【Hardware Design】Hardware Study Notes 1--Introduction and Selection of Components

Written in the front: This note comes from Wang Gong’s hardware engineer training course. Students who want to learn hardware can go to Tencent Classroom to search directly. I put the course link at the end of the article. The following is my summary of knowledge points, hardware learning It is still recommended that you read more component manuals and do more hands-on operations.

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1. Resistance

1.1 Classification of resistors

There are three basic types of commonly used chip resistors: metal film resistors, thin film chip resistors, and thick film chip resistors. These three resistors look similar on the surface and may have similar purchase specifications.

• Differences in manufacturing process: Thick film resistors generally use screen printing technology, while thin film resistors use vacuum evaporation, magnetron sputtering and other processes. The difference in material and process between thick film resistors and thin film resistors directly leads to the difference in performance of the two resistors.
• Thick film resistors generally have poor precision, 10%, 5%, and 1% are common precisions, while thin film resistors can achieve 0.01% one-thousandth precision, 0.1% one-thousandth precision, etc.
• At the same time, the temperature coefficient of thick film resistors is difficult to control and is generally relatively large. Similarly, thin film resistors can have a very low temperature coefficient, so that the resistance value of the resistor changes very little with temperature, and the resistance value is stable and reliable. Therefore, thin film resistors are commonly used in various instruments, medical equipment, power supplies, electrical equipment, electronic digital products, etc.

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1.2 Selection of resistors

factors to consider

• Parameters: resistance, precision, rated power (package), rated voltage, maximum working voltage, temperature coefficient
• Aging factor: The percentage of the relative change in the resistance value of the resistor under the rated power long-term load, which is a parameter indicating the life span of the resistor.
• Noise: Including thermal noise and current noise.

specific choice

• Calculate the resistance value of the required resistor and calculate the power consumption consumed by the resistor, leaving a certain margin. Select the appropriate series and package according to the resistance value and power consumption. 根据算出的阻值，选择最接近的标称值电阻;根据功耗需求，选择合适的封装.
• Try to choose commonly used, low-cost or common resistors in BOM. For example, for some applications that are not sensitive to resistance, such as pull-up or pull-down resistors, existing resistors in the BOM can be selected to reduce the types of components in the BOM.

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2. Capacitance

Definition: Capacitance (or capacitance) is a physical quantity that expresses the ability of a capacitor to hold charges.

The essence of capacitance is to store charge and release charge:

the actual capacitance is equivalent to:

the actual capacitance has ESR (equivalent series resistance) and ESL (equivalent series inductance) .
For the definition, you can read this article:
Understanding of Capacitor ESR and ESL

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2.1 Ceramic Capacitors

definition:

Ceramic capacitor (ceramic capacitor) is a general term for capacitors with ceramics as the medium, also known as ceramic dielectric capacitors and ceramic dielectric capacitors.

Features:

• Advantages: high voltage resistance, good insulation, stable performance;
• Disadvantages: small capacity;

Types of Ceramic Capacitors:

• 一类为温度补偿型NPO介质
  NPO, also known as COG, has the most stable electrical performance and basically does not change with temperature, voltage, and time. It is an ultra-stable, low-loss capacitor material type, suitable for high-frequency and ultra-high frequency that require high stability and reliability. , VHF circuit
• 二类介电常数型X7R介质
  X7R is a strong dielectric, so it can produce capacitors with higher capacitance than NPO dielectric. The performance of this kind of capacitor is relatively stable. With the change of temperature and voltage time, its unique performance change is not significant. It belongs to the type of stable capacitor material. In medium and high frequency circuits.
• 三类为半导体型X5R介质
  X5R has a high dielectric constant and is often used in the production of large-capacity capacitors with large specific capacitance and high nominal capacity. However, its capacity stability is better than that of X7R, and its capacity and loss are more sensitive to test conditions such as temperature and voltage. It is mainly used in oscillation, coupling, filtering and bypass circuits in electronic complete machines.

