“Not only is the code readable, but so is the schematic. In many cases, the schematic diagram is not only for yourself, but also for others. If the readability is poor, it will bring a series of communication problems. Therefore, it is necessary to develop a good habit and make a standardized schematic diagram. In addition, a good schematic will also consider testability, maintainability, BOM normalization, etc.
Not only is the code readable, but so is the schematic. In many cases, the schematic diagram is not only for yourself, but also for others. If the readability is poor, it will bring a series of communication problems. Therefore, it is necessary to develop a good habit and make a standardized schematic diagram. In addition, a good schematic will also consider testability, maintainability, BOM normalization, etc.
As shown in the figure above, use lines to divide the entire schematic into areas, and write functional descriptions in each area, such as: power supply, STM32, etc.
This allows people to understand the entire schematic more clearly and quickly, and it is easy to find the circuit according to the problem during debugging and maintenance.
2 Label key parameters
As shown in the figure above, the maximum output current is marked, so that it is convenient for others to know whether the power supply can bear the load when modifying the circuit.
Other parameters can also be written, such as: input voltage range, applicable temperature range, and even truth tables in digital circuits.
3 Notes on Resistors/Capacitors/Inductors/Beads
As shown in the figure above, each resistor is written with the resistance value and accuracy. For high-power resistors, the power can also be written, depending on the specific situation.
Generally, 1% accuracy is required for the sampling resistor on the switching power supply and the resistor on the op amp, and 5% accuracy can be used for the pull-up and pull-down resistors.
Note: The resistance value here is not recommended to be written as 102, but directly written as 1K. Try not to let others do this conversion, or others won’t do it.
As shown in the figure above, the capacitance and withstand voltage of each capacitor are written. For high-precision capacitors, the accuracy or material can also be written.
Such as: ceramic capacitor materials are X7R, Y5V, NP0 and so on.
Note: The capacitance value here is not recommended to be written as 105, but directly as 1uF.
Write the inductance value and saturation current for each Inductor as shown in the figure above.
(4) Magnetic beads
As shown in the figure above, each magnetic bead is written with the resistance value and the corresponding frequency.
(5) Other components
Other components are also similar to resistors, capacitors, etc., such as: crystal oscillator 8MHz 50ppm, etc., to draw inferences from other facts.
As shown in the figure above, increase the L1 inductance so that it can be disconnected during maintenance and easier to troubleshoot. Inductors, magnetic beads, or 0R resistors can be used here, depending on the situation.
But there are also special cases. If the load is particularly large, the required power of the components connected in series is very large, and the cost increases too much, and it is not worth it. In this case, it can be omitted.
If a chip such as a QFP64 package is connected behind, the power is relatively small, and components can be connected in series, because there will be more cases of poor QFP soldering.
The BOM table is the bill of materials. Try to reduce the types of materials as much as possible, which can reduce the workload of buyers and reduce many problems in production.
There are two pull-up resistors, one is 4.7K and the other is 10K. If this resistance has little effect, they can be combined into 10K.
Decoupling capacitors are also the same operation, to draw inferences from other facts.
6 Symbols for power and ground
As shown in FIG. For dual power supply systems, write a plus or minus sign on the power supply symbol, and for a single power supply system, you can write only a plus sign.
But don’t use VCC, otherwise others will have to observe how many V is powered.
As shown in the figure above, there is only one ground plane, so use GND. If there are digital ground and analog ground, use AGND and DGND.
There are also some systems with video ground, audio ground, etc., which also use different symbols.
Note: Do not hide the network names such as GND, it will be prone to problems.
7 test points
As shown in the image above, test points have been added. The test point is also a circular pad with bare copper.
In some QFP, BGA, QFN packaged chips, some pins are difficult to measure with an oscilloscope. At this time, test points can be added to facilitate operation.
8 Network label
As shown in the figure above, PC7 and PC6 are the IIC interfaces connected to OLED12864. The OLED prefix is added to the network label here to reduce the conflict of the network label and increase the readability. Similarly, the network connected to the temperature sensor can write DS18B20_DATA, and the component name is added to the network label. Other chips operate in the same way.
Use capital letters for all network labels.
9 Fault Tolerance/Compatibility
In the early stage of design, or inadvertently, or the construction schedule is too fast, there is not so much time to study whether the connection method on the circuit is correct.
At this time, some reserved circuits can be used to improve the fault tolerance of the entire board.
As shown in the figure above, assuming that the engineer is not sure whether it is RX to TX or RX to TX, four resistors can be used to implement both connections. (NC is not connected)
When R11 and R14 are soldered, and R12 and R13 are not soldered, it is the connection method of RX to RX and TX to TX.
Without soldering R11 and R14, when soldering R12 and R13, it is the connection method of RX to TX and TX to RX.
After the debugging is passed, remove the four resistors and connect them with the correct connection method. This will not only guarantee the construction period, but also will not make mistakes.
If a board is not sure whether to use F103 of STM32 or F407, at this time, it can be made into a compatible design.
As shown in the figure above, a 0R resistor can be placed in the circle. When using F103, solder the 0R on, but not the capacitor. When using F407, solder the capacitor on, and 0R is not soldered.
Of course, other circuits can also be reserved.
Two characters, NC and NF, often appear on the schematic diagram, as shown in the figure below, which means no connection or welding.
NC=Not connect not connect.
NF=Not Fix does not install.
Of course, NC can also be expressed as normal close, which is often used in relays and contactors.
To be identified on a case-by-case basis.
Version 11 Changes
In many cases, a circuit is not successful in one version. It goes through many versions, and each version has changes. This should be clearly marked.
As shown in the picture above, it is clearly pointed out that the V2 version changed C12 to 10uF, so that in case of problems, it is easy to trace back.
12 floating pins
Floating pins should also be marked with an X.
In many cases, requirements are constantly changing. If you only design for the current needs, once there are changes in the future, you will have to re-design the board.
Therefore, it is necessary to add some reserved pins and circuits to quickly verify whether the function of the whole board meets the new requirements. As shown in the figure below, some IO ports are reserved.
Some interfaces are not foolproof, that is, there are two or more connection methods.
As shown in the figure above, there are two connection methods for 4P DuPont lines (together, not separated):
red, black, and yellow;
Yellow white black red.
Then, the socket is defined as shown in the above figure on the schematic diagram, then there will be a connection method that will cause the power supply to be reversed, which may burn the components.
If the schematic is designed like this, the components will not be burned, because the 3.3V power supply is only added to the GPIO port.
To be fool-proof, you can use a fool-proof seat, such as: USB socket, aviation interface, etc.
There is also another method, the symmetrical design method.
As shown in the figure above, the arrangement of pins is symmetrical, that is, no matter how it is connected, there is no problem, but the cost will increase.
There is also a method suitable for the interface of the DC power supply, which is generally used by the access control system.
As shown in the figure above, adding a rectifier bridge, no matter how +13.4V and PGND are connected, the correct +12V and GND can be generated on pins 1 and 3.
Of course, this method also considers cost and power consumption.
15 The flow of the signal
Some analog circuits need to indicate the flow direction of the signal.
As shown in the figure above, the signal flow in the directional coupler is indicated.
16PCB Routing Recommendations
If the PCB is not drawn by you, you can indicate the PCB routing rules or suggestions on the schematic diagram.
As shown in the figure above, the processing method of a pair of differential lines on the PCB is marked.
17 Not used means negated
Once “” is used, it may be blocked by the wire and then invisible, resulting in a possible incorrect network connection.
Consider using “#” to indicate negation.