SCON Register in 8051

The serial control register is an 8-bit register and its address is 0X98.  it can be Bit and Byte Addressable. the serial control register is used to configure and control serial port.

R1 -

Set, to indicate a byte has been received by 8051 and placed in SBUF.

T1 -

Set, when 8051 finishes transfer of 8-bit character.

RB8 -

Receive bit 8. This is the 9th bit received in modes 2 .and 3. If  SM2=0, RB8 is the stop bit.

TB8 -

Transmit bit 8. This is the 9th bit transmitted in modes 2 and 3.

REN -

Receiver enable/disable bit. Set HIGH to receive data on the RxD pin.

SM2 -

Enables multiprocessor communication in modes 2 and 3. In mode 2 or 3, if SM2 is set to 1 then RI will not be activated if the received 9th data bit (RB8) is 0. In mode 1, if SM2=1 then RI will not be activated if a valid stop bit was not received. In mode 0, SM2 should be 0.

SM1 -

This is the serial mode selection bit as shown below.

SM0 -

This is the serial mode selection bit as shown below.

Dalton Atomic Theory

John Dalton was an English chemist, physicist, and meteorologist.

He is best known for introducing the Atomic theory into chemistry, and for his research into colour blindness, sometimes referred to as Daltonism in his honour. Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. Dalton based his theory on the law of conservation of mass and the law of constant composition.
Theory :-

• All the matter is made up of very tiny particles called atoms which looks like a hard solid ball.
• All the items of same element are identical in size mass and chemical properties.
• Chemical reactions only involved the rearrangement of atoms. atoms are not created or destroyed during chemical reactions.
• Atoms of one element are different from the atoms of all other elements.
• Compounds of different elements are formed when atoms of different elements combine in a fixed ratio.
• atoms cannot be divided and it neither be created nor be destroyed.

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PCON Register in 8051

The power control register is an 8-bit register and its address is 0X87.  it can be Byte Addressable. the power control register is used to control 8051 power modes.

IDL, idle mode bit -

If IDL bit is set then it will activate idle mode of 8051 to save power.

PD, power down bit -

If PD bit is set when it will power down 8051.

GF0 and GF1 -

These are the general purpose flag bits.(i.e. bit0 and bit1)

SMOD, serial mode bit -

This bit is used to determine the serial communication port baud rate with timer 1.

Baudrate = Oscillator frequency in Hz / N[256-TH1].

The value of N, in above equation, is determined based of SMOD. 

If SMOD = 0 then N = 384.

If SMOD = 1 then N = 192.

Double baud rate bit. If Timer 1 is used to generate baud rate and SMOD = 1, the baud rate is doubled when the serial port is used in modes 1, 2, or 3.

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PSW Register in 8051

Program status word is also known as Flag Register. it is an 8-bit register. its address is 0XD0. it can be Bit and Byte Addressable. it can contains 6 Flag bits i.e. CY (carry), AC (auxiliary carry), P (parity), and OV (overflow) and 2 user definable bits. The PSW.5 and PSW.1 bits are general purpose status flag bits.

CY, the carry flag -  

This flag is set whenever there is a carry out from the D7 bit. This flag bit is affected after an 8-bit addition or subtraction. It can also be set to 1 or 0 directly by an instruction such as "SETB C" and "CLR C" stands for "set bit carry" and "CLR C" for "clear carry".

AC, the auxiliary carry flag -    

If there is a carry from D3 to D4 during an ADD or SUB operation, this bit is set; otherwise, it is cleared. This flag is used by instructions that perform BCD (Binary Coded Decimal) arithmetic.

FO -

General purpose flag bit available for the user.

RS1 and RS0 -

These are the register bank select bits as shown below.

OV -

Overflow flag is set if there was an arithmetic overflow. For signed numbers, results greater than 127 or less than -128 will set OV flag. For unsigned numbers this can be ignored.

PSW.1 -

User definable flag

P, the parity flag -

The parity flag reflects the number of 1s in the A (accumulator) register only. If the A register contains an odd number of 1s, then P=1. Therefore, P=0 if A has an even number of 1s.