Classification symbols consist of three parts, 第一部分为最低工作温度，第二部分为最高工作温度，第三部分为随温度变化的容差.

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2.2 Tantalum Capacitors

definition:

An electrolytic capacitor that uses an anodized oxide of a metal button as a medium;

Classification:
liquid button capacitors and solid button capacitors; in-line button capacitors and patch button capacitors;

Features:

• Good temperature performance; ESL is small , almost zero; small size; ESR is smaller than aluminum electrolytic capacitors with the same rated voltage ;
• high price;
• Poor withstand voltage capability , high temperature derating use, applied to power filtering, low frequency bypass and signal coupling.

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2.3 Aluminum electrolytic capacitor

definition:

An electrolytic capacitor is a kind of capacitor in which a dense oxide film with good insulation is formed by metal through anodic oxidation as a medium. Metals include aluminum, tantalum, niobium, and titanium. Therefore, electrolytic capacitors are divided into aluminum electrolytic capacitors, tantalum electrolytic capacitors, and the like.

Features:
• 容量大、体积大;
• 频率特性差，在高频率下等效容量很小;
• 漏电流比较大;
• Large ESL and ESR;
• Extremely unstable performance at very high temperature and low temperature;
• Applied to power filter, low frequency bypass and signal coupling;

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2.4 Capacitor selection

Parameters considered:

• Capacity and error: the maximum deviation range allowed between the actual capacitance and the nominal capacitance.
  Pay attention to the accuracy level in the selection, expressed in letters: D——±0.5%, F——±1%, G——±2%, J——±5%, K——±10%, M——± 20%.
• Rated working voltage: The capacitor can work stably and reliably for a long time in the circuit, and the maximum DC voltage it can withstand.
• Insulation resistance: Indicates the magnitude of leakage. Generally, the larger the insulation resistance, the better, and the smaller the leakage. The
  insulation resistance of electrolytic capacitors is generally small.
• Temperature coefficient: within a certain temperature range, the relative change value of capacitance for every 1°C change in temperature. The smaller the temperature coefficient, the better.
• Frequency characteristics: The nature of the electrical parameters of a capacitor changing with the frequency of the electric field. For capacitors working under high frequency conditions, since the dielectric constant is smaller at high frequencies than at low frequencies, the capacitance also decreases accordingly. Losses also increase with frequency.
• Equivalent series resistance (ESR): The ESR of products with larger losses is larger; as the capacity increases, the ESR of products will become smaller; the ESR of button capacitors is particularly small.

Method of choosing

According to the following formula:

select the appropriate capacitor to minimize the impedance at a specific frequency, which is convenient for filtering.

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3. Inductance

3.1 Definition and Introduction

definition:

Inductance is a property of a closed circuit and a physical quantity. When the current passes through the coil, a magnetic field induction is formed in the coil, and the induced magnetic field will generate an induced current to resist the current passing through the coil.
It is a circuit parameter that describes the induced electromotive force effect in this coil or in another coil due to the change of coil current. Inductance is 自感和互感the general term for. A device that provides inductance is called an inductor.
When two inductance coils are close to each other, the change in the magnetic field of one inductance coil will affect the other inductance coil, and this effect is mutual inductance. The size of the mutual inductance depends on the degree of coupling between the self-inductance of the inductance coil and the two inductance coils. The components made using this principle are called transformers.

effect:

• Through DC, resistance to AC
• Block the change of current and keep the working current of the device stable
• filtering

Features:
The resistance when the DC signal passes through the coil is the resistance of the wire itself, and the voltage drop is very small;
when the AC signal passes through the coil, a self-induced electromotive force will be generated at both ends of the coil, and the direction of the self-induced electromotive force is opposite to that of the applied voltage, hindering exchange through.
Therefore, the characteristic of an inductor is to pass DC and block AC. The higher the frequency, the greater the coil impedance .

