How Thyristors (SCR) Work

Basically an SCR ( Silicon Controlled Rectifier) which is also known by the name Thyristor works quite like a transistor. The device gets its name (SCR) due to its multi layered semiconductor internal structure which refers to the "silicon" word in the beginning of its name.

The second part of the name "Controlled" refers to gate terminal of the device, which is switched with an external signal in order to control the device's activation, and hence the word "Controlled".

And the term "Rectifier" signifies the rectification property of the SCR when its gate is triggered and power is allowed to flow across its anode to cathode terminals, this may be similar to the rectification with a rectifier diode.

The above explanation makes it clear how the device works like a "Silicon Controlled Rectifier".

Although an SCR rectifies like a diode, and imitates a transistor due to its triggering feature with an external signal, an SCR internal configuration consists of a four layer semiconductor arrangement (P-N-P-N) which are made up of 3 series PN junctions, unlike a diode which has a 2-layer (P-N) or a transistor which includes a three layer (P-N-P/N-P-N) semiconductor configuration.

You may refer to the following image for understanding the internal layout of the explained semiconductor junctions

Another SCR property that distinctly matches with a diode is it's unidirectional characteristics which allows current to flow only in one direction through it, and block from the other side while it's switched ON, having said that SCRs have another specialized nature which allows them to be operated as an open switch while in the switched OFF mode.

This two extreme switching modes in SCRs restricts these devices from amplifying signals and ths these cannot be used like transistors for amplfying a pulsating signal.

The silicon controlled rectifiers or the SCRs just like Triacs, Diacs, orUJT's which all have the property of  performing like rapidly switching solid state AC switches while regulating a given AC potential or current.

So for engineers and hobbyists these devices become an excellent solid state switch option when it comes to regulating AC switching devices such lamps, motors, dimmer switches with maximum efficiency.

An SCR is a 3 terminal semiconductor device which are assigned as Anode, Cathode and the Gate, which in turn are internally made with 3 P-N junctions, having the property to switch at a very high speed.

Thus the device can be switched at any desired rate and discretely set  ON/OFF periods, for implementing a particular average switch ON or switch OFF time to a load.

Technically, the layout of an SCR or a thyristor can be understood by comparing it to a couple of transistors (BJT) connected in the back to back order, so as to form like a complementing regenerative pair of switches, as shown in the following image:

A Thyristors Two Transistor Analogy

 

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR2 feeds directly into the base of the PNP transistor TR1, while the collector current of TR1 feeds into the base of TR2.

These two inter-connected transistors rely upon each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current. So until one of the transistors is given some base current nothing can happen even if an Anode-to-Cathode voltage is present.

Simulating the SCR topology with a two transistor integration  reveals the formation to be in a manner such that the collector current of the NPN transistor is supplying straight to the base of the PNP transistor TR1, while the collector current of TR1 is connecting the supply with the base of TR2.

The simulated two transistor configuration seem to interlock and compliment each others conduction by receiving the base drive from the collector emitter current of the other, this makes the gate voltage very crucial and ensures that the shown configuration can never conduct until a gate potential is applied, even in the presence of the anode to cathode potential may be persistent.

 In a situation when the device's anode lead is more negative than its cathode, allows the N-P junction to remain forward biased, but ensuring the outer P-N junctions to be reverse biased such that it acts like a standard rectifier diode. This property of an SCR enables it to block a reverse current flow, until a significantly high magnitude of voltage which may be beyond its beak down specs is inflicted across the mentioned leads, which forces the SCR to conduct even in the absence of a gate drive.

 The above refers to a critical characteristics of thyristors which can causes the device to get triggered undesirably through a reverse high voltage spike and/or an high temperature, or a rapidly increasingly dv/dt voltage transient.

 Now suppose in a situation where the Anode terminal experiences more positive with regards to its cathode lead, this helps the outer P-N junction to become forward biased, although the central N-P junction continues to remain reverse biased. This consequently ensures that the forward current is also blocked.

Therefore in case a positive signal induced across the base of the NPN transistor TR2 results the passage of the collector current towards the base f TR1, which in trun forces the collector current to pass towards the PNP transistor TR1 boosting the base drive of TR2 and the process gets reinforced.

 The above condition allows the two transistors to enhance their conduction until the point of saturation owing to their shown regenerative configuration feedback loop which keeps the situation interlocked and latched.