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Adder

An adder is a digital logic circuit in electronics that implements addition of numbers. In many computers and other types of processors, adders are used to calculate addresses, similar operations and table indices in the ALU and also in other parts of the processors. These can be built for many numerical representations like excess-3 or binary coded decimal. The adders are classified into two types -

1. Half adder

2. Full adder

Half adder

The half adder adds two binary digits called as augend and addend and produces two outputs as sum and carry. XOR is applied to both inputs to produce sum and AND gate is applied to both inputs to produce carry.

0+0 = 0
0+1 = 1
1+0 = 1
1+1 = 10

These are the least possible single-bit combinations. But the result for 1+1 is 10, the sum result must be re-written as a 2-bit output. Thus, the equations can be written as
0+0 = 00
0+1 = 01
1+0 = 01
1+1 = 10

The output ‘1’of ‘10’ is carry-out. ‘SUM’ is the normal output and ‘CARRY’ is the carry-out.

From above table it is cleared that  1-bit adder can be easily implemented with the help of the XOR Gate for the output ‘SUM’ and an AND Gate for the ‘Carry’. When we need to add, two 8-bit bytes together, we can be done with the help of a full-adder logic. The half-adder is useful when you want to  add one binary digit quantities. A way to develop a two-binary digit adders would be to make a truth table and reduce it.  When you want to make a three binary digit adder, do it again. When you decide to make a four digit adder, do it again. The circuits would be fast, but development time is slow.

The simplest expression uses the exclusive OR function as Sum=AÅB. An equivalent expression in terms of the basic AND, OR, and NOT is: SUM=A|.B+A.B’

Full adder

This adder is difficult to implement than a half-adder. The difference between a half-adder and a full-adder is that the full-adder has three inputs and two outputs, whereas half adder has only two inputs and two outputs. The first two inputs are A and B and the third input is an input carry as C-IN. When a full-adder logic is designed, you string eight of them together to create a byte-wide adder and cascade the carry bit from one adder to the next.

The output carry is designated as C-OUT and the normal output is designated as S.

With the truth-table, the full adder logic can be implemented. You can see that the output S is an XOR between the input A and the half-adder, SUM output with B and C-IN inputs. We take C-OUT will only be true if any of the two inputs out of the three are HIGH.

So, we can implement a full adder circuit with the help of two half adder circuits. At first, half adder will be used to add A and B to produce a partial Sum and a second half adder logic can be used to add C-IN to the Sum produced by the first half adder to get the final S output.

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Unijunction Transistor

Unijunction Transistor :

The Uni junction Transistor or UJT for short, is another solid state three terminal device that can be used in gate pulse, timing circuits and trigger generator applications to switch and control either thyristors and triac’s for AC power control type applications.
Like diodes, uni +junction transistors are constructed from separate P-type and N-type semiconductor materials forming a single (hence its name Uni-Junction) PN- junction within the main conducting N-type channel of the device.

Although the Unijunction Transistor has the name of a transistor, its switching characteristics are very different from those of a conventional bipolar or field effect transistor as it can not be used to amplify a signal but instead is used as a ON-OFF switching transistor. UJT’s have unidirectional conductivity and negative impedance characteristics acting more like a variable voltage divider during breakdown.

Like N-channel FET’s, the UJT consists of a single solid piece of N-type semiconductor material forming the main current carrying channel with its two outer connections marked as Base 2 ( B₂ ) and Base 1 ( B₁ ). The third connection, confusingly marked as the Emitter ( E ) is located along the channel. The emitter terminal is represented by an arrow pointing from the P-type emitter to the N-type base.

 The Emitter junction is positioned along the channel so that it is closer to terminal B₂ than B₁. An arrow is used in the UJT symbol which points towards the base indicating that the Emitter terminal is positive and the silicon bar is negative material. Below shows the symbol, construction, and equivalent circuit of the UJT.

 Symbol and Construction :

The symbol for the uni junction transistor (N type) looks as shown in beloe fig. it is quite similar to that of the junction field effect transistor (JFET), except that it has a bent arrow representing the Emitter( E ) input. While similar in respect of their ohmic channels.

  The N-type channel basically consists of two resistors R_B2 and R_B1 in series with an equivalent (ideal) diode, D representing the p-n junction connected to their center point. This Emitter p-n junction is fixed in position along the ohmic channel during manufacture.

 Resistance R_B1 is given between the Emitter, E and terminal B₁, while resistance R_B2 is given between the Emitter, E and terminal B₂. As the physical position of the p-n junction is closer to terminal B₂ than B₁ the resistive value of R_B2 will be less than R_B1.