Impedance Z = 2πfL,
f: frequency, unit Hz, Hz
L: inductance value, unit Henry, H

The impedance of the capacitor Z = 1/(2πfC), C is the capacitance,
so the higher the frequency of the capacitor, the smaller the impedance.

Filter:
Therefore, low-pass filters and high-pass filters can be designed based on the characteristics of capacitors and inductors

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3.2 Classification of inductance

Commonly used inductors can be divided into the following types:

There is also a chip inductor:

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3.3 Parameters of the inductor

Inductance:

The inductance is represented by L, the basic unit is H (Henry), and the inductance units in engineering are μH and mH.

Inductance reflects the ability of an inductor to store magnetic field energy. Its size is related to the number of turns of the inductance coil, the geometric size, whether there is a magnetic core (iron core), and the magnetic permeability of the magnetic core. The inductance for high-frequency circuits is relatively small, and the inductance for low-frequency circuits is relatively large.

Rated current:
The maximum current that the inductor can pass without damage for a long time.
For inductors with the same inductance, the thicker the wire diameter of the wound coil, the greater the rated current of the inductor
带有磁芯的电感器工作电流过大时，将引起电感量降低、线圈烧毁.

DC resistance (DCR): The
DC internal resistance of the inductor coil winding is at $10^{-3}-10^{On\; the\; order\; of\; 2\Omega }$ .

• The larger the inductance of the same series of inductors, the more turns of the coil, and the correspondingly larger internal resistance;
• For an inductor with the same number of turns, the larger the diameter of the wire used to wind the coil, the smaller the internal resistance;
• For an inductor with the same inductance, the smaller the internal resistance, the better;
• The higher the copper content of the coil wire, the lower the internal resistance of the inductor.

While aluminum wire inductors and "copper-clad iron" inductors reduce production costs, the internal resistance of the inductor increases significantly.

The internal resistance of an inductor with a large inductance can be tested with a multimeter at low resistance; the internal resistance of an inductor that is too small can only be accurately measured by a special milliohm meter or a digital bridge.

Quality factor (Q):

The quality factor is defined as the ratio of the stored energy to the consumed energy of the inductor, that is, the ratio of the inductive reactance of the coil to the DC internal resistance of the coil:

Q值越大，则电感器的功率损耗越小。

Error range:
Except that the error of the inductor used in the oscillation circuit needs to be controlled within 0.5%, the error range of the general inductor is acceptable within ±10% to 20%.
The error level of the inductor is as follows:

For example:
the parameter mark of the small-volume chip inductor shown in the figure below is 4R7M, the inductance of this inductor is 4.7μH, and M means that the error range is ±20%.

Distributed capacitance:
The actual equivalent model of the inductor is shown in the figure below.

The distributed capacitance C and the self-inductance L are in parallel relationship, which determines the natural frequency (resonant frequency) f0 of the inductor.

为避免自激，应确保电感器的工作频率远小于其固有频率

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4. Magnetic beads

4.1 Introduction of Magnetic Beads

definition:

The function of magnetic beads and magnetic rings is mainly to eliminate the noise existing in the transmission line structure (circuit).

Please pay attention to 消除the word " ". The capacitor itself can play the role of filtering, and the combination of the inductor and the capacitor can also play the role of filtering, but this kind of filtering does not really eliminate the noise, but grounds the noise.

• The role of magnetic beads is also filtering, but unlike capacitors and inductors, magnetic beads can reflect noise in a certain frequency band, and can absorb noise and convert it into heat energy in a certain frequency band.
• A magnetic bead is a resistor whose impedance varies with frequency;
  at low frequencies, the impedance is low; as the frequency increases, the impedance gradually increases and gradually shows the resistance function; the
  working principle of the ferrite bead is to absorb and heat through the impedance The form dissipates the energy of the unnecessary frequency band, thereby filtering out the noise

Example of use:

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4.2 Parameters of magnetic beads

Uses:
• Magnetic beads are often connected in series in electronic circuits to suppress noise in the circuit.
• Power line
• High frequency line, such as clock line, RGB line
• Oscillating loop
• Loop generated by ringing signal
• Grounding loop

Specifications:

The difference between magnetic beads and inductors:

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5. Diodes

5.1 Definition

Diode characteristic curve

The use of diode generally uses its forward conductivity and reverse breakdown characteristics.