Thus as soon as the SCR is triggered, it allows a current to flow from its anode to cathode with only a minimal forward resistance of around coming in the path, ensuring an efficient conduction and operation of the device..

When subjected to an AC, the SCR may block both the cyles of the AC until the SCR is offered with a triggering voltage across its gate and cathode, which instantly allows the positive half cycle of the AC to pass across the anode cathode leads, and the device begins imitating a standard rectifier diode, but only as long as the gate trigger stays switched ON, the conduction breaks the moment gate trigger is removed.

The enforced voltage-current or I-V characteristics curves for the activation of a silicon controlled rectifier may be witnessed in the following image:

Thyristor I-V Characteristics Curves

However For a DC input, as soon as the thyristor is triggered ON, due to the explained regenerative conduction it undergoes a latching action such that the anode to cathode conduction holds on and keeps conducting even if the gate trigger is removed.

Thus for a DC power the gate completely loses its influence once the first triggering pulse is applied across the gate of the device ensuring a latched current from its anode to cathode. It may be broken by momentarily breaking the anode/cathode current source while the gate is completely inactive.

 SCR are not designed to be perfectly analogue like the transistor counterparts, and therefore cannot be made to conduct at some intermediate active region for a load which may be somewhere between complete conduction and compete switch OFF.

This is also true because the gate trigger has no influence on how much the anode to cathode can be made to conduct or saturate, thus even a small momentary gate pulse is enough to swing the the anode to cathode conduction into a full switch ON.

The above feature enable an SCR to be compared and considered like a Bistable Latch possessing the two stable states, either a complete ON or  a complete OFF. This is caused due to the two special characteristics of the SCR in response to an AC or a DC inputs as explained in the above sections. 

 How to Use the Gate of an SCR to Control its Switching

 As discussed previously, once an SCR is triggered with a DC input and its anode cathode is self latched, this may be unlocked or switched OFF either by momentarily removing the anode supply source (anode current Ia) fully, or by reducing the same to some significantly low level below the specified holding current of the device or the "minimum holding current" Ih.

This implies that thr Anode to Cathode minimum holding current should be reduced until the thyristors internal P-N latching bond is able to restore its natural blocking feature into action.

Therefore this also means that in order to make an SCR conduct with a gate trigger it's imperative that the anode to cathode load current is over the specified  "minimum holding current" Ih, otherwise the SCR might fail to implement the load conduction, therefore if IL is the load current, this must be as IL > IH.

However as already discussed in the previous sections, when an AC is used across the SCR Anode.Cathode pins, ensures that the SCR is not allowed to execute the latching effect when the gate drive is removed.

This is because the AC signal switches ON and OFF within its zero crossing line which keeps the SCR anode to cathode current to turn off at every 180 degrees shift of the positive half cycle of the AC waveform.

This phenomenon is termed as "natural commutation" and imposes an crucial feature to an SCR conduction. Contrary to this with DC supplies this feature becomes immaterial with SCRs.

But since an SCR is designed to behave like a rectifier diode it responds effectively only to the positive half cycles of an AC and remains reversed biased and entirely unresponsive to the other half cycle of the AC even in the presence of a gate signal. Tis implies that in the presence of a gate trigger, the SCR conducts across its anode to cathode only for the respective positive AC half cycles and stays muted for the other half cycles.

Due to the above explained latching feature and also the cut-of during the other half cycle of an AC waveform, the SCR can be effectively used for chopping phase AC cycles such that the load can be switched at any desired (adjustable) lower power level.

Also known as phase control, this feature can be implemented through an external timed signal applied across the gate of the SCR. This signal decides after how much delay the SCR may be fired once the AC phase has commenced its positive half cycle.

So this allows only that portion of the AC wave to be switched which is being passed after the  gate trigger..this phase control is among the main features of a silicon controlled thyristor.

A Thyristor phase control may be understood by looking at the images below.

The first diagram shows an SCR whose gate is permanently triggered, as may be seen in the first diagram this allows the complete positive waveform to get initiated from start to finish, that from across the central zero crossing line.

Thyristor Phase Control

At the outset of each positive half-cycle the SCR is “OFF”. On the induction of the gate voltage activates the SCR into conduction and allows it to be entirely latched “ON” throughout the positive half cycle. When the thyristor is switched on at the start of the half-cycle (θ = 0o), the connected load (a lamp or any similar) would be “ON” for the whole positive cycle of the AC waveform (half-wave rectified AC) at an elevated average voltage of 0.318 x Vp.

