Altec Peerless 4722 MC Step-up transformer

A step up transformer basically increases the magnitude of primary applied voltage that is increases the amplitude of voltage wave form. A voltage amplifier does exactly the same.

Than a very strange but thinkable question comes what is the difference between the two and can we use a small step up transformer in place of voltage amplifier and vice-versa?

Differences

How amplifier works - The concept

Transformer is not an amplifier, because:

The output and input powers are same and there is not any another source other than the signal (that is incoming AC voltage), Amplifier can amplify the signal voltage without reducing the output current.

Transformer follows the principle of induction where as Amplifier follows the principle of boosting the signal (voltage or current). Actually, the amplifier generates a completely new output signal based on the input signal. We can understand these signals as two separate circuits.

The output circuit is generated by the amplifier’s power supply, which draws energy from a battery or power outlet.

Light Detectors - Introduction And Purpose (picture by OSRAM)

Detectors of electromagnetic radiation in the spectral range from ultraviolet to far infrared are called light detectors. From the standpoint of a sensor designer, absorption of photons by a sensing material may result either in a quantum or thermal response.

Therefore, all light detectors are divided into two major groups that are called quantum and thermal. The quantum detectors operate from the ultraviolet to mid-infrared spectral ranges, whereas thermal detectors are most useful in the mid- and far-infrared spectral range where their efficiency at room temperatures exceeds that of the quantum detectors.

Solid-state quantum detectors (photovoltaic and photoconductive devices) rely on the interaction of individual photons with a crystalline lattice of semiconductor materials. Their operations are based on the photoeffect that was discovered by A. Einstein, and brought him the Nobel Prize.

In 1905, he made a remarkable assumption about the nature of light: that, at least under certain circumstances, its energy was concentrated into localized bundles, later named photons.

When a photon strikes the surface of a conductor, it may result in the generation of a free electron. Part (φ) of the photon energy E is used to detach the electron from the surface; the other part gives its kinetic energy to the electron.

Photoelectric effect

The photoelectric effect can be described as:

hυ = φ + K_m

Figure 1 - Photoeffect in a semiconductor for high-energy (A) and low-energy (B) photons.

where φ is called the work function of the emitting surface and K_m is the maximum kinetic energy of the electron upon its exiting the surface. Similar processes occur when a semiconductor p-n junction is subjected to radiant energy:

The periodic lattice of crystalline materials establishes allowed energy bands for electrons that exist within that solid. The energy of any electron within the pure material must be confined to one of these energy bands, which may be separated by gaps or ranges of forbidden energies.

If light of a proper wavelength [sufficiently high energy of photons strikes a semiconductor crystal, the concentration of charge carriers (electrons and holes) in the crystal increases, which manifests in the increased conductivity of a crystal:

σ = e ( µ_e n + µ_h p)

where e is the electron charge, µ_e is the electron mobility, µ_h is the hole mobility, and n and p are the respective concentrations of electrons and holes.

Figure 1A shows energy bands of a semiconductor material, where Eg is the magnitude in electron volts (eV) of the forbidden band gap. The lower band is called the valence band, which corresponds to those electrons that are bound to specific lattice sites within the crystal. In the case of silicon or germanium, they are parts of the covalent bonding which constitute the interatomic forces within the crystal. The next higher-lying band is called the conduction band and represents electrons that are free to migrate through the crystal. Electrons in this band contribute to the electrical conductivity of the material. The two bands are separated by the band gap, the size of which determines whether the material is classified as a semiconductor or an isolator.

The number of electrons within the crystal is just adequate to completely fill all avail-able sites within the valence band. In the absence of thermal excitation, both isolators and semiconductors would therefore have a configuration in which the valence band is completely full and the conduction band completely empty. Under these imaginable circumstances, neither would theoretically show any electrical conductivity.

In a metal, the highest occupied energy band is not completely full. Therefore, electrons can easily migrate throughout the material because they need achieve only a small incremental energy to the above occupied states. Metals, therefore, are always characterized by a very high electrical conductivity. In isolators or semiconductors, on the other hand, the electron must first cross the energy band gap in order to reach the conduction band and the conductivity is, therefore, many orders of magnitude lower.

Table 1: Band Gaps and Longest Wavelengths for Various Semiconductors

For isolators, the band gap is usually 5 eV or more, whereas for semiconductors, the gap is considerably less (Table 1). Note that the longer the wavelength (lower frequency of a photon), the less energy is required to originate a photoeffect.

