Basic of multiplexing
Today this is a digital era, every thing is digital in its figure. Here the most fascinating thing, which can be seen is; the displays. Now the displays are faster and less costly. Did you ever think of their reason to be so, as their functions are same as previous?
To know about how this all thing happened, we are required to know some basic of designing displays, first and foremost the multiplexing.
Now what does this multiplexing means? Multiplexing is the word, formed using the word multiplex or multiplexer. You must be recalling the definition of the multiplexer, what was that, that was a device which has ‘2n’ inputs and with ‘n’ selection lines, to select which line to be connected to the output line. Also the output was only one line. Now you must be getting the idea about what is a multiplexer in the practical form. Practically it is a device which converts the several inputs to the common output on the basis of the selection lines. Now suppose if we are switching the selection line rapidly in a very high switching speed, then we are getting the continuous signal comprising all the inputs. So this in trouble-free form means the multiple accessing. In multiplexing, we make possible for different sources to access the same components of circuit for a small time.
Multiplexing can be of different types based on the field which we are taking consideration on i.e. time (TDMA) or frequency (FDMA). Today another type of multiplexing has arrived, popularly known as CDMA (code division multiple accesses). CDMA is used mainly in the field of communication. But TDMA can also be used for other purposes also, like your displays and to cut down the expenditure of the channel, by transmitting the data in a serial manner, to lessen the number of data lines.
The basic
Now we should also know the types of the multiplexing that are usually employed in most of the electronics. There are basically two types of multiplexing methods i.e. time division multiple access and frequency division multiple access. But here we are concerned with the key type of multiplexing i.e. time division multiple accesses (TDMA) used in the displays. In time division format we are supplying the current to every component but for a shorter time so that our eyes can’t recognize the fluctuation.
To know about, what the TDMA is; we’ll consider the daily observation. There can be two ways of eating anything. For clarification let us assume that you and four of your friends are going for eating the gol gappa, in a stall at the side of the road.
Now in the first way, the stall man supplies you the first, then to your next friend, then next and so on. Till the time you ends your previous one, you are supplied with the one another and the same happens with your friends.
The second way is that, initially the stall man will only serve you and remains idle till you end, on ending only he supplies you the next. And all your friends are idle for the time you eat. The same happens with your friends and so on.
Now which form of serving did you liked? Surely the first one, as it saves the time and serves more. This is TDMA in basic.
But in technical language, in TDMA the time is divided among the destinations and the same channel is utilized with same band width. Here in displays we make use of the fact that our eyes can guess only 16 blinks per seconds, one above it will make us blind to predict.
Now coming to the main point, that how could we design a time divided multiplexed display. Here we are concerned in the designing of the seven segment displays, as it is used most in all the electronic gazettes. For that basic components required with there properties are stated further.
But before all we must know that what are the data lines and what are the address lines. The data lines are that set of wires, which are connected to the supply and the segment of the display in parallel. We pass a current through them, when we wish to make the selected segment glow. These are used for the selection of the segments, of the seven segment displays. And the address lines are connected to the control circuitry for selecting, which display should glow. So data lines select the segments and the address lines select the display, for glowing.
They both seem to be the same, but you’ll clearly differentiate between them after words.
As we all know that in a digital circuit nothing is performed without triggering (applying the pulse), so at first stage we have put the pulse generator, generating pulse at a frequency determined by the component values. A counter counts the pulses applied in a cyclic manner, giving the output in 3 or 4 bit. Now decoder further distributes the lines to 23 or 24, to get only a single line, having the current at a time. Now this current drives the display i.e. only a single display is ON at a single time (just for the mili or micro seconds, depending on the frequency of the pulses).
Now for the period a display is ON, data circuit scans, that if any of the seven segments is to be ON. If so, then it makes it glow, by passing the current through the data lines, in the similar fashion as done in the address lines.
So in this manner, the small voltage (usually 5V) can drive hundreds of the LED’s (segments of the display), without any disturbance. The circuit complexity can be largely reduced by using the microcontrollers. A microcontroller can be programmed to give the desired output as required.
Decoder
This, in theoretical form, is a combinational circuit having ‘n’ inputs and ‘2n’ outputs. But in practical form, it is a device which gives all the combinations of the input bits i.e. if the input bits are ‘3’ then the output will be ‘8’ combinations (as in IC 74HC138).
