Practical Tips | Why are schematics always difficult to draw? Here are 17 tips to know

It is not only the code that has to be readable, but also the schematic. In many cases, the schematic is not only for yourself, but also for others to see. If the readability is poor, it will cause a series of communication problems. Therefore, we must develop good habits and make a standardized schematic. In addition, an excellent schematic will also consider testability, maintainability, BOM normalization, etc.

As shown in the figure above, use lines to divide the entire schematic diagram into areas, and write functional descriptions in each area, such as: power supply, STM32, etc.

This allows people to understand the entire schematic diagram more clearly and quickly, and it is also easy to find the circuit based on the problem during debugging and maintenance.

As shown in the figure above, the maximum output current is marked, so that when others modify the circuit, they can know whether the power supply can bear the load.

Other parameters can also be written, such as: input voltage range, applicable temperature range, and even truth tables in digital circuits.

(1) Resistance

As shown in the figure above, each resistor has its resistance value and precision written on it. For high-power resistors, the power can also be written on it, depending on the specific situation.

Generally, the sampling resistors on the switching power supply and the resistors on the operational discharge circuit should use 1% accuracy, and the pull-up and pull-down resistors can use 5% accuracy.

Note: It is not recommended to write the resistance value as 102, but 1K directly. Try not to let others do this conversion, or they will not know how to calculate it.

(2) Capacitance

As shown in the figure above, each capacitor has its capacitance and withstand voltage written on it. For high-precision capacitors, you can also write the precision or material.

For example: the materials of ceramic capacitors include X7R, Y5V, NP0, etc.

Note: It is not recommended to write the capacitance value as 105, but to write it directly as 1uF.

(3) Inductance

As shown in the figure above, each inductor has its inductance value and saturation current written on it.

(4) Magnetic beads

As shown in the figure above, each ferrite bead has its resistance value and corresponding frequency written on it.

(5) Other components

Other components are similar to resistors, capacitors, etc., such as crystal oscillator 8MHz 50ppm, etc. You should learn by analogy.

As shown in the figure above, the L1 inductor is added so that it can be disconnected during maintenance, making it easier to troubleshoot. An inductor, a magnetic bead, or a 0R resistor can be used here, depending on the specific situation.

However, there are special cases. If the load is particularly large, the power of the components required to be connected in series is very large, and the cost increase is too much, which is not cost-effective. In this case, it can be omitted.

If the chip connected later is a QFP64 packaged chip with relatively low power, you can connect the components in series, because poor soldering of QFP is more common.

The BOM is a list of materials. Trying to have as few types of materials as possible can reduce the workload of buyers and also reduce many problems in production.

There are two pull-up resistors here, one 4.7K and one 10K. If the resistance value does not have much impact, you can combine them into 10K.

The operation of decoupling capacitors is similar, so you should learn from it.

As shown in the figure above, for a dual power supply system, you need to write the positive and negative signs on the power symbol, and for a single power supply system, you can only write the positive sign.

But never use VCC, otherwise others will have to observe how many V the power supply is when they see it.

As shown in the figure above, if there is only one ground plane, use GND. If there are digital ground and analog ground, use AGND and DGND.

Some systems also have video ground, audio ground, etc., which also need to use different symbols.

Note: Do not hide network names such as GND, as this may cause problems.

As shown in the figure above, a test point is added. The test point is a round pad made of bare copper.

In some QFP, BGA, and QFN packaged chips, some pins are difficult to measure with an oscilloscope. In this case, you can add test points to facilitate operation.

As shown in the figure above, PC7 and PC6 are IIC interfaces for OLED12864. The network number here adds the OLED prefix to reduce network number conflicts and increase readability. Similarly, the network for the temperature sensor can be written as DS18B20_DATA, and the component name is added to the network number. The same operation is performed for other chips.

All network labels use uppercase letters.

In the early stages of design, either carelessness or a tight deadline means there is not enough time to study whether the circuit connections are 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, if the engineer is not sure whether it is RX to TX or RX to TX, four resistors can be used to achieve these two connection methods. (NC means no connection)

When R11 and R14 are soldered but R12 and R13 are not soldered, the connection is RX to RX and TX to TX.

Without soldering R11 and R14, when soldering R12 and R13, the connection is RX to TX, and TX to RX.

After the debugging is passed, remove these four resistors and connect them in the correct way. This can ensure the construction period and avoid mistakes.

If you are not sure whether to use STM32 F103 or F407 on a board, you can make a compatible design.

As shown in the figure above, a 0R resistor can be placed in the circle. When using F103, solder the 0R resistor and leave the capacitor unsoldered. When using F407, solder the capacitor and leave the 0R resistor unsoldered.

Of course, other circuits can also be reserved, and we should learn from them.

The characters NC and NF often appear on schematic diagrams, as shown in the figure below, which means no connection or soldering.

NC=Not connect.

NF=Not Fix Do not install.

Of course, NC can also stand for normal close, which is mostly used in relays and contactors.

It depends on the specific situation.

Often a circuit is not successful in just one version. It will go through many versions, and each version has changes. These should be clearly marked.

As shown in the figure above, it is clearly pointed out that the V2 version changes C12 to 10uF so that it is easy to trace in case of problems.

Dangling pins should also be marked with an X.

Often, requirements are constantly changing. If you only design for current requirements, you will have to redesign once there are changes in the future.

Therefore, it is necessary to add some reserved pins and circuits to quickly verify whether the functions of the entire board meet 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 ways to connect them.

As shown in the figure above, there are two ways to connect 4P Dupont wires (combined, not separated):

• Red, black, white and yellow;

• Yellow, white, black and red.

Then, if the socket is defined as shown in the figure above on the schematic diagram, there will be a way of connecting the power supply in reverse, which may burn out the components.

If the schematic is designed like this, the components will not be burned out because the 3.3V power supply is only added to the GPIO port.

To prevent mistakes, you can use a fool-proof socket, such as a USB socket, aviation interface, etc.

There is also another method, the symmetrical design method.

As shown in the figure above, the pin arrangement is symmetrical, which means that no matter how you connect it, there will be no problem, but the cost will increase.

There is another method applicable to the DC power supply interface, which is generally used in access control systems.

As shown in the figure above, by 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 approach also needs to consider cost and power consumption.

Some analog circuits need to indicate the direction of signal flow.

As shown in the figure above, the signal flow in the directional coupler is indicated.

If you don't draw the PCB, you can mark the PCB routing rules or suggestions on the schematic.

As shown in the figure above, the processing method of a pair of differential lines on the PCB is indicated.

Once "" is used, it may be blocked by the wire and then cannot be seen, which may cause the network to be connected incorrectly.

Consider using “#” to indicate negation.