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how to use I/O ports of PIC 18F family devices correctly

Fri, 13 Apr 2012 22:50:31 +0600

From Data sheet:
LAT Registers
The LATx register associated with an I/O pin eliminates the problems that could occur with read-modify-write instructions. A read of the LATx register returns the values held in the port output latches, instead of the values on the I/O pins. A read-modify-write operation on the LAT register, associated with an I/O port, avoids the possibility of writing the input pin values into the port latches. A write to the LATx register has the same effect as a write to the PORTx register.

The differences between the PORT and LAT registers can be summarized as follows:

•A write to the PORTx register writes the data value to the port latch.
•A write to the LATx register writes the data value to the port latch.
•A read of the PORTx register reads the data value on the I/O pin.
•A read of the LATx register reads the data value held in the port latch.

i.e.
When you want to read whether some external hardware is driving a pin high or low, you must set the pin to input mode (with relevent bits of TRIS register set to 1), and you must read PORTx. That read tells you if the actual voltage at the pin is high or low.

When you want to drive a pin high or low, you must set the pin to output (with relevent bits of TRIS register set to 0); you should write the bit to the LATx register.

(Writing that bit to the PORTx register may seem to do the right thing: that pin will - eventually - go high or low as commanded. But there are many cases - such as when some other pin on that port is connected to an open-collector bus - that writing to one bit of the the PORTx register will mess up the state of the other pins on that port, leading to difficult-to-debug problems).

Sending SMS through SIM300 example

Sun, 25 Mar 2012 08:04:59 +0600

Sending SMS/ text message through SIM300 module needs only 2 AT commands.
First you should connect SIM300 module to computer or microcontroller through its serial port.

then send following commands.

AT+CMGF=1 // this command will instruct SIM300 module to enter text mode. after giving command you should press enter key or send ascii 0x0D charactor.

Then send

AT+CMGW="xxxxxxxxxx" // xxxxxxxxxx is the phone number. after this command press enter or send ascii 0x0D charactor through MCU.

then you will get ">" promt. then type message and press + or send 0x0A 0x0D and 0x1A charactors through MCU to SIM300.

Thats all about sending SMS through SIM300 module.

Thermocouples

Mon, 13 Feb 2012 06:13:37 +0600

Thermocouples are well known as temperature sensors.

The working principle of Thermocouples
Thermocouples are made with two different conductors. When the two ends of the conductors are in different current an electrical voltage is generated. This voltage value varies with the temperature difference between two ends. So if we know the temperature of one end we can identify the temperature of the other end. By calibrating particular thermocouple we can use it as a thermal sensor.
To keep one end in a known temperature (usually cold end) an artificially cold junction is made by using a thermally sensitive device such as a thermistor or diode to measure the temperature of the input connections at the instrument. There are different combinations of conductors used so that numbers of type such as k type,E type,J type and etc. among these k type is widely use in industry.
Specifications
 Type of conductors use.
 Calibration type
Whether the type is K, J, T etc
 Temperature range.
K type has range of 0 to +1100°C
 Limit of error
K type +or-2%

Application
 Temperature measurement for kilns
 Gas turbine exhaust
 Diesel engines
 Industrial processes such as automated machines with heaters.

Photo Diodes

Mon, 13 Feb 2012 06:06:25 +0600

Photodiode is a sensor that can convert light into current or voltage. These photodiode are use in solar panel in large arrays too. Voltage or current varies with the light so that we can detect a light by using them.

The working principle of Photodiodes
Photodiode have semiconductor junction normally known as PN junction or PIN junction which has a thin intrinsic semiconductor region. So when this expose to the light it excites an electron. This will create free electrons and positive charged holes. This occurs in the junction's depletion region by flowing electrons. This makes a current and voltage according to the light.Normally a window or optical fiber connection allow light to reach the sensitive part of the device.