When the photon of frequency υ₁ strikes the crystal, its energy is high enough to separate the electron from its site in the valence band and push it through the band gap into a conduction band at a higher energy level. In that band, the electron is free to serve as a current carrier. The deficiency of an electron in the valence band creates a hole which also serves as a current carrier.

This is manifested in the reduction of specific resistivity of the material. On the other hand, Figure 1B shows that a photon of lower frequency υ₂ does not have sufficient energy to push the electron through the band gap. The energy is released without creating current carriers. The energy gap serves as a threshold below which the material is not light sensitive. However, the threshold is not abrupt. Throughout the photon-excitation process, the law of conservation of momentum applies.

The momentum and density of hole-electron sites are higher at the center of both the valence and conduction bands, and they fall to zero at the upper and lower ends of the bands. Therefore, the probability of an excited valence-band electron finding a site of like momentum in the conduction band is greater at the center of the bands and is the lowest at the ends of the bands. Therefore, the response of a material to photon energy increases from E_g gradually to its maximum and then falls back to zero at the energy corresponding to the difference between the bottom of the valence band and the top of the conduction band. A typ-ical spectral response of a semiconductive material is shown in Figure 2.

Figure 2 - Spectral response of an infrared photodiode

The light response of a bulk material can be altered by adding various impurities. They can be used to reshape and shift a spectral response of the material. All devices that directly convert photons of electromagnetic radiation into charge carriers are called quantum detectors and are generally produced in a form of photodiodes, phototransistors, and photoresistors.

When comparing the characteristics of different photodetectors, the following specifications usually should be considered:

NEP (noise-equivalent power) is the amount of light equivalent to the intrinsic noise level of the detector. Stated differently, it is the light level required to obtain a signal-to-noise ratio equal to unity.

Because the noise level is proportional to the square root of the bandwidth, the NEP is expressed in units of W/√Hz:

D* refers to the detectivity of a detector’s sensitive area of 1 cm² and a noise bandwidth of 1 Hz:

Detectivity is another way of measuring the sensor’s signal-to-noise ratio. Detectivity is not uniform over the spectral range for operating frequencies; therefore, the chopping frequency and the spectral content must be also specified. The detectivity is expressed in units of cm √Hz/W.

IR cutoff wavelength (λc) represents the long-wavelength limit of the spectral response and often is listed as the wavelength at which the detectivity drops by
10% of the peak value.

Maximum current is specified for photoconductive detectors (such as HgCdTe) which operate at constant currents. The operating current never should exceed the maximum limit.

Maximum reverse voltage is specified for Ge and Si photodiodes and photoconduc-tive cells. Exceeding this voltage can cause the breakdown and severe deteriora-tion of the sensor’s performance.

Radiant responsivityis the ratio of the output photocurrent (or output voltage) divided by the incident radiant power at a given wavelength, expressed in A/W or V/W.

Field of view (FOV) is the angular measure of the volume of space where the sensor can respond to the source of radiation.

Junction capacitance (C_j) is similar to the capacitance of a parallel-plate capacitor. It should be considered whenever a high-speed response is required. The value of C_j drops with reverse bias and is higher for the larger diode areas.

In the market, there are several types of capacitors that have been manufactured. Although all capacitors work essentially the same way, key differences in the construction of different capacitor types makes an enormous difference in their properties.

Each capacitor type has its own set of characteristics and applications from small delicate trimming capacitors up to large power metal-can type capacitors used in high voltage power correction and smoothing circuits.

Capacitor dimensions

The dielectric material between the two plates is the main element of the capacitor that gives rise to the different properties of the different types of capacitors. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor and its applications.

Some capacitors have the metal plates rolled up into a cylinder to form a small package which makes them look like tubes. Some capacitors are sealed using an epoxy resin after being constructed from ceramic materials.

Electrolytic Capacitors

Electrolytic capacitors are high voltage capacitors which produce high value of capacitance in a small component at the expenses of wide tolerance in the marked value and the necessity of connecting the capacitor so that one terminal is always positive. When very large capacitance values are required, electrolytic capacitors are generally used.

Electrolytic Capacitors

Due to their large capacitance and small size, they are also used in DC power supply circuits to help reduce the ripple voltage or for coupling and decoupling applications. High-capacity electrolytic, also known as supercapacitors or ultracapacitors, have applications similar to those of rechargeable batteries.