As the initialization of the gate switch ON is raised along the half cycle (θ = 0o to 90o), the connected lamp is lighted for a smaller amount time and the net voltage brought to the lamp likewise proportionally less dropping its intensity.

Subsequently it is easy to exploit a silicon controlled rectifier as an AC light dimmer and in many different additional AC power applications for example: AC motor-speed control, heat control devices and power regulator circuits, and so on.

Up till now we have witnessed that a thyristor is fundamentally a half-wave device that is able to pass current in only the positive half of the cycle whenever the Anode is positive and prevents current flow just like a diode in cases where the Anode is negative, even if the gate current remains active.

Nevertheless you may find many more variants of similar semiconductor products to choose from which originate under the title of “Thyristor” designed to operate in both directions of the half cycles, full-wave units, or could be switched “OFF” by the Gate signal.

This kind of products incorporate “Gate Turn-OFF Thyristors” (GTO), “Static Induction Thyristors” (SITH), “MOS Controlled Thyristors” (MCT), “Silicon Controlled Switch” (SCS), “Triode Thyristors” (TRIAC) and “Light Triggered Thyristors” (LASCR) to identify a few, with so many of these devices accessible in many different voltage and current ratings which makes them interesting to be used in purposes at very high power levels.

Thyristor Overview

Silicon Controlled Rectifiers known generally as Thyristors are three-junction PNPN semiconductor devices that could be considered two inter-connected transistors which you can use in the switching of mains operated heavy electrical loads. They are characterized to be latched-“ON” by a single pulse of positive current applied to their Gate lead and can keep on being “ON” endlessly until the Anode to Cathode current is reduced below their specified minimum latching measure or reversed.

Static Attributes of a Thyristor

Thyristors are semiconductor equipment configured to function only in the switching function.

Thyristor are current controlled products, a tiny Gate current is able to control a more substantial Anode current.

Enables current only once forward biased and triggering current applied to the Gate.

The thyristor operates similar to a rectifyier diode whenever it happens to be activated “ON”.

Anode current has to be more than sustaining current value to preserve conduction.

Inhibits current passage in case reverse biased, irrespective of whether or not Gate current is put on.

As soon as turned “ON”, gets latched “ON” performing regardless if a gate current being applied but only in case the Anode current is above latching current.

Thyristors are fast switches which you can use to substitute electromechanical relays in a number of circuits as they simply do not have any vibrating parts, no contact arcing or have problems with deterioration or grime. But additionally to simply switching substantial currents “ON” and “OFF”, thyristors can be accomplished to manage the RMS value of an AC load current without dissipating a considerable amount of power. An excellent example of thyristor power control is in the control of electrical lighting, heaters and motor speed.

In the next tutorial we will look at some basic Thyristor Circuits and applications using both AC and DC supplies.

Automatic Bathroom/Toilet Engage indicator

The post explains a very simple automatic bathroom/Toilet Engage indicator circuit which can be installed with any relevant door bolt for implementing the actions. The idea was created and submitted by Mr. Sandipan.

The Design

Hi Swagatam,

How are doing ? I'm doing good.

I have one more idea that need your help. I would like to implement is at my home ( just a simple useful fun ).

Here is my idea :

Automatic Bathroom/Toilet Engage indicator :

Requirements :

1. If a bathroom/toilet is engaged/ locked from inside ( in use ), a LED indicator ( Placed outside) should glow.

2. If the bathroom/toilet is locked from outside ( Not in use ), the indicator should not glow.

3. The bathroom/Toilet can be locked from inside using one Sliding door bolt or simple door bolt or both.

Based on the above requirement, I design a very simple Circuit using one LED, 2 AA battery and use door locks as switch. Here is the circuit diagram

So, if any bolt or both bolt are used from inside, it complete the circuit and LED will glow.

The problem is, a lot of wiring is visible in back side of the door. I have tested this circuit using breadboard but not with Bathroom/Toilet door :( .

Could you please suggest any other sensor based idea/circuit ?  

Thanks

Sandipan

How to Generate Sinewave PWM

Acquiring a sinewave output from an inverter can be the most crucial and the most advantageous feature for rendering maximum efficiency to the unit, in terms of its output quality.

However achieving a sinusoidal sine wave output could be quite complex and may not be recommended for inverters, because electronic devices normally do not "like" exponentially rising currents or voltages. Since inverters are essentially made by using solid state electronic devices, a sinusoidal waveform is normally avoided. Electronic power devices when forced to operate with sinusoidal waves produce inefficient results since the devices tend to get relatively more hot compared to when operated with square wave pulses.