Specifications
 Sensitivity A measure of the effectiveness
 Active area diameter
The active area of the detector
 Operating temperature
 Light Current and Dark current.
Photo current is the sum of above 2 current. So dark current must be minimized to increase the Sensitivity
 Fall time and Rise time
Time taken to react in a light variation

Application
 Photo detectors
 Inside compact disc players
 receivers for infrared remote control devices
 In camera light meters
 For optical communications

PTC THERMISTORS

Sun, 12 Feb 2012 20:05:11 +0600

PTC thermistors have been used in a wide variety of applications over the years. PTC thermistor applications make use of the characteristics inherent in their composition. Generally, applications are broken up into two distinct categories that utilize different characteristics of the PTC. The first category is those applications that utilize the voltage-current or current-time characteristics of the PTC thermistor. These are sometimes known as self-heated applications. The other general category is zero power or sensing applications. Unlike the NTC thermistor, the resistance versus temperature characteristic of a PTC thermistor is not well defined and applications that utilize the resistance versus temperature characteristic tend to utilize only a small portion of the R vs. T curve and utilize broader tolerances than that of NTC thermistors.

APPLICATIONS
1. PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.

2. PTC thermistors are used as timers in the degaussing coil circuit of most CRT displays and televisions. When the display unit is initially switched on, current flows through the thermistor and degaussing coil. The coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the point that the degaussing coil shuts off in under a second. For effective degaussing,it is necessary that the magnitude of the alternating magnetic field produced by the degaussing coil decreases smoothly and continuously, rather than sharply switching off or decreasing in steps; the PTC thermistor accomplishes this naturally as it heats up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive.
3. NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.
4. NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application.
5. NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.
6. NTC thermistors can be also used to monitor the temperature of an incubator.
7. Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging

ADVANTAGES
1. The thermistors are more sensitive than the RTD's and thermocouples
2. The thermistors have quick response time
3. Large temperature sensitivity, of the order of -4% to -6% per °C.
4. Available in many physical forms for probe configurations.
5. Not subject to standards or constrained by material types which gives many options for customization of mechanical and electrical characteristics.
6. Tight tolerances available (typically +/-0.1°C, +/- 0.2°C and customized values).
7. Interchangeable devices eliminate need for individual sensor calibrations.
8. Standard two-wire connection system.
9. Good electrical noise immunity.
10. Easy to interface to electronic instrumentation.
11. Can be customized to optimize thermal response.
12. Relationship between resistance and temperature of NTC thermistor sensors can be modeled easily by standard equations.
13. Temperature range can be extended from -100°C to + 300°C with special constructions.

DISADVANTAGES
1. Linearity of a thermistor is not good
2. Since they are made of semiconductor, thermistors are susceptible to permanent deceleration at high temperatures than are RTD's or the thermocouples.
3. Operating temperature limited to approximately -60 to +300 ºCelsius
4. Current source required

Inductive proximity sensors

Sun, 12 Feb 2012 14:39:22 +0600

This proximity sensor detects the presence of metallic objects. Operating principle of this sensor is based on a coil and high frequency oscillator that creates a field in the close surrounding of the sensing surface. The presence of metal in the operating area causes a change in the oscillation amplitude. This change is identified by a threshold circuit. Threshold circuit changes the output of the sensor. The operating distance of the sensor depends on the coil’s size as well as the target’s shape, size and material.

Gyroscope.

Sun, 12 Feb 2012 14:25:35 +0600

Technically, a gyroscope is any device that can measure angular velocity. As early Gyroscopes are available that can measure rotational velocity in 1, 2, or 3 directions. 3-axis gyroscopes are often implemented with a 3-axis accelerometer to provide a full 6 degree-of-freedom motion tracking system. Gyroscopes have cause from mechanical-inertial spinning devices including of rotors, axles, and gimbals to various incarnations of electronic and optical devices There are three basic types of gyroscope can identified as,

 Vibrating Structure Gyroscope
 Rotary gyroscopes
 Optical Gyroscope

Gyroscope Function,
Gyroscopes function differently depending on their type of its. Traditional spinning gyroscopes work on the basis that a spinning object that is tilted perpendicularly to the direction of the spin. Optical gyroscopes are most commonly ring laser gyroscopes. These devices send two lasers around a circular path in opposite directions.
Optical gyroscopes are a most improvement to the spinning mass gyroscopes because of there is no wear, greater reliability and smaller in the size and weight.
After the inventing of laser ring gyroscopes, a lot of properties were desired. MEMS vibrating mass gyroscopes aimed to create smaller and high sensitive devices.

