a beginner using pic controller
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A Beginners data logger project using PIC12F683 microcontroller
It is a very simple data logger project based on PIC12F683 microcontroller. The microcontroller
reads temperature values from a temperature sensor on a regular interval basis and stores
them into its internal EEPROM memory. The recorded temperatures can be later transferred to
a PC through serial interface. I originally published this project on electronics-lab.com last
summer. I thought this could be a very good learning project for beginners, and so I am posting
it here for Embedded Labs readers too.
Finished temperature logger powered from a 9V battery
Theory
The sensor used in this project is DS18B20. It is a digital temperature sensor manufactured by
Dallas Semiconductor (now MAXIM) that can measure temperature ranging from -55C to
+125C with an accuracy of 0.5C over the range of -10C to +85C. The sensor provides the
measured temperature (C) output in user-configurable 9, 10, 11, or 12-bit data corresponding
to the desired resolution of 0.5, 0.25, 0.125, and 0.0625 C. The sensor communicates with a
host microcontroller over a 1-wire bus. Readers are suggested to refer thedatasheet on
Maxims website for details on this sensor. Please keep in mind that there are three versions of
this sensors, namely DS1820, DS18S20, and DS18B20, and they have some architectural
differences. All of them have the same pin configuration and therefore the circuit diagram would
be same for all three types. However, some modification in the software may be required while
the microcontroller reads the temperature data from them.
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PIC12F683 has 256 bytes of internal EEPROM. Each temperature reading is stored as a byte,
which means only the eight most significant bits of DS18B20 output is recorded. Therefore, the
temperature resolution is decreased down to 1 C. This temperature logger can store up to 254
temperature values (254 bytes) in its internal EEPROM. The remaining two EEPROM locations
are used to store the sampling time and number of samples information. Three tact switches are
used to provide user inputs for controlling the operation of the data logger.
Circuit Diagram
The PIC microcontroller uses its internal clock source operated at 4.0 MHz. The DS18B20
sensor is interfaced to GP0 pin (7) of the microcontroller. An LED connected to the GP2 pin
serves as the only display in the circuit to indicate various actions of the data logger. For
example, it blinks every time a sample is recorded into EEPROM. The circuit is powered with +5
V derived from a 9V battery using an LM78L05 regulator IC. The LM78L05 circuit is a very
common circuit and therefore, it is not shown here.
Circuit diagram of data logger
The three tact switches provide the following functions.
Start: Starts data logging
Stop: Stops the logging procedure
Send/Reset: Transfers data to PC through serial port. However, if it is held pressed for 2
sec or more, the EEPROM locations are cleared and ready for new recordings.
Selection of sampling time
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This data logger offers three options for sampling interval: 1 sec, 1min, and 10 min. The
selection is made through the same three tact switches. Here is how it works. Suppose if 10 min
sampling time is needed, then first turn OFF the power, hold the Send/Reset button pressed,
turn the power ON, and wait till the LED glows. Once the LED glows, release the button, and the
sampling interval is set to 10 min. The new sampling time will be updated to EEPROM location
0 so that in case of power failure, the previous sampling time will be restored. Similarly, the useof Start or Stop button instead of the Send/Reset one sets the sampling time to 1 sec, or 1
min respectively. With 10 min sampling interval, this data logger can record temperature
samples over 42 hours.
Serial Interface to PC
Transferring data to PC through serial port requires a voltage translation circuit to convert the
TTL logic levels from PIC12F683 to appropriate RS232 voltage levels. A regular PNP transistor
with few other passive components can do this job. The RS232 standard uses a -3 to -12 V for
Logic 1 and +3 to +12 V for Logic 0. The required negative voltage is stolen from the TX pin of
the RS232 port on PCs side which is unused as there wont be any data transfer from the PC toPIC12F683. Under idle condition the TX pin on PCs side is held high (-12 V). The two figures
below describe the operation of converting TTL levels for 1 and 0 to corresponding RS232
levels. The positive terminal of the 10 uF capacitor is grounded because its negative terminal
has to deal with a more negative voltage.
Translating TTL Logic 1 level to RS232 Logic 1 level
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Translating TTL Logic 0 level to RS232 Logic 0 level
Finished project board
Software
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The firmware for PIC12F683 is developed in C using mikroC Pro for PIC compiler from
mikroElektronika. PIC12F683 doesnt have built-in hardware UART module for serial
communication but the mikroC compiler has built-in library routines to implement software
UART through any digital I/O pin of PIC microcontroller. Due to wide use of DS18B20 sensor for
temperature measurements, mikroC also provides 1-wire library routines for controlling the
operation of this sensor. The built-in functions for communicating with a 1-wire sensor are,
Ow_Reset is used for reseting sensor;
Ow_Read is used for receiving data from sensor; and
Ow_Write is used for sending commands to sensor.
MikroC also provides EEPROM library for read and write operations to internal EEPROM
locations. The overall programming is made much easier with the help of these built-in library
functions of mikroC. The source code with HEX files can be downloaded from the links below.
Download mikroC project files (for DS18B20 sensor)
Download mikroC project files (for DS1820 sensor)
The configuration bits setup for PIC12F683 can be done through Edit Project window in mikroC.
The figure below shows the required settings for this project.
Programming sequence
When the circuit is powered on, the LED blinks for 3 times that shows the data logger is turned
on and being initiated. The program checks for any key pressed for setting sampling interval. If
yes, it identifies the key and store the appropriate sampling interval information to EEPROM
location 0. The LED is turned on to indicate that the sampling interval has been set and its time
to release the pressed key. The program is now inside the main loop. The three switches (Start,
Stop and Send/Reset) operate in Interrupt-on-change mode, which means, any time the button
http://www.mikroe.com/eng/products/view/7/mikroc-pro-for-pic/http://embedded-lab.com/uploads/HexFiles/PIC12F/DataLogger_DS18B20.ziphttp://embedded-lab.com/uploads/HexFiles/PIC12F/DataLogger_DS1820.ziphttp://embedded-lab.com/blog/wp-content/uploads/2011/04/ProjectSetting.jpghttp://www.mikroe.com/eng/products/view/7/mikroc-pro-for-pic/http://embedded-lab.com/uploads/HexFiles/PIC12F/DataLogger_DS18B20.ziphttp://embedded-lab.com/uploads/HexFiles/PIC12F/DataLogger_DS1820.zip -
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is pressed, an interrupt is generated. Pressing the Start button begins data recording. Each time
a temperature sample is recorded in to the EEPROM, the LED blinks to indicate that the logging
process is going on. Pressing the Stop button will interrupt this process, while Send will initiate
the serial transfer of recorded samples through GP1 pin (6). If the Send button is pressed for
more than 2 sec, the entire EEPROM locations will be cleared.
On PCs side, the hyperterminal program is used to receive logged temperature data from
PIC12F683. The rate of data transfer is set to 9600 baud on both end. The figure below shows
the hyperterminal settings for this project.
Hyperterminal settings on PC
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Connecting to PC's serial port
Data received by the hyperterminal program
Conclusion and future work
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A simple data logger project was demonstrated using a PIC12F683 micrcontroller and DS18B20
temperature sensor. A number of improvements can be done with this project, such as adding
an external EEPROM for expanding its recording capacity. A separate application program can
be developed on PCs side to receive the serial data from the logger and generate temperature
vs time plots. A real time clock chip can also be added in the project to keep record of the actual
time stamp. A bigger PIC with more I/O pins would be more appropriate for implementing theseadditional features.