Bluetooth baseband protocol principle

Summary of the main principles of Bluetooth technology

People use more and more electronic devices, and with the increase of information exchange between electronic devices, the connection and winding of cables has become very messy. In order to save cables and simplify the connection between devices, it is necessary to design a technology to eliminate the "last" connection.

In 1994, in a research project called "Multi-Communicator Link (Multi-Communicator Link)" by the Mobile Communications Department of Sweden's Ericsson, engineers discovered a low-frequency wireless band that can be used without permission, and developed A small wireless transceiver chip, using radio frequency technology to achieve low-cost, low-power wireless interconnection between mobile phones and surrounding devices, they named this interconnection technical specification Bluetooth (Bluetooth).

1. The concept of Bluetooth

Bluetooth (Bluetooth) technology is actually a short-range wireless communication technology. The use of "Bluetooth" technology can effectively simplify the communication between mobile communication terminal devices such as handheld computers, notebook computers and mobile phones, and can also successfully simplify the communication between these devices and the Internet, making these modern communication devices and Internet data Transmission becomes faster and more efficient.

Bluetooth technology has the following obvious technical characteristics: it can transmit voice and data at the same time; it uses a universal frequency band; low cost, low power consumption and low radiation; it can be applied to various electronic devices; it has network characteristics, etc.

2. Bluetooth device structure

The principle is to embed a small and low-power radio transceiver chip into traditional electronic equipment. The Bluetooth chip includes a radio transceiver and a link controller (LC). The wireless transceiver is the core of the Bluetooth device, and the radio frequency band used is between ISM2.4GHZ and 2.48GHZ. The control connection consists of two parts: software connection - Link Manager (LM) and hardware - Link Controller (LC). LM executes link setting, supervision and configuration; it is responsible for connection, establishment and teardown of links and security control. LC implements data sending and receiving. Logical LC and adaptation protocol have the functions of completing data disassembly, controlling service quality and multiplexing protocol. This layer protocol is the basis for the realization of other layers of protocols. Figure 1 shows the main operation and functions of the wireless transceiver. The Bluetooth link controller implements the baseband communication protocol and related processing procedures. Figure 1 also summarizes the main functions of the baseband, which is responsible for frequency hopping and the transmission of Bluetooth data and information frames.

3. Bluetooth baseband layer protocol system

1. Bluetooth frequency hopping technology

Frequency hopping technology is the data sent on each time slot in the physical channel, constantly jumping from one channel to another. The master device and the slave device will hop from the current channel to the next channel according to the same frequency hopping sequence as each other. The frequency hopping sequence is determined by the 48-bit BD_ADDR (Bluetooth device address) address in the master device.

2. Bluetooth device address

There is a unique 48-bit BD_ADDR (Bluetooth Device Address) address in the Bluetooth device. This address can be said to be the computing core of Bluetooth technology. Almost all control parameters responsible for the normal operation of the Bluetooth system, such as frequency hopping sequence, channel access code, and encryption key, are obtained from this address. The addresses involved in the device still include AM_ADDR (Active Member Address) active member address, PM_ADDR (Parked Member Address) waiting member address, and AR_ADDR (Access Request Address) access request address.

3. Bluetooth data transmission type

Bluetooth technology can send voice and data at the same time, because Bluetooth technology supports both circuit switching and packet switching data transmission methods. In the Bluetooth technology standard, the transmission of circuit switching is called SCO link, and the transmission of packet switching is called ACL link.

SCO Link Connection-Oriented Synchronous Transmission (Synchronous Connection-Oriented, SCO) link belongs to the synchronous transmission type of circuit switching. Circuit switching means that once the connection between the master device and the slave device is established, the system will reserve time slots at fixed intervals for the master device and the slave device regardless of whether there is data to be sent, and other slave devices cannot use the time slot on this connection to send data. SCO is a point-to-point symmetric connection, that is, the connection is established between a master device and a slave device. SCO is more suitable for voice transmission.

ACL link connectionless asynchronous transmission (Asychronous Connection-Less, ACL) link belongs to the asynchronous transmission type of packet switching. Packet switching is to cut the high-level data into segments for switching. The ACL link can occupy any time slot to transmit data, but it can only transmit on the time slot not used by the SCO link. The ACL link is suitable for transmitting bursty data information, and its master device can establish ACL links with multiple slave devices at the same time, which is a point-to-multipoint asymmetric connection.

4. Bluetooth piconet and scatternet

After two Bluetooth devices establish a connection, a personal area of a piconet is formed. Each piconet has one and only one master device, and one or more slave devices at the same time, and they can switch roles with each other. Each piconet can only have 7 active slave devices, because in the Active state, the master device assigns an active member address AM_ADDR to each connected slave device, and the master device uses this address to identify different slave devices in the piconet equipment. AM_ADDR consists of 3 bits, so there can only be up to 8 devices in a piconet. In other words, up to 7 slave devices are active. Each piconet has a maximum of 255 sleeping slaves. Because the Bluetooth device entering the Park state from the Active state will get a PM_ADDR address, and PM_ADDR is composed of 8 bits, so it can accommodate up to 256 Bluetooth devices in the Park state. A piconet consists of a shared channel through which its members communicate. This channel consists of an explicit frequency hopping sequence, which is tracked by the members of the piconet in a synchronized manner and controlled by the master device.