Reverse breakdown is divided into 齐纳击穿and 雪崩击穿.

• 齐纳击穿: It is reversible, that is, when the applied voltage is removed, the characteristics of the device can be restored. (for Zener diodes)
• 雪崩击穿: The applied voltage value of the breakdown, called the breakdown voltage.

When the current passes through the tube, the tube core will heat up, and the temperature will rise. When the temperature exceeds the allowable limit, the tube core will be overheated and damaged.

Leakage current
When the diode is reverse biased, there will still be a weak reverse current flowing through the diode, which is called leakage current.
The smaller the reverse current, the better the unidirectional conductivity of the tube.

The capacitance between the two poles of the diode: barrier capacitance and diffusion capacitance.

Reverse recovery time
When the diode transitions from the on state to the off state, the diode needs to release the stored charge first before blocking the reverse current. This discharge time is called the reverse recovery time, during which the current flows reversely through the diode.
That is, the time from when the forward conduction current is 0 to entering the completely cut-off state.

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5.2 Zener tube

definition:

Zener diode, English name Zener diode, also known as Zener diode. Using the reverse breakdown state of the pn junction, the current can change in a wide range while the voltage is basically unchanged, and the
diode that acts as a voltage regulator is made.

characteristic:

• Zener diode is a semiconductor device with high resistance until the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance decreases to a small value, and the current increases in this low resistance region And the voltage is kept constant.
• Zener impedance is ideally zero, but in reality Zener diodes have a certain impedance. Therefore, Zener diodes only operate within a limited current range.
  

Related parameters:

• V _Z (Nominal Zener Voltage) Nominal stable voltage: 是稳压二极管最重要参数.
• I _ZT stable current: the current value passing through the tube when the Zener tube generates a stable voltage. When it is lower than this value, although the voltage regulator tube is not unable to stabilize the voltage, the effect of voltage stabilization will become worse; Consumes more power.
• Z _ZT represents the impedance of the Zener diode at the nominal voltage regulation value and the nominal voltage regulation current value.
• Izk represents the current at the inflection point _of the reverse breakdown of the Zener diode.
• Z _zk represents the impedance at the inflection point of the reverse breakdown of the Zener diode.
• V _R represents the reverse voltage of the Zener diode
• I _R represents the leakage current of the Zener diode under reverse voltage

Precautions:

• Zener diodes must not be connected in parallel as this would increase the allowable power dissipation.
• If two Zener diodes are connected in parallel, the diode with the lower Zener voltage will conduct most of the current and so may exceed its allowable power dissipation.

Application:
Zener diodes are suitable for protecting power lines, power control lines, and can protect internal circuits and ICs from overvoltage caused by hot swapping, etc.

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5.3 Schottky Diodes

Definition:
Schottky diode: named by Dr. Schottky (Schottky), the abbreviation of Schottky Barrier Diode
(Schottky Barrier Diode, abbreviated as SBD). It is made on the principle of metal-semiconductor junction formed by the contact of metal and semiconductor.

characteristic:

• It has a lower forward voltage than ordinary PN junction diodes.
• is with extremely low junction capacitance and faster switching speed (10ns)
• The reverse current is larger than the general PN junction diode

application:

1. Dual Power Switching
   
2. logic and
   
3. Freewheeling
   

The Schottky diode is connected in reverse parallel in the circuit. When the inductance coil is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through the Schottky diode. The diode that plays this role is called a freewheeling diode.