 These two series resistances produce a voltage divider network between the two base terminals of the unijunction transistor and since this channel stretches from B₂ to B₁, when a voltage is applied across the device, the potential at any point along the channel will be in proportion to its position between terminals B₂ and B₁. The level of the voltage gradient therefore depends upon the amount of supply voltage.

When used in a circuit, terminal B₁ is connected to ground and the Emitter serves as the input to the device. Suppose a voltage V_BB is applied across the UJT between B₂ and B₁ so that B₂ is biased positive relative to B₁.

 Operation of a UJT : 

This transistor operation starts by making the emitter supply voltage to zero, and its emitter diode is reverse biased with the intrinsic stand-off voltage. If VB is the voltage of the emitter diode, then the total reverse bias voltage is VA + VB = Ƞ VBB + VB. For silicon VB = 0.7 V, If VE gets slowly increases to the point where VE = Ƞ VBB, then IE will be reduced to zero. Therefore, on each side of the diode, equal voltages results no current flow through it, neither in reverse bias nor in forward bias.

 When the emitter supply voltage is increased rapidly, then the diode becomes forward-biased and exceeds the total reverse bias voltage (Ƞ VBB + VB). This emitter voltage value VE is called the peak-point voltage and is denoted by VP. When VE = VP, emitter current IE flows through the RB1 to the ground, that is, B1. This is the minimum current required for triggering the UJT. This is called the peak-point emitter current and is denoted by IP. Ip is inversely proportional to the Inter-base voltage, VBB.

 Now when the emitter diode starts conducting, charge carriers are injected into the RB region of the bar. As the resistance of a semiconductor material depends upon doping, the resistance of RB decreases due to additional charge carriers.

 Then the voltage drop across RB also decreases, with the decrease in resistance because the emitter diode is heavily forward biased. This in turn results in larger forward current, and as a result charge carriers are injected and it will cause the reduction in the resistance of the RB region. Thus, the emitter current goes on increasing until the emitter power supply is in limited range.

 VA decreases with the increase in emitter current, and UJT have the negative resistance characteristic. The base 2 is used for applying external voltage VBB across it. The terminals E and B1 are the active terminals. UJT usually gets triggered by applying a positive pulse to the emitter, and it can be turned off by applying a negative trigger pulse.

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Uni-junction Transistor Relaxation Oscillator

Uni-junction Transistor Relaxation Oscillator :

When a voltage Vs (supply voltage) is firstly applied, the uni junction transistor is "OFF" and the capacitor C1 is fully discharged but begins to charge up exponentially through resistor R3. As the Emitter of the UJT is connected to the capacitor, when the Vc (charging voltage) across the capacitor becomes greater than the diode volt drop value, the p-n junction behaves as a normal diode and becomes forward biased triggering the UJT into conduction. The UJT is “Turn ON”. At this point the Emitter to B1 impedance collapses as the Emitter goes into a low impedance saturated state with the flow of Emitter current through R1 taking place.

As the ohmic value of resistor R1 is very low, the capacitor discharges rapidly through the UJT and a fast rising voltage pulse appears across R1. Also, because the capacitor discharges more quickly through the UJT than it does charging up through resistor R3, the discharging time is a lot less than the charging time as the capacitor discharges through the low resistance UJT.

When the voltage across the capacitor decreases below the holding point of the p-n junction ( V_OFF ), the UJT turns “OFF” and no current flows into the Emitter junction so once again the capacitor charges up through resistor R3 and this charging and discharging process is constantly repeated while there is a supply voltage, Vs applied.

UJT Oscillator Waveforms :

 The relaxation oscillator continually switches "ON" and "OFF" without any feedback. The frequency of operation of the oscillator is directly affected by the value of the charging resistance R3, in series with the capacitor C1 and the value of η. The output pulse shape generated from the Base1 (B1) terminal is that of a sawtooth waveform and to regulate the time period, you only have to change the ohmic value of resistance, R3 since it sets the RC time constant for charging the capacitor.

The time period, T of the sawtoothed waveform will be given as the charging time plus the discharging time of the capacitor. As the discharge time, τ₁ is generally very short in comparison to the larger RC charging time, τ₂ the time period of oscillation is more or less equivalent to T ≅ τ₂. The frequency of oscillation is therefore given by ƒ = 1/T

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