Application of wireless transmitter chip A7105 in RF short-distance communication
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1: Comparison of the three mainstream RF solutions and their advantages and disadvantages
1): Bluetooth solution (IEEE802.15)
Bluetooth is a radio technology that supports short-distance communication between devices (generally within 10m). It can exchange wireless information between many devices including mobile phones, PDAs, wireless headsets, laptops, and related peripherals. Using "Bluetooth" technology can effectively simplify the communication between mobile communication terminal devices, and can also successfully simplify the communication between devices and the Internet, so that data transmission becomes faster and more efficient, broadening the road for wireless communication. Bluetooth uses a decentralized network structure, fast frequency hopping and short packet technology, supports point-to-point and point-to-multipoint communication, and works in the globally common 2.4GHz ISM (i.e. industrial, scientific, and medical) frequency band. Its data rate is 1Mbps. It uses a time-division duplex transmission scheme to achieve full-duplex transmission.
The biggest feature of the information age is the more convenient and faster information dissemination. Based on this, technicians are also working hard to develop better information data transmission methods. Bluetooth is not just a simple technology for mobile phones and the entire IT industry, but a concept. When the Bluetooth Alliance made a solemn promise to the future, the entire industry was shocked. Throwing aside the constraints of traditional connections and thoroughly enjoying the fun of freedom, the promise that Bluetooth gives us is enough to cheer us up.
The Bluetooth protocol allows data to be transferred between 1 master device and up to 7 slave devices at a maximum transfer rate of 723 kbit/s. However, the actual rate will be less than this value.
Gaussian frequency shift keying (GFSK) modulation mode uses 83 1Mbps channels in the 2.4G band. GFSK uses Gaussian filtering on the baseband signal before sending it to the carrier. It can smooth high levels ("1") and low levels ("0"), and can provide a narrower and "cleaner" spectrum for the transmitted signal compared to the direct method of frequency shift keying (FSK).
There are three basic power levels for Bluetooth devices: Class 1 (100m line of sight), Class 2 (10m) and Class 3 (2-3m). Currently, Class 2 devices are commonly used.
Each device in a Bluetooth network has a unique 48-bit identification number. The first device to identify (usually within 2 seconds) becomes the master device, and is then set to use the frequency band 1600 times per second. All other devices in the network will lock and synchronize with this master device. The master device transmits in the even time slot, and the slave device responds in the odd time slot. The slave device in the network will be assigned an address and listen to its own time slot and address information.
Slave devices can also enter low power "detection", "hold" and "stop" modes. In detection mode, the device only listens in the designated detection slot, but maintains synchronization. In hold mode, the device listens to determine whether it needs to be activated. In stop mode, the device abandons its address. Although holding and stop modes can extend battery life and save power, it also means that the device loses synchronization and re-establishing the link will require waiting time, which can take several seconds, which is undoubtedly a disadvantage if the user requires a quick response.
2): ZigBee (IEEE802.15.4)
Zigbee is synonymous with the IEEE 802.15.4 protocol. The technology specified by this protocol is a short-range, low-power wireless communication technology. The name comes from the bee's figure-eight dance. Since bees (bees) rely on flying and "buzzing" (zig) to shake their wings to communicate with their companions about the location of pollen, that is, bees rely on this method to form a communication network in the group. Its characteristics are short distance, low complexity, self-organization, low power consumption, low data rate, and low cost. It is mainly suitable for use in the fields of automatic control and remote control, and can be embedded in various devices. In short, ZigBee is a cheap, low-power short-range wireless networking communication technology.
In the process of using Bluetooth technology, people found that although Bluetooth technology has many advantages, it still has many defects. For the fields of industry, home automation control and industrial telemetry and remote control, Bluetooth technology is too complex, power consumption is high, distance is short, and the network scale is too small. However, industrial automation has an increasingly strong demand for wireless data communication. Moreover, for industrial sites, this wireless data transmission must be highly reliable and resistant to various electromagnetic interferences in industrial sites. Therefore, after long-term efforts, the ZigBee protocol was officially launched in 2003. In addition, Zigbee uses the communication protocol Home RF Lite for home networks that it has studied before.