Ceramic Capacitors

Ceramic or Disc Capacitors are manufactured by coating 2 sides of a small porcelain or ceramic disc with silver and are then stacked together to create a capacitor. They are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage. Although small in physical size, they have high dielectric constant.

Ceramic Capacitors

They are non-polarized devices and the exhibit large non-linear changes in capacitance against temperature.

Ceramic capacitors have multiple layers in order to ensure that sufficient levels of capacitance can be obtained with a single capacitor package. Although other styles are available, the 3 main types of ceramic capacitors include leaded disc ceramic capacitors, multilayer surface mount chip ceramic capacitors and specialist microwave bare leadless disc ceramic capacitors.

Film Capacitors

Film Capacitors

The most commonly available of all types of capacitors are the film capacitors which consist of a relatively large family of capacitors with the difference being in their dielectric properties.

They can come in an assortment of shapes and case styles including wrap & fill (oval & round), epoxy case (rectangular & round) and metal hermetically sealed (rectangular & round).

Tantalum Capacitors

Tantalum Capacitors

Similar to electrolytic capacitors, tantalum capacitors are also polarized but uses tantalum within the construction of the capacitor in order to provide extremely high levels of capacitance for any given volume. They offer a form of capacitor that provides a very high capacity density.

They are available in both wet (foil) and dry (solid) electrolytic types.

Originally published at EEWeb

TKxP Power MOSFETs in DPAK Package from Toshiba

Power MOSFETs are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors.

The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current on the order of nanoamperes. Hence, the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Althoug h in steady state the gate draws virtually no current, this is not so under transient conditions.

The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents. The circuit symbol of a power MOSFET is shown in Fig . 1.

Power MOSFET circuit symbol

Power MOSFETs are majority carrier devices, and there is no minorit y carrier storage time. Hence, they have exceptionally fast rise and fall times. They are essentially resistive devices when turned on, while bipolar transistors present a more or less constant V_CE(sat) over the normal operating range.

Power dissipation in MOSFETs is Id² R_DS(on) , and in bipolars it is I_C V_{CE (sat)} . At low currents, therefore, a power MOSFET may have a lower conduction loss than a comparable bipolar device, but at higher currents, the conduction loss w ill exceed that of bipolars. Also, the R_DS(on) increases w ith temperature.

An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In MOSFETs, R_DS(on) increases with temperature, and thus the current is automatically diverted away from the hot spot. The drain body junction appears as an antiparallel diode between source and drain.

Thus, power MOSFETs w ill not support voltage in the reverse direction. Although this inverse diode is relatively fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as 100ns. Since MOSFETs cannot be protected by fuses, an electronic protection technique has to be used.

With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional MOSFETs. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating w ithin its specification range at all times, its chances for failing catastrophically are minimal. However, if its absolute maximum rating is exceeded, failure probability increases dramatically. Under actual operating conditions, a MOSFET may be subjected to transients – either externally from the power bus supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the absolute maximum ratings.

Such conditions are likely in almost every application, and in most cases are beyond a designer’s control. Rugged devices are made to be more tolerant for over-voltage transients. Ruggedness is the ability of a MOSFET to operate in an environment of dynamic electrical stresses, without activating any of the parasitic bipolar junction transistors.

The rugged device can w ithstand hig her levels of diode recover y d_v=d_t and static d_v =d_t .

Figure 1 - Common form of resistor

You may be asked: “what is the power rating of the resistors you want to buy?” when buying a resistor to build a certain circuit. For the most standard class of resistors, you may simply be given ¼ Watt resistor.

The power rating of a resistor is the specification given with a resistor that serves to tell the maximum amount of power that the resistor can withstand.

Thus, if a resistor has a power rating of ¼ Watts, ¼ Watts is the maximum amount of power that should be fed into the resistor.

When an electrical current passes through a resistor, electrical energy is lost by the resistor in the form of heat and the greater this current flow, the hotter the resistor will get. Heat is generated as the current passes through electrical components. The heat is usually negligible and unnoticed in a circuit if the current is small enough and suitable for the circuit. A substantial amount of heat in a circuit can be created if the current is large enough.

The reason why resistors are given power ratings is because current can melt components and possibly create shorts in a circuit if the maximum allowable amount of power that can pass through the resistor is not specified.

