A high-speed signal refers to a large difference in the level of each point on the transmission path. The distinction between high speed and low speed is not only related to the signal frequency, but also related to the length of the transmission path.

The higher the signal frequency, the shorter the length of the low-speed and high-speed watershed, and vice versa.

1.3 Hardware Design Process

1.3.1 Demand Analysis

(1) Overall performance requirements: such as packet forwarding capability, processing delay, maximum processing bandwidth, CPU processing capability, etc. In response to these requirements, the selection of CPU, memory, switching chips and other devices can be carried out initially.

(2) Functional requirements

(3) Cost requirements

(4) Interface requirements

(5) Power consumption requirements

1.3.2 Outline Design

Such as data packet forwarding capability, processing delay, maximum processing bandwidth, CPU processing capability, etc. In response to these requirements, the selection of CPU, memory, switching chips and other devices can be carried out initially.

1.3.3 Detailed design

The electronic design engineer is responsible for the definition of each bus interface signal, CPU storage space allocation, clock and reset circuit device selection and its topology, interrupt link topology, and detailed block diagram of the power supply circuit (the generation mode of each power supply, voltage value, current value, etc.), the filtering method of key power supply, the function of logic device and its register manual, the definition of user interface on the panel and the connection relationship of interface signal, the selection of indicator light device and its connection relationship, and finally draw the schematic diagram and Generate a bill of materials. Later in the detailed design phase, the development of the test plan should begin.

1.3.4 Debugging

General steps for board debugging:

(1) Appearance inspection

(2) Static impedance test

(3) Static power supply test

(4) Clock test

(5) Functional module test

1.3.5 Testing

(1) List of test equipment

(2) Construction diagram of the test environment

(3) Power supply test

(4) Signal integrity and timing of each interface signal

(5) Functional test of each common interface

(6) Test of reset link

(7) Clock-related tests such as crystal oscillators, clock drivers, and phase-locked loops

(8) Miscellaneous tests such as indicator lights, board in-position signals, slot numbers, etc.

(9) Flow test

1.3.6 Production change

1.4 Schematic design - more standardized

(1) On the first page of the schematic diagram, draw the overall frame diagram of the board, the power architecture block diagram, the clock topology diagram, the reset link topology diagram, the interruption link topology diagram, the boundary scan link diagram, etc. If there are many ports on the board, add port descriptions. If the topology of the I2C bus is complex, add the address of each I2C device.

(2) Near the output end of the power circuit on the schematic diagram, the voltage value and current value of the power supply should be marked (note the current value and show it to the PCB engineer).

(3) Annotate the layout requirements when drawing the schematic diagram.

(4) Draw the capacitive filter circuit of the schematic diagram according to the arrangement order of the capacitors on the PCB.

(5) The rate and routing layer of key signals should be marked on the schematic diagram. If there is a relationship between the lengths of the signal lines, it is also recommended to mark it on the schematic diagram.

(6) High heat dissipation and heat-sensitive devices should be marked on the schematic diagram. If there are special placement requirements, they can also be noted on the schematic diagram.

(7) On the schematic diagram, annotations should be made on the jumper wires and the configuration method of the welding device.

2.1 Resistance

2.1.2 Resistor Application Points

(1) Resistance value;

(2) size;

(3) Rated power; use at least 20% derating;

(4) Accuracy, when used to set the working parameters of the device, select a high-precision resistor, such as 1%.

2.2 Capacitance

2.2.2 The function and analysis of capacitance

1. The role of capacitors in high-speed circuits

(1) Charge buffer pool

The load of the power supply in the high-speed circuit changes dynamically, and the current and power consumption of the high-speed operating device are constantly changing. In this case, it is necessary to provide a buffer pool for the device to provide a stable voltage for the device when the current consumption changes.

(2) Important release path for high-frequency noise

The state of the high-speed circuit is constantly switched between 1 and 0, and the direction of the current is constantly switched between output and input. The high-speed switching of the state will generate a lot of noise interference on the circuit. The frequency of the interfering signal is the 2nd and 3rd multiplication of the effective signal. These high-frequency interferences need to be released to a relatively stable ground plane on the power transmission path. Z=1/jwc, when the frequency is high, the impedance of the capacitor is low, which becomes an important discharge path for high-frequency noise.

