Embedded hardware - read schematic

Learn the first lesson of hardware : Learn to read schematic diagrams

It is very important for embedded software engineers and programmers to understand the schematic diagram . Please note before going into details , For all Embedded designers 、 It is very important to understand the schematic diagrams and symbols created and used by hardware engineers to describe their hardware design . No matter how complex the hardware design is , No matter how much practical experience you have in designing real hardware , These diagrams and symbols are the key to understand complex hardware design quickly and effectively . They also contain information about devices that are compatible with embedded programmer design and hardware , And teach programmers how to successfully communicate with hardware engineers about the hardware requirements of software .

The following describes the hardware diagrams used in several different types of projects .

• block diagram :

  Block diagrams usually present the main components of a board at a system architecture or higher level ( processor 、 Bus , Input / Output 、 Memory ) Or a component ( Such as processor ). In short , The square diagram is a basic overview of the implementation details of the hardware . Although the block diagram can reflect the actual physical layout containing these main components , But it mainly presents how different components or units in components work as a whole at the system architecture level . It is the simplest way to describe and reproduce the components of a system . The symbols used in the block diagram are simple , For example, a square or rectangle is used to represent a chip , The bus is represented by a straight line . Block diagrams often lack sufficient detail 、 It is not enough for the software designer to write all the underlying software that can accurately control the hardware ( Without going through a lot of painful debugging 、 test 、 Error or even burn down the hardware ). However , They are very useful in conveying a basic overview of the hardware 、 It also provides a basis for creating more detailed hardware diagrams .

​ chart 1 AMD/ National Semiconductor Corporation x86 Reference board

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• Schematic diagram :

  The schematic diagram is to provide all devices in a circuit or the interior of a single component ( From the processor to each resistor ) More detailed view of the circuit diagram . The schematic diagram does not present the physical layout of the circuit board and components , Instead, it provides data flow information in the system , Define the specified flow direction of various signals - which signals are transmitted in different lines on the bus , It finally appears on the pins of the processor , wait . In the schematic diagram , Use Schematic symbols To describe all the components in the system . They usually don't look like the physical devices they represent , Surface is a simplified representation of physical devices based on some kind of schematic symbol standard . When trying to determine the actual working principle of the system or by debugging the hardware 、 When writing and debugging software to manage hardware , Schematic diagrams are the most useful for both hardware and software designers .

​ chart 2 Schematic example

​ chart 3 Common schematic symbols

• Wiring diagram :

  This diagram is used to show the bus connection relationship between the primary and secondary components on the circuit board or inside the chip . In the wiring diagram , The vertical and horizontal lines are used to represent the signal lines of the bus , And use schematic symbols or simpler symbols ( Physically similar to on-board components or in-component units ). These diagrams can represent an approximate description of the physical layout of components or circuit boards .

• Logic diagram :

  Logic diagrams use logic symbols ( And 、 or 、 Not 、 Exclusive or etc ) And logic input / Output (1 and 0) To represent various circuit information . It does not replace schematic diagrams , But it can help simplify certain types of circuits to understand how they perform their functions .

• Sequence diagram :

  The sequence diagram shows the sequence diagram of various input and output signals in the circuit and the relationship between these different signals . They are The user manual and data manual of hardware are the second most common after the square block diagram .

  No matter what kind of graph it is , In order to understand the picture , First, learn the standard symbols used in the diagram 、 Conventions and rules . surface 1 Examples of symbols used in the sequence diagram and the inputs associated with each symbol are shown in / Convention of output signal .

  ​ surface 1 Sequence diagram symbol table
  

chart 4 Is an example of a sequence diagram , Each line represents a different signal . About the symbolic expression of the rising and falling of the signal in the figure , The rise time is expressed by the time required for the signal to change from low level to high level , The falling time is expressed by the time required for the signal component to change from high level to low level ( The whole time that the slash in the symbol goes through ). When comparing two signals , The delay time is measured at the center of the rising edge or falling edge of the two compared signals . In the figure 4 In the first falling symbol signal in , On the signal B and C as well as A and C There is a delay of descent time between . Compare pictures 4 Medium signal A and B The first falling symbol of , The sequence diagram shows that there is no delay between the two .

chart 4 Example of sequence diagram

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