Figure 2 - Resistor ratings

The Resistor Power rating is sometimes called the Resistors Wattage Rating and is defined as the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance. Depending upon the size, construction, and ambient operating temperature, the power rating of resistors varies a lot from less than one tenth of a watt to many hundreds of watts. For an ambient temperature of +70 degrees Celsius, most resistors have their maximum resistive power rating given.

Since the standard power ratings of 0.25W or 0.5W are suitable for most circuits, the power ratings of resistors are rarely quoted in parts lists. It should be clearly specified in the parts list for the rare cases where a higher power is required.

The Resistor Power Triangle

From Ohm’s Law, a product of power is produced when a voltage is dropped across a resistor and a current passed through the resistor. A Power Triangle superimposes the 3 quantities of power, voltage and current into a triangle since power is always consumed if a resistor is subjected to a voltage or if it conducts a current. The image below shows the power dissipated as heat in a resistor at the top and the current and the voltage at the bottom.

Figure 3 - Power triangle

The above expression for the resistor power can produce two possible alternative variations if two of the values are known. The 3 standard formulas can be used to calculate the power dissipation of any resistor.

Where,

V – is the voltage across the resistor in volts
I – is the current flowing through the resistor in amperes
R – is the resistance of the resistor in Ohms

NEC 2SD555 NPN POWER TRANSISTOR VCB=400V 10A 200W TO-3

Power transistors are used in applications ranging from a few to several hundred kilowatts and switching frequencies up to about 10 kHz. Power transistors used in power conversion applications are generally npn type. The power transistor is turned on by supplying sufficient base current, and this base drive has to be maintained throughout its conduction period. It is turned off by removing the base drive and making the base voltage slightly negative (within -V_BE(max)).

The saturation voltage of the device is normally 0.5 to 2.5 V and increases as the current increases. Hence, the on-state losses increase more than proportionately with current. The transistor off-state losses are much lower than the on-state losses because the leakage current of the device is of the order of a few milliamperes.

Because of relatively larger switching times, the switching loss significantly increases with switching frequency. Power transistors can block only forward voltages. The reverse peak voltage rating of these devices is as low as 5 to 10 V.

Power transistors do not have I²t withstand capability. In other words, they can absorb only very little energy before breakdown. Therefore, they cannot be protected by semiconductor fuses, and thus an electronic protection method has to be used. To eliminate high base current requirements, Darlington configurations are commonly used. They are available in monolithic or in isolated packages. The basic Darlington configuration is shown schematically in Figure 1. The Darlington configuration presents a specific advantage in that it can considerably increase the current switched by the transistor for a given base drive.

Figure-1 - A two-stage Darlington transistor with bypass diode.

The V_CE(sat) for the Darlington is generally more than that of a single transistor of similar rating with corresponding increase in on-state power loss.

During switching, the reverse-biased collector junction may show hot-spot breakdown effects that are specified by reverse-bias safe operating area (RBSOA) and forward-bias safe operating area (FBSOA).

Modern devices with highly interdigited emitter base geometry force more uniform current distribution and therefore considerably improve secondary breakdown effects. Normally, a well-designed switching aid network constrains the device operation well within the SOAs.

Video Tutorials

The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic (low saturation voltage) of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal zero or slightly negative.

The IGBT has a much lower voltage drop than a MOSFET of similar ratings.

The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors.

However, care should be taken not to exceed the maximum power dissipation and specified maximum junction temperature of the device under all conditions for guaranteed reliable operation. The on-state voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low onstate voltage, a sufficiently high gate voltage must be applied.

In general, IGBTs can be classified as punch-through (PT) and nonpunch-through (NPT) structures, as shown in Figure below. In the PT IGBT, an Nþ buffer layer is normally introduced between the P⁺ substrate and the N^¯ epitaxial layer, so that the whole N¯ drift region is depleted when the device is blocking the off-state voltage, and the electrical field shape inside the N¯ drift region is close to a rectangular shape.

Because a shorter N¯ region can be used in the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V.

(a) Nonpunch-through IGBT, (b) punch-through

IGBT equivalent circuit

High voltage IGBTs are realized through a nonpunch-through process. The devices are built on an N^¯ wafer substrate which serves as the N^¯ base drift region. Experimental NPT IGBTs of up to about 4 KV have been reported in the literature. NPT IGBTs are more robust than PT IGBTs, particularly under short circuit conditions. But NPT IGBTs have a higher forward voltage drop than the PT IGBTs. The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of parallel-connected IGBTs are (1) on-state current unbalance, caused by VCE(sat) distribution and main circuit wiring resistance distribution, and (2) current unbalance at turn-on and turn-off, caused by the switching time difference of the parallel connected devices and circuit wiring inductance distribution.