So the next best option for implementing a sine wave from an inverter is by the way of PWM, which stands for Pulse width modulation.

PWM is an advanced way (digital variant) of putting forth an exponential waveform through a proportionately varying square pulse widths whose net value is calculated to exactly match the net value of a selected exponential waveform, here "net" value refers to the RMS value. Therefore a perfectly calculated PWM with reference to a given sine wave can be used as a perfect equivalent for replicating the given sinewave.  

Furthermore, PWMs become ideally compatible with electronic power devices (mosfets, BJTs, IGBTS) and allow these to run with minimal heat dissipation.

However generating PWMs waveforms is normally considered complex, and that's because the implementation is not easy to simulate in ones mind.

Even I had to go through some brainstorming before I could correctly simulate the function through some intense thinking and imagining.

The best, and the conventional method of generating a sinewaver PWM is by feeding a couple of exponentially varying signals to the input of an opamp for the required processing. Among the two input signals one needs to be much higher in its frequency compared to the other.

The IC 555 can also be used effectively for generating sine equivalent PWMs, by incorporating its built-in opamps and an R/C triangle ramp generator circuit.

The following discussion will help you to understand the entire procedure.

New hobbyists and even the professionals will now find it quite easy to understand regarding how sine wave PWMs are implemented by processing a couple of signals by using an opamp, let's figure it out with the help of the following diagram, and simulation.

As mentioned in the previous section, the procedure involves the feeding of two exponentially varying waveforms to the inputs of an opamp.

Here the opamp is configured as a typical comparator, so we can assume that the opamp will instantly start comparing the instantaneous voltage levels of these two superimposed waveforms the moment these appear or are applied to its inputs.

In order to enable the opamp to implement the required sine wave PWMs correctly at its output, it's imperative that one of the signals has a much higher frequency than the other. The slower frequency here is the one which is supposed to be the sample sine wave which needs to be imitated (replicated) by the PWMs.

Ideally, both the signals should be sinewaves (one with a higher frequency than the other), however the same can be also implemented by incorporating a triangle wave (high frequency) and a sine wave (sample wave with low frequency).

As can be seen in the following images, the high frequency signal is invariably applied to the inverting input (-) of the opamp, while the other slower sinewave is applied to the non-inverting (+) input of the opamp.

In a worst case scenario, both the signals can be triangle waves with the recommended frequency levels as discussed above. Still that would help you to achieve a reasonably good sinewave equivalent PWM.

The signal with the higher frequency is termed as the carrier signal, while the slower sample signal is called the modulating input.

 Referring to the figure above, we can clearly visualize through plotted points the various coinciding or overlapping voltage points of the two signals over a given time span. The horizontal axis signifies the time period of the waveform, while the vertical axis indicates the voltage levels of the two simultaneously running, superimposed waveform.

The figure informs us regarding how the opamp would respond to the shown coinciding instantaneous voltage levels of the two waveforms and produce a correspondingly varying PWM at its output.

The procedure is actually not so difficult to imagine. The opamp simply compares the fast triangle wave's varying instantaneous voltage levels with the relatively much slower sinewave (this can also be a triangle wave), and checks the instances during which the triangle waveform voltage may be lower than the sine wave voltage and responds by instantly creating high logic at its outputs, which is sustained as long as the triangle wave potential continues to be below the sine wave potential, and the moment the sine wave potential is detected to be lower than the instantaneous triangle wave potential, the outputs reverts with a low and sustains until the situation reverts.

This continuous comparison of the instantaneous potential levels of the two superimposed waveforms over the two inputs of the opamps results in the creating of the correspondingly varying PWMs which may be exactly the replication of the sine waveform applied on the non-inverting input of the opamp.

The following image shows the slo-mo simulation of the above operation:

Here we can witness the above explanation being implemented practically, and this is quite how the opamp would be executing the same (although at a much fater rate, in ms).

The upper figure shows a slightly more accurate sine wave PWM depiction than the second scrolling diagram, this is because in the first figure I had the comfort of the graph layout in the background whereas in the second simulated diagram I had to plot the same without the help of the graph coordinates, therefore I might have missed a few of the coinciding points and therefore the outputs looks a little inaccurate compared to the first one.