It is used here to select the segments, which we wish, should glow. We apply the address lines to the output of this, so that we can easily control the function by only changing small number of bits (to the input of the decoder). Like this for having the 16 seven segments display we are required to have only 4 controlling bits which reduces the number of wires required. Depending on the number of the displays it can be 3 inputs and 8 outputs or 4 inputs 16 outputs decoder. If the second one is not available then two 3 inputs 8 outputs can be arranged to make the 4 inputs 16 outputs. The demultiplexer IC can also be used as the decoder by applying ‘1’ to the single input and treating the selection lines as the inputs for the decoder.
Transistor
We know that a transistor (usually n-p-n, due to greater mobility of the electrons) is a versatile and the basic component in the electronics. Here we make the use of the property of the transistor that it can act as a switch, depending on the input voltage applied to the base of the transistor.
We use the transistor as a switch, in the common emitter configuration. As said earlier the input voltage controls the operation, as, if the input voltage is less than the cut-in voltage of the transistor, the base current is zero, hence the collector current. And the transistor is in OFF condition (open circuit). When the base current is increased so the collector current also increases and the condition arrives when the transistor is driven in saturation. This is the condition when the transistor is in ON state (close circuit).
Counter
As the name suggests that this is a combinational circuit that counts the number of pulses applied to it. As required, these can be designed in any modes (mode defines the number of states a counter goes). We usually prefer the ring counter, in which the first state is repeated after the last state. Depending on the bit designations it has the modes i.e. for the 4 bit counter the maximum states are 16. Decimal equivalent of its output defines the seven segment display to be lightened up. As required, it can be 3 bit (for eight displays) or 4 bit (for sixteen displays). By adding some extra components it can be designed to start the counting from in between.
Pulse generator
To number the addresses, we use a pulse source which will continuously supply the pulses at high frequency (normally in MHz). This frequency determines the speed of the circuit. This can be an astable multivibrator, made using discrete components or using the 555 timer, or any programmed timer. This supplies the pulses to the counter for counting.
Above is the basic schematic showing the arrangement of the components for a time divided displays.
To know about how this all thing happened, we are required to know some basic of designing displays, first and foremost the multiplexing.
Now what does this multiplexing means? Multiplexing is the word, formed using the word multiplex or multiplexer. You must be recalling the definition of the multiplexer, what was that, that was a device which has ‘2n’ inputs and with ‘n’ selection lines, to select which line to be connected to the output line. Also the output was only one line. Now you must be getting the idea about what is a multiplexer in the practical form. Practically it is a device which converts the several inputs to the common output on the basis of the selection lines. Now suppose if we are switching the selection line rapidly in a very high switching speed, then we are getting the continuous signal comprising all the inputs. So this in trouble-free form means the multiple accessing. In multiplexing, we make possible for different sources to access the same components of circuit for a small time.
Multiplexing can be of different types based on the field which we are taking consideration on i.e. time (TDMA) or frequency (FDMA). Today another type of multiplexing has arrived, popularly known as CDMA (code division multiple accesses). CDMA is used mainly in the field of communication. But TDMA can also be used for other purposes also, like your displays and to cut down the expenditure of the channel, by transmitting the data in a serial manner, to lessen the number of data lines.
The basic
Now we should also know the types of the multiplexing that are usually employed in most of the electronics. There are basically two types of multiplexing methods i.e. time division multiple access and frequency division multiple access. But here we are concerned with the key type of multiplexing i.e. time division multiple accesses (TDMA) used in the displays. In time division format we are supplying the current to every component but for a shorter time so that our eyes can’t recognize the fluctuation.
To know about, what the TDMA is; we’ll consider the daily observation. There can be two ways of eating anything. For clarification let us assume that you and four of your friends are going for eating the gol gappa, in a stall at the side of the road.
Now in the first way, the stall man supplies you the first, then to your next friend, then next and so on. Till the time you ends your previous one, you are supplied with the one another and the same happens with your friends.
The second way is that, initially the stall man will only serve you and remains idle till you end, on ending only he supplies you the next. And all your friends are idle for the time you eat. The same happens with your friends and so on.