The NPT IGBTs can be paralleled because of their positive temperature coefficient property.

Written by Robert Green

Engineers have been using Hall effect measurements to characterize materials since Edwin Hall discovered the phenomenon in 1879. The Hall effect is the generation of a voltage, called the Hall voltage, across a sample of a material when that sample is exposed to a combination of a magnetic field through the sample and a current along the length of the sample (see Figure 1).

Figure 1 - The Hall effect

In the electronics industry, integrated circuit manufacturers and crystal manufacturers use Hall effect measurements in materials research, device development, and device manufacturing. In addition to measuring the Hall voltage, companies use Hall effect measurement systems to determine quite a few material parameters, including carrier mobility, carrier concentration (n), Hall coefficient (RH), resistivity, magnetoresistance ®, and the conductivity type (N or P).

The recent interest in Hall effect measurements is being driven by the search for next-generation semiconductors that can overcome the barriers of size and speed posed by silicon-based materials. The goal is maximizing carrier mobility with materials that can create smaller components and minimize heat dissipation. There is great hope that nanomaterials and single atomic layer crystals such as graphene or graphene-based material structures will be the solution. To characterize the properties of these new materials, including analyzing carrier mobility, Hall effect measurements are essential.

A basic Hall effect measurement system will likely include the following:

A constant-current source. For low resistivity material samples, the source must be able to output from milliamps to amps of current. For high resistivity samples (such as intrinsic semiconductors), the constant current source may have to be able to go as low as 1 nanoamp, but in general, a source capable of producing from 10 microamps to 100 milliamps will suffice.

A high input impedance voltmeter. The voltmeter used must be able to make accurate measurements anywhere from 1 microvolt to 100V. High resistivity materials may require ultra-high input Z or differential measurements. A permanent magnet or an electromagnet. These are typically available with ranges from 500 to 5000 gauss.
A sample holder.

Depending on the application, your test system might also include some other equipment. The recommended technique to get the best quality measurements requires that multiple measurements be made at multiple edges of the sample; thus, a switching matrix is recommended to reliably acquire all measurements. Also, because Hall mobility is dependent on sample temperature, you may want a temperature measurement probe capable of 0.1°C resolution.
Measuring Mobility

To determine carrier mobility (µH), you first measure the Hall voltage (VH) by forcing both a magnetic field perpendicular to the sample and a current through the sample. Accurate measurements of both the sample thickness (t) and its resistivity (ρ) are also required. With just these five parameters (B, I, VH, t, and ρ), you calculate the Hall mobility using the following formula:

μH= |VHt| / BIρ

Figure 2 - Measurement configurations for both the Hall effect voltage (a) and resistivity measurement (b). The resistivity measurement is made without a magnetic field

To obtain results with high confidence, we recommend that you take eight different measurements. You reverse the source current polarity, source on additional terminals, and reverse the direction of the magnetic field. If the voltage readings differ substantially, it’s advisable to recheck the test setup to look for potential sources of error.

For more on this topic, see the Keithley application note, “Hall Effect Measurements in Materials,” downloadable from Keithley’s website: http://www.keithley.com/data?asset=55773.

Thyristor 2.5A; Peak Repetitive Off-State Voltage, Vdrm: 600V; Current Igt: 500µA

The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpn device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device.

The turn-off is achieved by applying a reverse voltage across the anode and cathode. The thyristor symbol and its volt-ampere characteristics are shown in Figure 1. There are basically two classifications of thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter-grade thyristor is the low turn-off time (on the order of a few microseconds) for the latter.

The converter-grade thyristors are slow type and are used in natural commutation (or phase-controlled) applications. Inverter-grade thyristors are used in forced commutation applications such as DC-DC choppers and DC-AC inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an external commutation circuit.

This requires additional commutating components, thus resulting in additional losses in the inverter.

Thyristors are highly rugged devices in terms of transient currents, di/dt, and dv/dt capability. The forward voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 1000 A, it seldom exceeds 3 V.

While the forward voltage determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device junction temperature at high operating frequencies. Because of this, the maximum switching frequencies possible using thyristors are limited in comparison with other power devices considered in this section.

