AN2278
Figure 1.
Configuration mode flowchart
Application Invoked
Software configuration
Enter min. threshold value
Enter max. threshold value
Button 5 pressed
Enter normal
mode
Figure 2.
Normal mode flowchart
NO
Button6
pressed?
YES
Timer interrupt
Read stored temperature
display on hyperterminal
All
values read and
displayed?
NO
5/9
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