Nevertheless, the operation is quite evident and distinctly brings out how an opamp is supposed to process a PWM sine wave by comparing two simultaneously varying signals at its inputs as explained in the previous sections.

Actually an opamo would process it much more accurately than the above shown simulation, may be  a 100 times better, producing a extremely uniform and well dimensioned PWMs corresponding to the fed sample. sinewave.

Sine Wave Inverter Circuit using Bubba Oscillator

In this post we learn how to make a simple sine wave inverter using bubba oscillator sine wave generator. The idea ws requested by Mr. Ritwik Naudiyal.

The Request 

Hello Sir!! I am a 4th year B.Tech Student Electrical Eng.

We are trying to make pure wave sine wave inverter using PWM and bubba oscillator for our Final project.also along with it a battery charging and auto cut off circuit would be needed

We want the inverter to work for day to day purposes.We would be grateful to you if u can give a working circuit fr this.

thnk u! 

 The Design

The proposed sine wave inverter using bubba oscillator may be understood with the help of the following points:

The stage comprising two 555 ICs are configured as PWM generators where IC1 forms a square pulse generator for the PWMs while IC2 forms the monostable PWM generator with respect to the modulation input applied at its pin5.

The sine wave modulation input at pin5 of IC2 is ahieved with the help of a bubba oscillator created by using four opamps from the IC LM324.

The generated sine wave pulses are fixed at precise 50 Hz and fed to pin5 of IC2 via a BJT common collector for further processing.

The 50 Hz for the bubba oscillator is set by selecting R precisely with the help of the following formula:

f = 1/2(3.14)RC 

IC2 compares the sine wave modulations at its pin5 with the square pulses at its pin2 and generates an equivalent PWM waveform at its pin3.

The flip flop stage reqired for switching the power stage is configured through a single IC 4017 whose outputs are appropriately integarted with the two high gain high current power BJT stage formed by Darlington TIP122 and TIP35.

The pin14 of the 4017 is clocked at around 200 Hz via pin3 of IC1 in order to achieve a 50 HZ switching across the power transistors.

The PWM modulation of the above 50 Hz switching is implemented with the help of the two 1N4148 diodes connected across the bases of the tIP122 and are switched in accordance with the PWM from pin3 of IC1

Assumed waveforms of the PWMs may be referred in the following image:

What's IGBT - How it Works

The insulated-gate bipolar transistor (IGTB) device is a type of three-terminal power semiconductor device which are basically used as electronic switch and is also known for providing a combination of  extremely fast switching and high efficiency in the more newer devices. 

A range of modern appliances such as VFDs (Vaiable Frequency Drives), VSFs (variable speed refrigerators), trains, stereo systems with switching amplifiers, electric cars, and air conditioners use insulated-gate bipolar transistor for switching the electric power. 

Symbol of depletion mode IGBT

In case the amplifiers use insulated-gate bipolar transistor often synthesize waveforms which are complex in nature along with low-pass filters and pulse width modulation; as insulated-gate bipolar transistor are basically design to turn on and off on a fast and rapid pace. 

The pulse repetition rates are boasted by the modern devices which consist of switching application and fall well within the ultrasonic range which are the frequencies which are ten times higher than the highest audio frequency handled by the device when the devices is used in the form of an analog audio amplifier. 

The MOSFETs consisting of high current and characteristics of a simple gate-drive is combined with the bipolar transistors which has low-saturation-voltage capacity by the IGTB. 

A single device is made by IGBT by combining the bipolar power transistor which acts as a switch and an isolated gate FET which acts as the control input. 

The insulated-gate bipolar transistor (IGTB) is majorly used in applications which consists of multiple devices which are placed in parallel to each other and most of the times have capacity of handling very high current which are in the range of hundreds of amperes along with a 6000V of blocking voltage, which in turn is equal to hundreds of kilowatts.use medium to high power such as induction heating, switched-mode power supplies, and traction motor control. Insulated-gate bipolar transistors which are large in size.

Insulated-gate bipolar transistor (IGTB) is a new and recent invention of the time. The first-generation devices which were invented and launched in 1980s and the early years of 1990s were found to have slow switching process relatively and are prone to failure through different modes such as latchup (where the device will continue to be switched on and not turn off till the current keeps on flowing through the device) and secondary breakdown (where when high current flows through the device, a localized hotspot present in the device goes into thermal runaway and as a result burns the device). 

There was a lot of improvement observed in the second-generation devices and the most new devices on the block, the third-generation devices are considered even better than the first tow generation devices. 