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5.4 LEDs

Performance:

• LED color The
  color of the LED is a very important indicator, which must be marked on every LED-related lighting product. At present, the colors of the LED mainly
  include red, green, blue, cyan, yellow, white and so on.
• LED current
  The forward limit (IF) current of LED is mostly 5-10mA. The luminous intensity of LED is only proportional to IF within a certain range. When IF>20mA, the enhancement of brightness can no longer be distinguished by the inner eye.
• LED voltage
  Generally speaking, the LED is a forward voltage, that is to say, the positive pole of the LED is connected to the positive pole of the power supply, and the negative pole is connected to the negative pole of the power supply. The voltage is related to the color, the voltage of red, yellow, yellow-green is between 1.7-2.0v. The voltage of white, blue and emerald green is between 3.0-3.6v.
• The reverse voltage V _Rm
  of the LED allows for an increased maximum reverse voltage. If the value is exceeded, the light-emitting diode may be damaged by breakdown.

Precautions:

• If the LED is driven by DC power supply voltage, a current limiting resistor must be connected in series in the circuit to prevent the tube from being burned due to excessive current passing through the LED. Note that the forward voltage drop of the LED is 1.7-2V.
• The reverse breakdown voltage of light-emitting diodes is relatively low, generally only a few volts. Therefore, when the LED is driven by an AC voltage, a rectifier diode can be connected in parallel with reverse polarity at both ends of the LDE so that the reverse bias voltage does not exceed 0.7V in order to protect the light-emitting diode.

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5.5 TVS Diodes

definition:

TVS—the abbreviation of transient voltage suppressor, the English full name is Transient Voltage Suppressor Diode.
When the two poles of the TVS are impacted by reverse transient high energy, it can change the high impedance between the two poles to low impedance at a speed of ps seconds, absorb surge power up to several thousand watts, and clamp the voltage between the two poles. Located at a predetermined value, it effectively protects the precision components in the electronic circuit from being damaged by surge pulses.

characteristic:

• Fast response (1ps)
• High transient power
• low leakage current
• Small breakdown voltage deviation
• Strong clamping ability
• The ability to withstand surge and suppress voltage is extremely strong.

Impulse voltage test level:

Related parameters:

• Reverse cut-off voltage V _RWM and reverse leakage current I _R : Reverse cut-off voltage V _RWM represents the highest voltage at which the TVS tube does not conduct, and there is only a small reverse leakage current I _R under this voltage .
• Breakdown voltage V _BR : the voltage when the TVS tube passes through the specified test current, which is the sign voltage indicating that the TVS tube is turned on.
• Maximum clamping voltage V _C : the voltage presented at both ends of the TVS tube when the pulse peak current I _{PP flows.}
• Pulse peak current I _PP : The maximum peak current of the 10/1000μs wave that the TVS tube allows to pass, and permanent damage may be caused if the current value exceeds this value.
• Dynamic resistance: R _DYN Dynamic resistance is the slope of the V–I curve between V _BR and V _C when the ESD protection diode reversely breaks down as the reverse voltage increases .
• C _T : equivalent capacitance of the diode. Interelectrode capacitance will affect the response time of TVS

Selection points:

• The maximum reverse clamping voltage V _C of TVS should be less than the damage voltage of the protected circuit;
• The rated reverse turn-off voltage V _RWM of the TVS must be greater than or equal to the maximum working voltage of the protected circuit. If the selected V _RWM is too low, the device may enter an avalanche or affect the normal operation of the circuit due to too much reverse leakage current;
• AC voltage can only use bidirectional TVS;
• The maximum peak pulse power PM of TVS must be greater than the possible peak pulse power of the protected circuit, and its peak pulse current should be greater than the transient surge current;
• When laying out the PCB of TVS, it should be far away from the protected device, and should be placed near the interface as much as possible;
• Junction capacitance is a key factor affecting the use of TVS in high-speed lines.
  