FIGURE 1 - (a) Thyristor symbol and (b) volt-ampere characteristics.

Thyristors have I²t withstand capability and can be protected by fuses. The nonrepetitive surge current capability for thyristors is about 10 times their rated root mean square (rms) current. They must be protected by snubber networks for dv/dt and di/dt effects. If the specified dv/dt is exceeded, thyristors may start conducting without applying a gate pulse. In DC-to-AC conversion applications, it is necessary to use an antiparallel diode of similar rating across each main thyristor.

Thyristors are available up to 6000 V, 3500 A.

FIGURE 2 - (a) Triac symbol and (b) volt-ampere characteristics.

A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol and volt-ampere characteristics are shown in Figure 2. Because of the integration, the triac has poor reapplied dv/dt, poor gate current sensitivity at turn-on, and longer turn-off time.

Triacs are mainly used in phase control applications such as in AC regulators for lighting and fan control and in solid-state AC relays.

Written by Ray Salemi

State machines are a foundation of digital design. Eventually we all reach the point where we need to control our digital algorithm, and we almost always turn to a state machine to do the job.

Because of this, many EDA tools recognize when the RTL designer is creating a state machine and use this information to improve their simulation and synthesis results. Once a tool recognizes a state machine in your design, it can deliver a list of features that are not available for generic logic. For example, synthesis tools can change the state machine encoding to improve synthesis results, while simulators can render the state machine and provide debugging and coverage information. Tools such as Mentor’s Precision High-Reliability synthesis engine can even add error correcting information to allow the state machine to power through single-event upsets and jump to the next correct state.

However, none of these features will work if the software can’t recognize the state machine. While EDA tool manufactures support a wide variety of state machine coding styles, it’s still possible to write code that can’t be recognized as a state machine, either by software tools or by humans.

This is the first of a series of articles that will talk about state machine today. We’re going to start with the most basic topic, naming our states and identifying the state variable.

Every state machine has a register that holds the state. This register feeds the logic that creates the state machine output. The register also combines with input signals to figure out the next state. In this article, we’ll examine techniques for coding a state machine to make it easy to debug and reuse. As an example, we’ll use a simple traffic light state machine:

The first thing you’ll notice about this state machine is that the states have descriptive names. The machine sits in the cars_go state until someone presses the walk_request button. Then it slows the cars, stops them, and lets the pedestrians walk. It then warns the people that the light is about to go back to green, and then goes back to green. This is a simple illustrative state machine. In a real-world state machine we’d need to use a counter to keep the states in each state long enough to be useful.

We want to capture the readability of the state names in our code. This will make it easier to debug, and we’ll be able to put the state variables into the waveform viewers of some simulators to see the current state on a wave form. We can create this name-to-state mapping in VHDL and in Verilog (SystemVerilog’s approach is similar to VHDLs).

In VHDL we create a state type that lists the names of the states. Then we declare our state variables to be of the state type. The code looks like this:

Lines 45-50 specify a VHDL type called “state_type” and create values that can be placed in that type. This serves two purposes: it makes the state machine easy to read, and it lets the compiler catch cases where you mistyped the value. This is more difficult to do if you were using raw numbers for your state types.

Notice that this code doesn’t specify the coding for the state machine; it leaves the coding up to the synthesis tool, or up to the needs of the designer. A designer could tell the synthesis tool to code this state machine as a one-hot if it was necessary to catch single event upsets or allow for error-correcting codes. Or the designer could have the synthesis tool use a binary encoding for a more compact solution.

We can do a similar thing with Verilog:

This code uses Verilog parameters to attach names to numbers, and then later code can use the names to put the numbers into the state diagram. The numbers in this example are suggested encodings for the states. Most synthesis tools can override these state encodings to create whichever encoding the designer wants to see.

For completeness, here is the same thing in SystemVerilog. Notice that SystemVerilog has adopted VHDL’s numberless approach:

All of these ways of writing the state machine make it much easier to write the next-state logic and the output logic. Here is the next state logic for this state machine written in Verilog and VHDL:

This is very easy to read—much easier than if we hand named the states “s0,” and “s1” as I have seen in some code. Now, when you come back to this state machine in six months, you’ll remember what you were doing when you designed it.

Today we looked at ways to create synthesis-friendly state machines that are easy to understand and reuse. In the next article, we will look at coding Mealy and Moore state machines and our options for dividing next-state logic and output logic.