(3) Realize AC coupling

Two devices interconnected through a high-speed differential pair work at different levels, and the transmitted differential pairs will carry DC components of different levels and cannot recognize each other. Capacitors can pass through AC and block DC to achieve the function of filtering out DC components. Namely AC coupling (AC Couple), DC isolation (DC Blocking).

AC coupling (AC Coupling) is through the DC blocking capacitor coupling, remove the DC component　　

DC coupling (DC Coupling) is straight-through, AC and DC pass together, it does not remove the AC component.

In AC coupling, capacitors are connected in series in the line with an impedance of 1/jwC. The smaller the capacitance, the greater the impedance to the low-frequency signal, which seriously attenuates the low-frequency signal.

Define Tc as the data cycle per bit, NUM as the maximum allowable number of 0 or 1 bits, the load impedance is R (generally 50Ω), and C is the capacitance of the AC coupling capacitor. Then there is an empirical formula:

F=800MHz, Tc=1.25ns, R=50Ω, according to the maximum length of 0 or 1 bits that may appear in the code analysis application, if the maximum connection is 85bits, set NUM=86, the minimum AC coupling capacitance The value requirements are:

Cmin=7.8*86*1.25ns/50Ω=16.77nF=0.01677nF

In the design, choosing a coupling capacitor of 0.01μF obviously cannot satisfy the empirical formula, resulting in data frame errors. When designing, it should be noted that the value of the coupling capacitor should not be too large. If the value is too large, it will not be able to meet the edge slope requirements of high-speed signal conversion. In high-speed design, the capacitance value of the coupling capacitor is generally taken as 0.1μF, which can not only meet the possible long 1 long 0 situation in the data frame, but also meet the requirements of high-speed signal conversion.

2. Equivalent circuit of capacitor

(1) Capacitive devices are not pure capacitors, but small circuits with components such as ESR, ESL, and Rleak.

(2) ESL depends on the type and package of the capacitor, and ESR depends on the working temperature, frequency, wire resistance and so on.

(3) In most cases, the smaller the ESR, the better. The high-frequency attenuation is small during DC coupling, and a low-impedance loop is provided for noise during filtering. Multiple capacitors connected in parallel can reduce ESR, which is equivalent to parallel ESR components. But there are exceptions, which need to be selected according to device requirements.

3. Characteristics of filter capacitor impedance changing with frequency

The function mechanism of the filter capacitor is to provide a low-impedance loop for noise and other interference. At the noise frequency point, the impedance of the filter capacitor is required to be small, that is, when the noise frequency falls near the resonance point, the filtering effect is the best.

According to the resonant frequency formula: F=(ESL×C)-1/2, the larger C and ESL, the lower the resonant frequency, that is, the worse the filtering effect of capacitance on high-frequency interference; the smaller C and ESL, the lower the resonant frequency The higher it is, the more suitable it is to filter out high-frequency interference.

(1) The impedance-frequency curve of a capacitive device is a smiling curve. The left side of the curve depends on the capacitance component, and the right side depends on the ESL component.

(2) When filter capacitors are connected in parallel to widen the low-impedance frequency band, not only the capacitance matching, but also the package matching must be considered. When multiple capacitors of the same type are connected in parallel, although the low impedance frequency band cannot be widened, the impedance at the resonance point can be reduced.

2.2.3 Capacitors commonly used in high-speed circuit design

1. Ceramic Capacitors

Its advantages are small size, low price, good stability, but small capacity. At present, the commonly used ceramic capacitors can have a small capacitance of tens of picofarads and a large capacitance of tens of microfarads. The X7R, X5R, etc. that are often mentioned are ceramic capacitors. The meaning of the symbols is as follows:

The ESR value of ceramic capacitors is generally small, which is suitable for high frequency filtering. The use of capacitors must be derated. X7R and X5R should be derated by at least 20%. Y5V is not recommended to be used in high-speed circuits and when the ambient temperature changes drastically.

2. Tantalum Capacitors/Polymer Tantalum Capacitors

Tantalum capacitors have the characteristics of good temperature characteristics, small ESL value, good high-frequency filtering performance, small size, saving PCB area, and large capacitance. Therefore, tantalum capacitors are generally used in occasions that require large-capacity capacitor filtering, such as filtering for high-energy-consuming devices such as CPUs.