In order to connect more than 8 active devices, multiple micro-networks must be established, and then the main device of each micro-network must be connected. This combined structure is a scatternet. Scatternets overlap in space and time. A slave device in one microgrid can be a slave device of multiple microgrids, or a master device of another microgrid, which makes communication between microgrids possible. Because there are only 79 frequency points, a scatternet has at most 10 microgrids.

5. The working status of the Bluetooth device

Bluetooth devices have different working states in different occasions. There are two main working states: connection state (Connection State) and waiting state (Standby State). When it is connected with other devices, it is called the connection state. At this time, the master device and the slave device use the same channel access code and the same frequency hopping sequence to communicate with each other. When it does not interact with other devices, it is called a waiting state. At this time, the device operates with the default system timing CLKN, and consumes very low power. Before the device enters the connection state from the waiting state, the device needs to perform a series of signal query and call procedures. The states where inquiries and calls are made are called intermediate states. Figure 4 shows the transitions between the three states.

(1) Bluetooth device connection status:

In order to save power consumption and reduce interference to other users, the slave device does not transmit data for a long time. When it wants to connect with the master device in the master-slave network, the slave device can choose to enter different connection states.

In the active (Active) state, the slave device is basically always listening to the transmission signal from the master device. In the Active state, the slave device has the AM_ADDR address and the same frequency hopping sequence as the master-slave network. Since the Active state has been receiving packets and is ready to send packets at any time, this state provides the fastest response. But it also consumes the most power.

The slave device is activated periodically in the Sniff state. The master device periodically sends packets to the slave device at a certain time interval, and the slave device only needs to receive the signal sent by the master device within these time intervals, but the slave device still retains AM_ADDR and the same frequency hopping sequence as the master-slave network. Compared with Active, Sniff mode consumes less power and responds slower.

In the hold (Hold) state, the slave device completely stops listening to packets within a specified time interval, which is determined by the agreement between the master device and the application program in the slave device, and the slave device will resume the original model. In Hold mode, the slave device will temporarily stop supporting the ACL link, but still supports the SCO link, so the slave device still has the AM_ADDR address and the same frequency hopping sequence as the master-slave network. The response in Hold mode may be slower than Sniff mode, but it can save more power.

In the dormant (Park) state, the slave device remains synchronized with the frequency hopping sequence of the master device, but is not active (the slave device in Active, Sniff, and Hold modes is considered active). In Park mode, the slave device will discard the AM_ADDR address and get the PM_ADDR and AR_ADDR addresses from the master device. In the master-slave network, the slave devices in Park mode have a specific PM_ADDR address, but AR_ADDR may be the same as other slave devices. When the master device wants to wake up a slave device in the Park state, it sends the PM_ADDR address of the slave device on the broadcast channel BC, and at the same time specifies the AM_ADDR address of the slave device after it is called the Active state. Through the broadcast channel BC, the master device can wake up multiple slave devices in the Park state at the same time. When the slave device wants to recover from the Park state to the Active state, it also requests the master device with the AR_ADDR address on the broadcast channel BC. After the master device receives it, it sends a control signal to wake up the slave device from the Park state.

(2) The intermediate state of the Bluetooth device:

When the master device does not know whether there are slave devices around, it must query the state to get the BD_ADDR address and internal timing of all the surrounding slave devices, and then enter the call state to connect with the slave devices. If the master device already knows the slave device to be connected, it can directly enter the call state to connect with the slave device.

In the figure, the device in step 1-3 enters the query state; in step 4-5, the slave device enters the query response state after receiving the query signal. After the end, the master device has received the FHS packet responded by the slave device, including the BD_ADDR address of the slave device , internal timing and device type; step 6-7, the master device enters the call state and establishes a connection with a specific slave device, but at this time the timing of the master and slave devices is not synchronized; step 8-10, the slave device enters the call after receiving the call signal Response state, return the ID packet as a response; Step 10-13 After receiving the ID packet, the master device enters the master device response state, and then sends a FHS packet to inform the master device of the BD_ADDR address, connection member address and other information. An ID packet is returned, the connection between the master and slave devices is successfully established, and both devices enter the connection state.

Summarize

Bluetooth technology has a wide range of applications and has been applied to various fields, such as various data and voice devices in local area networks: PCs, dial-up networks, notebook computers, printers, fax machines, digital cameras, mobile phones and high-quality earphones. The Bluetooth wireless communication method connects the above-mentioned devices into a piconet (Piconet), and multiple piconets can be connected to each other, and various devices can communicate anytime and anywhere.

The development of Bluetooth is not the development of one industry, but the common development of multiple industries. It needs the promotion of various industries to have a longer-term development. With the development of the times and the improvement of technology, the development of Bluetooth technology has a bright future. Bluetooth will have a significant impact on our life and work.

The above are the details of the Bluetooth baseband protocol principle introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. If you have Bluetooth product design and development needs, you can trust us. We have rich experience in custom development of smart electronic products. We can evaluate the development cycle and IC price as soon as possible, and can also calculate the PCBA quotation. We are a number of chip agents at home and abroad, including MCU, voice IC, BLE Bluetooth IC, dual-mode Bluetooth module, and wifi module. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth development, wifi technology, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the research and development of electronic education products.