The third-generation devices consist of MOSFETs with speed rivaling, and tolerance and ruggedness of excellent level. The devices of second and third generation consists of pulse rating which are extremely high which make them very useful in order to generate large power pulses in various areas such as plasma physics and particle. Thus the second and third generation devices have superseded mostly all the older devices such as triggered spark gaps and thyratrons used in these areas of plasma physics and particle.

These devices also hold attraction for the hobbyist of high-voltage due to their properties of high pulse ratings and availability in the market at low prices. This enables the hobbyist to control huge amounts of power in order to drive devices such as coilgums and Tesla coils.

Insulated-gate bipolar transistors are available at affordable price range and thus act as an important enabler for hybrid cars and electric vehicles.

By Surbhi Prakash

How to Write Codes: PIC Tutorial 11

We dealt with a lot of basics within the last tutorial, therefore I feel the time has come that we composed

our first program.

The program we will write would count the quantity of occasions we turn a switch on, and then exhibit the number. The program would count from 0 to 9, viewable on 4 LEDs in binary form, along with the input or interrupt will likely be on RB0. The number one thing we must conduct is inform the PIC to leap over the address in which the Program Counter points to whenever an interrupt takes place. You will observe that We are employing an unique method of exhibiting hexadecimal numbers. Before I happened apply F9h in which h indicated hexadecimal. We could write this as 0xF9, which is the structure we are going to employ from now on.

Now we need to tell the PIC that we are going to use interrupts, and we are using RB0 pin 6 as an interrupt pin:

bsf INTCON,7;GIE – Global interrupt enable (1=enable)

bsf INTCON,4;INTE - RB0 interrupt enable (1=enable)

I am going to clear the interrupt flag just in case (I never trust anything!)

bcf INTCON,1;INTF - Clear flag bit just in case

Currently we must establish our 2 ports. Keep in mind that as we are now utilizing RB0 as an interrupt pin, this needs to be established as an input:

We are going to use a variable called COUNT to store the number of switch counts. We could just simply increment the value on Port A, but you will see why I am using a variable when we write our interrupt routine.

Therefore, our principal program is composed, and at this point we must inform the PIC how to proceed whenever an interrupt takes place. Within this example, our interrupt will probably be the switch. Just what we would like the PIC to is one to the adjustable COUNT every time the switch is confined. Nevertheless, we just wish to show the how many occasions the switch shuts from 0 to 9. Above, I stated we might be able to have simply incremented the value on Port A every time there is an interrupt. However, Port A has 5 bits, in case we simply incremented the port, we are going to possess the highest count of 31. There are a couple of explanations why I selected not to move up to 31.

Initially, we will employ a 7-segment screen, which could at the most only go from 0 to 15 (0 to F in hex). Next, I additionally wish to show you a few of the arithmetic commands which you stumbled on in the past few lessons. Therefore we will continue with our interrupt routine. Currently the firstly we must accomplish is briefly store the details of our w register, since we have been applying this to shift the contents of COUNT to PORTA. In case we don’t save it, in that case we might be able to deliver a totally different number because of our arithmetic. Therefore let’s accomplish that first:

At this point we we understand if the value of COUNT is 9 or more. Just what we need to accomplish now is if COUNT is more than 9, place it back to 0, or else return to the main program to ensure that we are able to deliver it to Port A. The BTFSS command since you understand would the subsequent

instruction in case the carry flag is scheduled i.e COUNT = 10:

The only thing which is remaining to do now is enter collectively as well as determine values to our constants, which we are able to perform right at the start of our program.

Each time you activate the switch on, the LEDs are going to count up in binary from 0000 to 1010

then back to 0000.

The following figure shows the circuit diagram compatible with the above explained code. Interestingly you will find that the timing capacitor has been included in the design. This is nice little ploy through which you get the freedom of avoiding the inclusion of the capacitor in case you don't have any with you during that time.

Here the capacitance comes into play via the stray capacitance across the oscillator pin and ground.

Of course it might not seem to be a very intelligent way of avoiding a capacitor practically since the stray value might vary with different given conditions.

Another section which can be witnessed in the circuit is the denouncing network across the switch. This prevents interference while mechanical switching and prevents the PIC from getting confused if the switching was a single toggle or multiple toggles.

1. 

Understanding Interrupts: PIC Tutorial 10

The topic of interrupts may well gonna be the lengthiest and toughest to go thru. You cannot find any uncomplicated method of detailing interrupts, however with a little luck towards the end of this part you may be able to apply interrupts into your own programs.