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6. Tertiary tube and MOS tube

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6.1 Triode

About the basics of tri-level tubes and MOS tubes:
[Hardware Design] Analog Electronics Basics II - Amplifying Circuit
Device Patterns:

The amplification effect of the triode is essentially to use a small base current signal to control the large current signal of the collector current, which is the function of "controlling the large with the small", rather than "magnifying the energy".

The voltage drop between the base and the emitter is the same as the forward voltage drop of the diode, which is 0.6-0.7V. When designing the circuit, set the base-emitter voltage of the transistor to >0.6V to make the diode between the base and emitter conduct, and then calculate other parts of the circuit.

Parameter Note h _FE is the magnification factor β

application:

• R1 and R3 are current limiting resistors, mainly the role of R2
• One is pull-down to determine the state, and the other releases the parasitic current of the triode,
• The larger R1, the smaller R2, the smaller the parasitic capacitance of the triode

calculate:

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6.2 MOS tube

Device pattern:
The identification of the three pins on the MOS tube symbol should grasp the key points

• G pole, easier to recognize.
• The S pole, whether it is a P-channel or an N-channel, is where the two lines intersect.
• The D pole, whether it is a P-channel or an N-channel, is the side of the separate lead.
• If the arrow points inward, it is the N-channel, and if it faces outward, it is the P-channel.
• Parasitic diode, N channel, from S pole to D pole; P channel, from D pole to S pole.

parameter:
极限参数：

• V _DSS is the maximum voltage that the drain and source of the device can withstand in the off state.
• ID represents the continuous current value that the drain can withstand. If the _current flowing exceeds this value, it will cause the risk of breakdown.
• E _AS represents the single-pulse avalanche breakdown energy, and the device can safely absorb the level of reverse avalanche breakdown energy.
• P _D represents the maximum power dissipation, the actual power consumption should be less than this parameter with a certain margin.
  Thermal resistance indicates the difficulty of heat conduction, the smaller the thermal resistance, the better the heat dissipation performance

使用参数：

• I _DSS represents the gate voltage V _GS =0 , and V _DS is a drain-source leakage current of a certain value, generally in the microampere level.
• I _GSS represents the gate drive leakage current, the smaller the better.
• V _GS(th) represents the turn-on voltage of the MOS.
• R _DS(ON) represents the on-resistance of the MOS. Generally, the smaller the on-resistance, the better. It determines the conduction loss of the MOS. The larger the on-resistance, the greater the loss.

等效电容：

• C _iss represents the input equivalent capacitance
• C _oss means output equivalent capacitance
• C _rss represents the reverse transfer capacitance

使用方向：

Four areas of MOS tube:

Unsaturated region (variable resistance region)
Corresponding working area before channel pre-pinch

条件: V _GS > V _GS(th) (on voltage), V _DS < V _GS - V _GS(th)
特性 : _ID is controlled by V _GS and V _DS at the same time .
The R _DS equivalent resistance formula is as follows:

Saturation region (constant current region)
The corresponding working area after the channel is pre-pinched off.

条件: V _GS > V _GS(th) , V _DS > V _GS - V _GS(th)
特点 : I _D is only controlled by V _GS and has approximately nothing to do with V _{DS .} Considering the channel length modulation effect, the output characteristic curve is slightly upturned with the increase of V _DS .

cut-off area
Working area below I _{D = 0.}

条件: V _GS < V _GS(th)
特点 : I _G ≈ I _D ≈ 0, which means that the three electrodes of the MOS tube are disconnected.

breakdown region
When V _DS increases to a certain value, the drain liner PN junction avalanche breaks down, and _ID increases sharply.

MOS tube for voltage on-off: U _G should be greater than 10V than U _S , and it must work in a non-saturated conduction state when it is turned on.

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7. Logic Devices

7.1 Introduction to Logic Devices

Hardware circuit design is inseparable from logic devices such as NAND gates, logic drivers, level shifters, and analog switches. Familiarity with the application of logic devices is an essential skill for hardware engineers.

