PCB design considerations of smart bracelet

As a popular product form in recent two years, the smart bracelet has attracted more and more attention. Although it cannot be accepted by all, its emergence has indeed made some changes in the PCB product market.

An intelligent bracelet is usually composed of RF circuit unit, clock circuit unit, memory circuit unit, sensor circuit unit and master MCU unit, while the circuit PCB is usually concentrated in a small range for single-sided or double-sided mounting, and the circuit board is mainly composed of four or six layers. The following figure shows the circuit PCB of the 37 degree smart bracelet found on the network.

Since so many functions are concentrated on a small PCB, we should pay special attention to the layout and wiring of the bracelet. Now we summarize some precautions for reference.

First of all, pay attention to the wiring protection in the partition layout.

From the PCB circuit board above, it can be seen that each part of the circuit of the smart bracelet (marked with boxes of different colors) has a good partition: since the smart bracelet is a collection of digital circuit components, as long as the matching resistance and capacitance are well distributed in the circuit design, the circuit modules with certain functions can be completed, which makes the circuit design more concise and easy to find. Although some sensor circuit units use analog circuit technology for data acquisition, once the module is designed as a module, data communication and information transmission can be completed through the corresponding connection interface.

In the layout of circuit modules, on the one hand, attention should be paid to the clock circuit and crystal oscillator circuit to reach the target through the shortest path; on the other hand, attention should be paid to avoiding data lines when routing the clock to prevent interference from affecting the stability of the system.

During PCB routing, it is necessary to protect the key routing, such as whether the clock generation circuit and crystal oscillator circuit are protected by copper coating, whether the ring ground protection is carried out, etc., which is generally protected in PCB design. For the crystal oscillator part, it is necessary to dig copper.

Secondly, handle the RF circuit well.

The smart bracelet needs to be linked with the mobile phone when it is used. Therefore, the radio frequency part is the key part. In this part of the design, special attention must be paid. Now the smart bracelets on the market are all wireless data transmission based on Bluetooth, so the emphasis is on the processing of Bluetooth RF. If the smart bracelet is only used for data transmission and does not need to transmit sound and music, then low-power Bluetooth is the best choice. In the design, the shape of the Bluetooth antenna, the antenna layout, and the shell material of the smart bracelet are all important factors that affect the performance of the smart bracelet. In the design process of smart bracelet PCB, an excellent RF antenna engineer is particularly important.

Thirdly, ESD protection shall be provided.

In the previous article, I also wrote some technologies and knowledge about circuit ESD. Today, I would like to talk about the importance of ESD to smart bracelet products. Different countries and regions have different standards for different products or the same product ESD requirements. In order to make the product pass relevant tests, ESD protection design shall be carried out during design, and ESD testing shall be carried out after the design is completed to ensure that the product can adapt to electronic testing in the local market.

Finally, the system upgrade interface is reserved.

Whether the function upgrade of a smart bracelet is convenient or not is of great significance to users or fans of smart bracelets. If the software of a product is upgraded at an appropriate time (it may be that the hardware design has reached some functions in the early stage and the software has not kept up with the PCB market, or it may be that some software bugs have been repaired), this indicates that the product is still in the process of continuous research and development, which is a good psychological compensation for users. Now, there are usually two ways to upgrade the software of the smart bracelet: through USB interface and wireless push. As for which method to adopt, it needs to be determined in the early planning and software and hardware planning.