The disadvantage of tantalum capacitors is their weak ability to withstand voltage and current. It is generally required that the working voltage of tantalum capacitors be derated by more than 50% relative to the rated voltage. Tantalum capacitors also have good self-healing capabilities. In one of the following three situations, the rated voltage of tantalum capacitors needs to be derated by more than 70% for use:

(1) The load presents strong sensibility;

(2) The series resistance is small;

(3) The transient current is large. It is easy to break down the metal tantalum dielectric, causing the tantalum capacitor to fail.

3. Aluminum electrolytic capacitors/polymer aluminum electrolytic capacitors

Aluminum capacitors have large capacity and high withstand voltage, but have poor temperature stability, poor precision, and poor high-frequency filtering performance, so they are only suitable for low-frequency filtering. Tantalum capacitors are not suitable for applications with large transient currents, in which case aluminum electrolytic capacitors are required. The voltage should be derated by at least 20%.

As the product usage time increases, the electrolyte inside the aluminum electrolytic capacitor will gradually dry up, the capacity will gradually decrease, the ESR will gradually increase, and the filtering effect will weaken. Therefore, in the selection of capacitors for high-speed circuit design, aluminum electrolytic capacitors should be avoided as much as possible.

4. OSCON capacitor

The appearance of OSCON capacitors resembles aluminum electrolytic capacitors. The advantages are that OSCON capacitors have a smaller ESR, better temperature stability than aluminum electrolytic capacitors, and a lower price than tantalum capacitors. The disadvantage is that for most OSCON capacitors, the pins are plugged in and the volume is relatively large.

In circuit design, the input and output parts of the DC/DC power supply often need to be equipped with tantalum capacitors with amplified capacity, and the cost is relatively high. In this case, an OSCON capacitor of the same value can be substituted at a fraction of the cost of the corresponding tantalum capacitor.

(1) Ceramic capacitors are small in size, low in price and good in stability, but small in capacity. Suitable for high frequency filtering.

(2) Tantalum capacitors have good temperature stability, small ESL value, good high-frequency filtering performance, small size, can save PCB area, and have large capacitance, but their ability to withstand impulse voltage and impulse current is weak.

(3) Aluminum electrolytic capacitors have large capacity and high withstand voltage, but have poor temperature stability, poor precision, and poor high-frequency filtering performance, and are only suitable for low-frequency filtering.

(4) In the application of capacitance, attention should be paid to the understanding of the impedance-frequency characteristic curve.

2.2.4 Decoupling Capacitors and Bypass Capacitors

The decoupling capacitor is used to provide a local "small pond" for the power supply of the device to ensure the stable operation of the device. Devices operating at high speeds will constantly generate rapidly changing charge demands. For this rapid demand, the power module cannot supply current to the device in time to supplement, and can only rely on the capacitors near the device to solve it. Capacitors can be understood as small ponds that are usually filled with water. Once the crops near the small pond lack water, they can be replenished immediately from the small pond without requiring help from distant water plants. The decoupling capacitor has another function, which is to provide a low-impedance path for the high-frequency noise generated by the high-speed operating device to flow into the ground plane nearby, so as to avoid these interferences from affecting other loads of the power supply.

The filter capacitor can be understood as the "big pond" of the power supply, which requires a larger capacity capacitor.

The function of the bypass capacitor is to provide a low-impedance path to the ground plane for the previous stage (such as interference such as high-frequency noise generated by the power supply), so as to avoid these interferences from affecting devices that are working at high speed.

It can be seen from the above description that there is no essential difference between decoupling capacitors and bypass capacitors. In terms of application, according to the formula Z=1/(2πF×C), where F is the operating frequency of the device, their role at high frequencies Both provide a low-impedance return path back to the ground plane for interference in the circuit.

Note:

(1) This article is mainly summarized in the book "High-speed Circuit Design Practice", plus some personal understanding. If there is any infringement, please contact to delete, if you like it, please like, follow and add collection.

(2) Many design cases are omitted in the summary. If you are interested, please move to the original book.

Application points of resistance and capacitance in high-speed circuit design

1.2 How to distinguish between high speed and low speed