We have separated the section into 2 stages. That is to enable separate the topic into sections, also to provide you with an handy plit up for easy understanding. What exactly is an interrupt? Surely, as the term indicates, an interrupt is a technique or a signal that prevents a microprocessor/microcontroller from whatever thing its performing that something different could happen. Allow me to give you an daily illustration. Think you are relaxing in your own home, conversing to another person. All of a sudden the phone sounds. You quit talking, and grab the telephone to talk to the caller. Once you have your telephone interaction, you decide to return to conversing to the individual before the telephone rang. It is possible to consider the principal routine while you chatt to somebody, the phone ringing brings about to disrupt your conversing, and the break in routine is the method of speaking on the phone. While the phone discussion comes to an end, then you go back to your primary routine of chatting. This illustration is precisely how an interrupt a processor to take action. The primary program is operating, carrying out certain function in a circuit, however when an interruption takes place the primary program halts while a different routine is performed. routine ends, the processor moves back to the primary routine just as before. The PIC possesses 4 sources of interrupt. They could be broken down into a couple of groups. Two are sources of interrupts which can be utilised outwardly to the PIC, whilst the other two are inner processes. Let me clarify the two external types here. The other two is going to be described in different tutorials once we arrive at timers and storing data. Should you check out the pin-out of the PIC, you will notice that pin 6 it is RB0/INT. At this point, RB0 is clearly Port B bit 0. The INT represents that it could as well be configures as an outside interrupt pin. Furthermore, Port B pins 4 to 7 (pins 10 to 13) may also be utilized for interrupts. Before we are able to employ the INT or another Port B pins, we must accomplish two tasks. Very first we must inform the PIC that we will utilize interrupts. Next, we must designate which port B pin we are going to be utilising as an interrupt rather than as an I/O pin. Inside the PIC you can find a register known as INTCON, and is at address 0Bh. In this register you will discover 8 bits that may be enabled or disabled. Bit 7 of INTCON is known as GIE. This is the Global Interrngupt Enable. Fixing this to 1 informs the PIC that we will employ an interrupt. Bit 4 of INTCON is known as INTE, INTerrupt Enable. Putting this bit to 1 conveys to the PIC that RB0 is going to be an interrupt pin. Configuring bit 3, called RBIE, informs the PIc that we are going to be utilizing Port B bits 4 to 7. At this point the PIC understands when this pin can be high or low, have to halt what it’s performing and proceed with an interrupt routine. At this point, we must inform the PIC whether or not the interrupt will likely be on the ascending edge (0V to +5V) or the dropping edge (+5V to 0V) transformation of the signal. Put simply, do we wish the PIC to interrupt each time the signal moves from low to high, or from high to low. By delinquency, this can be established to be placed on the rising edge. The edge ‘triggering’ is scheduled up in an additional register called the OPTION register, at address 81h. The bit we are enthusiastic about is bit 6, often referred to as INTEDG. Setting this to 1 triggers the PIC to disrupt on the mounting edge (default state) and setting it to 0 stimulates the PIC to disrupt on the sliding edge. If you would like the PIC to activate on the rising edge, then you certainly don’t have to do anything to this bit. At this point, sadly, the Option register is in Bank 1, meaning that we enjoy to modify from bank 0 to bank 1, set the bit in the Option register, after that return to bank 0. The key here is to accomplish every bit of the Bank 1 registers in a single strike, for example establishing the port pins, after that returning to Bank 0 if you are done. Fine, consequently we have notified the PIC which pin will probably be the interrupt, and where edge to trigger, what goes on in the program and the PIC any time the interrupt happens? A couple of stuff take place. Very first, a ‘flag’ is scheduled. This informs the internal processor of the PIC that an interrupt has transpired. Next, the program counter (which I talked about within the previous tutorial) tips to a specific address within the PIC. Let’s swiftly check out all these individually. Interrupt Flag In our INTCON register, bit 1 is the interrupt flag, called INTF. At this point, whenever any interrupt arises, this flag will likely be fixed to 1. When there isn’t an interrupt, the flag is placed to 0. As well as that is just about all accomplishes. At this point you may be pondering ‘what is the point?’ Surely, even though this flag is scheduled to 1, the PIC is not able to, and will not, react to another interrupt. Therefore, let’s express that we bring about an interrupt. The flag will likely be fixed to 1, and the PIC might go to our routine for working the interrupt. When this flag wasn’t fixed to 1, and the PIC was permitted to continue answering the interrupt, then continuously pulsing the pin could keep the PIC returning to the beginning of our interrupt routine, and by no means completing it. Returning to my illustration of the telephone, it’s similar to lifting the telephone, and immediately once proceeding to discuss it begins ringing yet again since another person wishes to speak with you. It is advisable to complete one dialogue, then grab the phone again to speak with the subsequent individual. You can find a small problem with this flag. Even though the PIC quickly sets this flag to 1, it doesn’t set it again 0! That activity must be exercised by the programmer – i.e. you. This can be effortlessly accomplished, since I’m certain presume, and needs to be achieved after the PIC has carried out the interrupt routine.