This standard is defined as IEEE802.15.4, which is also a simple data protocol with high reliability. This includes a response mechanism to notify each transmission and other technologies to maintain the reliability of information. ZigBee does not require the synchronization function of Bluetooth, so it reduces power consumption to a certain extent.
Like Bluetooth, ZigBee operates in the ISM 2.4GHz band (5MHz 16 channels). This standard also provides versions in the European 868MHz (single channel) and US 915MHz (2MHz 10 channels) bands. It guarantees a maximum rate of 250kbit/s.
3): Proprietary solution (A7105 solution, similar to nRF solution)
The proprietary solution adopts the channel mode of Bluetooth. The proprietary solution divides the frequency band between 2.400~2.483GHz into 166 channels with a bandwidth of 500KHz, while Bluetooth is divided into 83 channels and ZigBee is 16 channels (see Figure 2). Compared with Bluetooth and ZigBee, this allows the A7105 proprietary solution to have more available frequencies when encountering interference from crowded frequency bands.
Interference handling
All three wireless technologies, Bluetooth, ZigBee and proprietary schemes, have mechanisms to reduce interference from RF devices operating in the same frequency band.
Bluetooth has a frequency hopping spread spectrum (FHSS) mechanism that ensures even coverage of 79 1MHz channels to avoid constant channel interference.
ZigBee uses its 16 frequency bands to deal with narrowband interference, so it is more susceptible to interference when there are other 802.11b/g devices present, which may require waiting for other devices to stop sending.
The proprietary solution takes a more flexible hybrid approach. Because of its modest output power, interference is unlikely to occur. To minimize current consumption and complexity, the proprietary solution does not use spread spectrum mode and simply transmits at a single frequency until a packet arrives when interference is encountered. If the frequency needs to be changed during transmission, it is simply a single-byte command sent via SPI.
With 166 500KHz channels, frequencies can be reallocated to avoid transmission frequencies used by other devices. Even in "hot spots" such as airports, frequency reallocation is not frequent, within a few minutes or even hours.
Comparison between the proprietary solution and the other two solutions
First, the disadvantages of using Bluetooth and ZigBee solutions: First, in order to comply with the standard, you have to meet the standard, which will cost you high NRE fees to start designing and testing compatibility. Second, due to its nature, the standard must be a "one size fits all" solution - in an increasingly competitive global market, your competitors have the same technology as you, and it is difficult to distinguish the advantages of your product. Finally, the standard provides little design flexibility; for example, the work of reducing power consumption on your RF product will be limited.
Advantages of proprietary solutions: low cost, nRF proprietary solutions do better than Bluetooth and ZigBee in terms of requiring a product to have long battery life, reliable communication, and low duty cycle.
Explanation of related nouns:
FSK (Frequency-shift keying) is an early modulation method used in information transmission. Its main advantages are: it is easy to implement and has good noise and attenuation resistance. It has been widely used in medium and low-speed data transmission. The most common is the dual-frequency FSK system that uses two frequencies to carry binary 1 and 0. Technically, there are two classifications of FSK, incoherent and coherent FSK. In incoherent FSK, the instantaneous frequency shifts between two discrete values named mark and space frequency, respectively. On the other hand, in coherent frequency shift keying or binary FSK, there is no discontinuity in the output signal.
Gaussian frequency shift keying GFSK - Gauss frequency Shift Keying, is to limit the spectrum width of the signal through a Gaussian low-pass filter before modulation. GFSK Gaussian frequency shift keying modulation is a digital modulation method that pre-modulates and filters the input data through a Gaussian low-pass filter and then performs FSK modulation. While maintaining a constant amplitude, it can control the spectrum of the modulated signal by changing the 3dB bandwidth of the Gaussian low-pass filter. It has the desired characteristics of wireless communication systems such as constant amplitude envelope, concentrated power spectrum, and narrow spectrum. Therefore, GFSK modulation and demodulation technology is widely used in many fields such as mobile communications, aviation and marine communications.