Load cells

Sun, 12 Feb 2012 14:24:33 +0600

It is a device That converts a forces in to electrical signals.This conversion will not happen in derectly and it happen in two stages through a mechanical arrangement. The force will measure as deformation and converts to electrical signal because the strain will change the effective electrical resistance of a wire. also A load cell usually consists of four strain gauges. The electrical signal output is typically in the order of a few millivolts and it wants amplification by an instrumentation amplifier before it used. The output of the transducer is injected into an algorithm to calculate the force applied to the transducer.So many types of load cells can be identified as bellow,
 Low profile load cells
 Bending beam load cells
 Canister load cells
 Hydraulic load cells
 Pneumatic load cells
 Compression load cells
 S beam load cells
 Platform and single point load cells

APPLICATIONS,
For aircraft component fatigue testing
Weighing equipment
For cranes & lifting equipments
For machine building & monitoring
For force checking & monitoring
For wind turbine testing and R&D...etc.

Photodiodes

Sun, 12 Feb 2012 14:23:02 +0600

Photodiode is an electrical device which is used to detect and convert light into an electrical energy signal through the use of a photo detector. Also the photodiode canbe used as a fiber optic element to deliver the light to the diode circuit and for convert it to electrical signal. Its function is basically same to the function of a solar panel or window.

There are some types of photodiode that can identified as,
 The p-n and p-i-n photodiodes
Those are the most commonly used, but
Avalanche photodiodes
Those are also available but lower than first case.
Working principle Photodiode also same as diode. it has PN junction. Therefore, it has intrinsic semi-conductive region. So when expose to the light it create positive holes and free electrons. When occur holes and electrons it tends to pass current through junction. Electronic/circuit symbol

APPLICATIONS
 Charge-coupled devices,
 Compact disc players,
 Photoconductors,
 Photomultiplier tubes.
 Smoke detectors.
 Infrared remote control devices.

Phototransistor Switch,

Sun, 12 Feb 2012 14:21:46 +0600

Phototransistors circuits may be adjusted for a selected sensitivity range and often do not require additional amplification. Mostly phototransistors are low cost visible and near IR photo detection, available with gains from 100to 1500, moderately fast response times and available in wide range .They can be applied in two modes, Active or Switch mode also.
Switch mode.
Means the phototransistor will either be “off” or “on” for a digital logic response to the object sensed.
Active Mode.
Means the phototransistor generates an output response based on the light or irradiance level. IC(on) will be proportional to the coupled light intensity.

APPLICATIONS
Remote Control of TV
Margin controllers in printers
Monitor paper position in copiers
Monitor paper stack height in copiers
LED light source
Security systems
Safety shields
Encoders...etc.

LDR

Sun, 12 Feb 2012 14:21:06 +0600

Light dependent resistor(LDR).

It can be also named as photo resistor or light dependent resistor and it is a component that Can sensitive to light. When light falls to the sensor then its resistance will changes gradually. Another thing is LDRs are easy to use and there are many LDR circuits’ available. And light dependent resistors are very useful especially in light or dark sensor circuits also. The resistance of the LDR will change over many orders of magnitude the value of the resistance falling as the level of light increases but normally the resistance of an LDR is very high value.