Memory Location

Whenever you initially power up the PIC, or in case there exists a reset, the Program Counter tips to address 0000h, that could be immedaitely at the outset of the program memory. But, in the event there is an interrupt, the Program Counter would indicate address 0004h. Therefore, while we are composing our program that will have interrupts, we firstly must inform the PIC to hop over address 0004h, and maintain the interrupt routine which begins at address 0004h discrete from the remainder of the program. This can be hassle-free to perform. Initially, we commence our program with a command known as ORG. This command indicates Origin, or start. We stick to it with an address. Since the PIC commences at address 0000h, we type ORG 0000h. After that we must bypass over address 0004h. We accomplish this by putting a GOTO instruction, accompanied by a label which tips to our primary program. We after that adhere to this GOTO command with one more ORG, this moment with the address 0004h. It will be after this command that we insert our interrupt routine. At this point, we might be able to possibly type in our interrupt routine straight following the second ORG command, or we are able to position a GOTO statement which points to the interrupt routine. It truly is related to option on your part. To inform the PIC it offers arrived at the conclusion of the interrupt routine we must position the command RTFIE towards the end of the routine. This command signifies return from the interrupt routine. While the PIC notices this, the Program Counter indicates to the final position the PIC was at before the interrupt occurred. We have established below a brief section of code to display the above:

There are a couple of stuffs you should be informed when utilizing interrupts. The initial tends to be that if you might be utilizing the identical register in your primary program and the interrupt routine, in mind that the details of the register will most likely alter when the interrupt takes place. For instance, let’s utilizing the w register to forward data to Port A primary program, therefore you can be additionally utilizing the w register in the interrupt routine to shift data from one destination to another. In case you are not cautious, the w register would include the last value it received while it had been in the interrupt routine, so when you return from the interrupt this information is going to be delivered to Port A rather than the value you possessed before the interrupt occurred.

The means around this is to momentarily save the details of the w register prior to when you utilize it once again in the interrupt routine. The second is the fact you can find a delay between when one interrupt takes place and when the subsequent one can arise. While you understand, the PIC possesses an exterior clock, which could possibly be a crystal or it could be a resistor-capacitor combo. No matter what the frequency of this clock, the PIC divides it by 4 after which employs this for it’s inner timing. For instance in case you have a 4MHz crystal linked to your PIC, in that case the PIC would perform the instructions at 1MHz. This interior timing is known as an Instruction Cycle. At this point, the data sheet claims (undoubtedly in diminutive print) that you need to enable 3 to 4 instruction rounds between interrupts. My would be to enable 4 rounds. The reason behind the delay is the PIC requires time to leap to the interrupt address, the flag, and arrive back away from the interrupt routine. Therefore, keep this in your mind if you work with an alternate circuit to activate an interrupt for the PIC. At this point, a point to is the fact that if you utilize bits 4 to 7 of Port B as an interrupt. You are unable to choose specific pins on Port B to function as an interrupt. Therefore, in case you allow these pins, they likely could be all obtainable. Therefore, for instance, you can’t simply have bits 4 and 5 – bits 6 and 7 will likely be empowered at the same time. What exactly is the purpose of getting four bits to represent an interrupt? Surely, you might have a circuit hooked up to the PIC, in case anyone of four lines go high, in that case this may be an issue that you require the PIC to influence instantly. One illustration of this could be a home security alarm, in which four sensors are linked to Port B pins 4 to 7. Any specific sensor can prompt the PIC to trigger an alarm, and the alarm signaling routine is the interrupt routine. This spares checking the ports constantly and permits the PIC to continue with different matters. Within the next tutorial, we are going to compose a program to manage an interrupt.