Logic LC is used to connect different LSI chips and circuit boards. They are also used to make minor modifications and adjustments to logic circuits, such as adding signal drive capability (i.e., buffering signals), shaping signal waveforms, adjusting signal output timing, and making minor changes to the system.

Standard logic ICs are classified into the following types with different electrical characteristics according to their structure (i.e. the manufacturing process used). Currently most 常用的是就兼具低功耗和低成本特点的CMOS逻辑IC.

• TTL (transistor-transistor logic)
  was originally widely used as bipolar logic for standard logic ICs.
  Compared with CMOS logic ICs, it provides higher current drive capability and operating speed, but consumes more power.
• CMOS logic (CMOS: Complementary MOSFET)
  combines p-channel and n-channel MOSFETs to achieve lower power consumption than TTL
  Initially slower than TTL but now offers higher operating speeds than TTL due to fine wafer manufacturing processes
• BiCMOS logic (bipolar CMOS)
  input stage and logic circuit use CMOS technology to reduce power consumption, and output stage uses bipolar transistors to improve current drive capability. MOS bipolar combination manufacturing process is complex and costly

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7.2 Introduction to Logic Levels

TTL, CMOS, LVTTL, LVCOMS, CML, ECL, PECL, LVPECL, LVDS, RS232 level, RS422 level, RS485 level, etc.

• The logic levels of TTL and CMOS can be divided into four categories according to the typical voltage: 5V series, 3.3V series, 2.5V series and 1.8V series.
• 5V TTL and 5V CMOS logic levels are common logic levels.
• Logic levels of 3.3V and below are called low-voltage logic levels, and LVTTL levels are commonly used.
  There are two low-voltage logic levels, 2.5V and 1.8V.

The logic levels of TTL and CMOS can be divided into four categories according to the typical voltage: 5V series, 3.3V series, 2.5V series and 1.8V series.

• 5V TTL and 5V CMOS logic levels are common logic levels.
• Logic levels of 3.3V and below are called low-voltage logic levels, and LVTTL levels are commonly used.
  There are two low-voltage logic levels, 2.5V and 1.8V.

The following TI official document "Logic Guide" shows a schematic diagram of logic levels:

• 输入高电平（Vih）: Guarantee the minimum input high level allowed when the input of the logic gate is high level. When the input level is higher than Vih, the input level is considered to be high level.
• 输入低电平（Vil）: Guarantee the maximum input low level allowed when the input of the logic gate is low level. When the input level is lower than Vil, the input level is considered to be low level.
• 输出高电平（Voh）: Guarantee the minimum value of the output level when the output of the logic gate is high level, and the level value when the output of the logic gate is high level must be greater than this Voh.
• 输出低电平（Vol）: Guarantee the maximum value of the output level when the output of the logic gate is low level, and the level value when the output of the logic gate is low level must be less than this Vol.
• Threshold level (Vt): There is a threshold level in digital circuit chips, which is the level when the circuit is just barely capable of flipping. It is a voltage value between Vil and Vih. For the threshold level of the CMOS circuit, it is basically one-half of the power supply voltage value, but to ensure a stable output, it must be required to input a high level > Vih , the input low level <Vil, and if the input level is above or below the threshold, that is, the region Vil～Vih, the output of the circuit will be in an unstable state. For general logic levels, the relationship of the above parameters is as follows: Voh > Vih > Vt > Vil > Vol.
• I _oh : The load current (source current) when the logic gate output is high level.
• I _ol : the load current (sink current) when the logic gate output is low level.
• I _ih : The current when the logic gate input is high level (sink current).
• I _il : The current when the logic gate input is low level (source current).
  The output pole of the gate circuit is directly drawn as the output terminal without connecting the load resistance in the integrated unit. This form of gate is called an open circuit gate. Open TTL, CMOS, and ECL gates are called open collector (OC), open drain (OD), and open emitter (OE) respectively. When using them, check whether they are connected to pull-up resistors (OC, OD gates) or pull-down resistors. (OE gate), and whether the resistance value of the resistor is appropriate. For an open collector (OC) gate, the pull-up resistor RL should meet the following conditions:
  (1) The pull current is as large as possible
  (2) The sink current is as small as possible