Now which form of serving did you liked? Surely the first one, as it saves the time and serves more. This is TDMA in basic.
But in technical language, in TDMA the time is divided among the destinations and the same channel is utilized with same band width. Here in displays we make use of the fact that our eyes can guess only 16 blinks per seconds, one above it will make us blind to predict.
Now coming to the main point, that how could we design a time divided multiplexed display. Here we are concerned in the designing of the seven segment displays, as it is used most in all the electronic gazettes. For that basic components required with there properties are stated further.
But before all we must know that what are the data lines and what are the address lines. The data lines are that set of wires, which are connected to the supply and the segment of the display in parallel. We pass a current through them, when we wish to make the selected segment glow. These are used for the selection of the segments, of the seven segment displays. And the address lines are connected to the control circuitry for selecting, which display should glow. So data lines select the segments and the address lines select the display, for glowing.
They both seem to be the same, but you’ll clearly differentiate between them after words.
As we all know that in a digital circuit nothing is performed without triggering (applying the pulse), so at first stage we have put the pulse generator, generating pulse at a frequency determined by the component values. A counter counts the pulses applied in a cyclic manner, giving the output in 3 or 4 bit. Now decoder further distributes the lines to 23 or 24, to get only a single line, having the current at a time. Now this current drives the display i.e. only a single display is ON at a single time (just for the mili or micro seconds, depending on the frequency of the pulses).
Now for the period a display is ON, data circuit scans, that if any of the seven segments is to be ON. If so, then it makes it glow, by passing the current through the data lines, in the similar fashion as done in the address lines.
So in this manner, the small voltage (usually 5V) can drive hundreds of the LED’s (segments of the display), without any disturbance. The circuit complexity can be largely reduced by using the microcontrollers. A microcontroller can be programmed to give the desired output as required.
Decoder
This, in theoretical form, is a combinational circuit having ‘n’ inputs and ‘2n’ outputs. But in practical form, it is a device which gives all the combinations of the input bits i.e. if the input bits are ‘3’ then the output will be ‘8’ combinations (as in IC 74HC138).
It is used here to select the segments, which we wish, should glow. We apply the address lines to the output of this, so that we can easily control the function by only changing small number of bits (to the input of the decoder). Like this for having the 16 seven segments display we are required to have only 4 controlling bits which reduces the number of wires required. Depending on the number of the displays it can be 3 inputs and 8 outputs or 4 inputs 16 outputs decoder. If the second one is not available then two 3 inputs 8 outputs can be arranged to make the 4 inputs 16 outputs. The demultiplexer IC can also be used as the decoder by applying ‘1’ to the single input and treating the selection lines as the inputs for the decoder.
Transistor
We know that a transistor (usually n-p-n, due to greater mobility of the electrons) is a versatile and the basic component in the electronics. Here we make the use of the property of the transistor that it can act as a switch, depending on the input voltage applied to the base of the transistor.
We use the transistor as a switch, in the common emitter configuration. As said earlier the input voltage controls the operation, as, if the input voltage is less than the cut-in voltage of the transistor, the base current is zero, hence the collector current. And the transistor is in OFF condition (open circuit). When the base current is increased so the collector current also increases and the condition arrives when the transistor is driven in saturation. This is the condition when the transistor is in ON state (close circuit).
Counter
As the name suggests that this is a combinational circuit that counts the number of pulses applied to it. As required, these can be designed in any modes (mode defines the number of states a counter goes). We usually prefer the ring counter, in which the first state is repeated after the last state. Depending on the bit designations it has the modes i.e. for the 4 bit counter the maximum states are 16. Decimal equivalent of its output defines the seven segment display to be lightened up. As required, it can be 3 bit (for eight displays) or 4 bit (for sixteen displays). By adding some extra components it can be designed to start the counting from in between.
Pulse generator
To number the addresses, we use a pulse source which will continuously supply the pulses at high frequency (normally in MHz). This frequency determines the speed of the circuit. This can be an astable multivibrator, made using discrete components or using the 555 timer, or any programmed timer. This supplies the pulses to the counter for counting.
Above is the basic schematic showing the arrangement of the components for a time divided displays.
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