APPLICATIONS
 Camera Light Meters.
Street lights.
 Clock Radios.
Alarm Devices.
Outdoor locks.
Light Interruption Detectors

Temperature sensors

Sun, 12 Feb 2012 14:19:55 +0600

Temperature sensors
1 .1Thermocouple.
The thermocouple is most commonly used type of temperature sensing device. It is a very simple device among sensors and it is easy to use and speed of response to changes in temperature, and also size of this sensor is very small. Important thing is it also have widest temperature range of all temperature sensors ( -200 0C to 20000C).

Basic thing is it is a junction between two dissimilar metals.and also six type of thermocouples can identify basically.
APPLICATIONS OF THERMOCUPLES
 Thermic fluid heaters
 Ovens
 Boilers
 Duct lines

Resistance type sensors,
These work on the principle that the electrical resistance of a conductor change with temperature of the conductor.

1.2 Thermistors.

Thermistor is a device which is used for sense the temperature. Thermistors have property of varies the resistance proportional to their Changes of temperature .There are two Kind of thermistors can identified as, Negative Coefficient of Resistance type(NTC) and Positive temperature Coefficient of resistance type (PTC).

NTC type,
In this case, the resistance is inversely proportional to temperature and the applications of this type of sensors as bellow,
 Resistance thermometers
 Digital thermostats
 Inrush-current limiting devices
 Automotive applications

PTC type,
This case is the reveres of NTC type thermestor. It means when increase temperature the resistance also will increase, not same as first case. And the applications also as bellow.
 Used in overload protect circuits
 Used in thermal switches
 Ordinary thermometers
Overall Practical Applications of Thermestors
 Fire alarms
 Electronic air conditioners
 Audio amplifiers
 Mobile phones
 Electric water heaters

sensor types

Sun, 12 Feb 2012 14:07:53 +0600

Almost any modern system use sensors to observe the environment and actuators to influence this environment. a basic description about sensors and actuators can express as bellow,

Thermal sensors,
Thermometer –Used to measure absolute temperature.
Thermocouple gauge– Used to measure temperature by its affect on two various metals
Calorimeter – Used to measure the heat of chemical reactions or physical changes and capacity of heat

Mechanical sensors,
Pressure sensor – To measure pressure
Barometer – measures atmospheric pressure inclined barometer
Liquid flow sensor – Used to measure flow rate of the liquid flow
Gas flow sensor – In addition to flow rate, it can measure velocity and direction also gasses
Accelerometer – measures acceleration

Electrical sensors,
Ohmmeter –Used to measure resistance of something
Voltmeter – measures voltage of something
Galvanometer – Used to measure amount of current
Chemical sensors,
Oxygen sensor – measures the percentage of oxygen in a gas or liquid.

Other types of sensors,

Optical sensors
Light sensors (photodetectors)
Photocells (photoresistor)
Infra-red sensor
Acoustic wave sensors
Motion sensors
Speedometer
Force sensors
Biological sensors.

SDCC + PIC16F877A: Problem with interrupt

Sun, 18 Dec 2011 08:16:44 +0600

The following is my code for part of my coding for pic16f877a. The problem is, after the interrupt routine (each time the usart receive a byte), the address returns to the beginning of main(). But I want it to return to the DESIRED_POSITION in the code below. How to do this? Thanks in advance.

When entering a char to the usart (using gpsim) after the DESIRED_POSITION is passed, the interrupt still return to the beginning of main().

void main () {
char c, *s, *s1, *response, prev_c;
int count=0, start_count = 0;
int stop = 0, resplen;

ADCON1 = 0x06; //Set all Port A pins
TRISA = 0xff; //as digital inputs
TRISB = 1 | NOR_LATCH_PINS; //Set RB0 and NOR latch pins as input
TRISC = 0xe0;
TRISD = 0xf0;

/* Interrupt settings */
GIE = 1;
PEIE = 1;
INTE = 1; //Enable RB0 interrupt
INTEDG = 1;

delay_ms (200);

usartInit ();
usartPuts ("AT+CPMS=\"SM\"\n");