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7.3 Logic Level Conversion

In practical application, some logic level conversion is sometimes required, such as converting TT1 level to CMOS level, but the thresholds of each logic level are different, so they cannot be directly interconnected with each other. Devices with different logic levels can be directly interconnected only when the following conditions are met at the same time.

• Condition one, the VOH of the sender is greater than the VIH of the receiver.
• The second condition is that the VOL of the sender is smaller than the VIL of the receiver.

As shown in the figure below, the level of 1.8V is converted to the level of 3.3V through the IC chip

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7.4 OC gate and OD gate

OC (OD) gate: open-drain output, simply put, the output is provided by the outside, and it only acts as a switch.

I2C is made up of many device lines and connections. If push-pull output (push-pull) is used, the following situation will inevitably occur. One device has high output and the other device has low output, that is, the PMOS of the left device is turned on and the right device The NMOS of the device is turned on, so that a short circuit is formed between VCC and GND. At this time, a large current will burn the device, and the consequences are disastrous. This phenomenon is called bus contention.

The characteristics of the open-drain output:

• Use the driving capability of the external circuit to reduce the power consumption inside the IC. Can drive loads higher than the chip supply voltage.
• Multiple open-drain output pins can be connected together. Form an "AND logic" relationship.
• When connected to a capacitive load, the falling delay is a transistor in the chip, which is actively driven, and the speed is faster; the rising delay is a passive external resistor, and the speed is slow. If high speed is required, the resistance selection should be small, and the power consumption will be large. Therefore, the choice of load resistance should take into account both power consumption and speed.
• The transmission level can be changed by changing the voltage of the pull-up power supply. This way you can perform arbitrary level conversions.

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7.5 Bus Hold

The bus holding circuit is added at the data input end of the IC. The bus-hold circuit consists of two inverters in a feedback loop, 当输入引脚处于开路状态（即悬空）时保持（锁存）其最后已知的状态.

The bus hold circuit has the following two electrical characteristics:

• Output: The bus holding current refers to the minimum current that can be supplied to the device or bus;
• enter:改变总线保持电路中保持的状态所需的最小过驱动电流。
• When we need to change its level, we need to pay attention to the input current

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7.6 Clamping Diodes

In order to protect the logic device, a clamping diode is often built between the input/output port and the power supply Vcc and GND. When the port level exceeds Vcc or GND, the diode can clamp the level within the limit range. Thus avoiding damage to the device.

Generally, when VCC is 5V, the conduction voltage of the clamp diode is 0.5V, and the maximum voltage of the logic device is generally 5.5V.

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7.7 Logic Device Considerations

• It is best not to connect logic devices in parallel, as it is easy to cause a short circuit.
  

• Generally, all unused inputs should be connected to VCC or GND .
  Since CMOS logic has a very high input impedance, any open input may cause erroneous output values ​​due to the influence of surrounding electric fields. Also, shoot-through current may flow at the midpoint of VCC and GND, causing increased current and, in worst case, device damage.
  All inputs without bus-hold capability unless otherwise noted in the data sheet. They have to be pulled up and down.

• Insert TVS diodes on the input side for electrostatic discharge (ESD) protection.
  If the applied voltage is higher than VCC or the voltage applied when the IC is off, the diode between the input terminal and the power supply may conduct.

• When using the partial power-down function, check whether the logic device has a clamping diode to prevent the clamping diode from being turned on during partial power-down, resulting in the logic device not being powered off.

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7.8 Calculation of Power Consumption

The power consumption of logic devices is divided into static power consumption P _s and dynamic power consumption _PL and _PD .

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