2. Main performance of RF IC (A7105)
Working frequency: 2400~2483MHz ISM band (global license-free band). Working distance: within 10m;
Channel distance: 500KHz, total number of channels: ~160, that is, the frequency range that can be effectively set is 2400~2483MHz, and a channel can be set every 500KHz interval. Different RF devices in the same space can be set to work in different channels through frequency hopping to reduce interference.
Low receiving power consumption: 500Kbps@16mA; Low sending power consumption: 0dBm@19mA; Sleep current: <1μA; Output power: 0dBm; Sensitivity: -110dBm@2.5KBPS, -104dBm@25KBPS, -97dBm@250KBPS, -93dBm@500KBPS, Data transmission rate: up to 500Kbps; Basic applications: mouse, keyboard, toys, etc.
Data transmission rate: up to 500Kbps;
Basic applications: mouse, keyboard, toys, etc.
3: RF system diagram and interface definition with MCU
For RF IC-A7015, its control is through SPI (3 or 4 wire) serial interface operation to read or write data (SCS, SCK, DIO or GIOx). If you want to use 4 wire serial interface, first make sure to use GIO1 or GIO2 pin for SPI data out.
Interface pin description between MCU and A7105:
SCS: SPI enable;
SCK: SPI clock signal;
SDIO: SPI data signal;
GIO1: multiplexed signal input/output 1, SPI data1;
GIO2: multiplexed signal input/output 2: SPI data2;
MCU and RF IC communicate via SPI. The format of SPI is as follows:
Four: Two data transmission modes of RF IC (A7105)
RF IC working mode: There are two working modes, one is direct mode and the other is FIFO mode. Different working modes can be set by the corresponding registers during initialization.
Direct mode: Provides users with an RF channel. The system at the Tx end transmits data to the RF DATA IO PIN. The RF only modulates the data and then transmits it. The RX end uses digital demodulation to restore the data.
FIFO mode: The timing is as follows:
1): Tx data transmission timing: first use SPI to write data into Tx FIFO (up to 64 bytes can be written), write command to make RF IC enter Rx mode, start transmitting data, and return to the original state after the transmission is completed.
2): Rx data transmission timing: Write command to make RF IC enter Rx state. When the same ID CODE is received, PIN RX_SYN will be set to 1. At this time, the received data begins to be written into Rx FIFO. After completing a data packet reception, it automatically leaves Rx and returns to the original state.
5. Implementation method of RF communication between A7105 and MCU
1. How to pair two RF ICs (link):
Before two RF ICs can communicate, they must be paired (Link) first. The two RF ICs use the same ID and channel when transmitting and receiving data, so that they can communicate normally.
When pairing, usually the Master and Slave use the same frequency. For example, when the Master is used as Tx, the frequency is set to 2.405GHz, and when the Slave is used as Rx, the frequency is set to 2.4055MHz. That is, Tx should have a higher bandwidth (500KHz) than Rx.
The steps of Link are as follows:
Host (Key/mouse side)
On the slave side, only when entering the pairing mode, it enters rx_mode to detect whether the ID code is received. If received, the working state is switched to Tx_mode, and the default data is sent to the host, indicating that the pairing is OK, and the received RF ID is saved.
2. Related processing of RF anti-interference
1): The difference between frequency hopping and spread spectrum
The frequency hopping step is 20MHz.
Direct Spread Spectrum: Direct Sequence Spread Spectrum (DS) is a method of spreading the signal spectrum at the transmitting end using a high-speed spread spectrum sequence, and despreading the signal at the receiving end using the same spread spectrum code sequence to restore the expanded spread spectrum signal to the original signal.
The direct sequence spread spectrum method directly modulates the carrier with a pseudo noise sequence. The data information to be transmitted needs to be channel coded and then modulated by a modulo 2 sum with the pseudo noise sequence to generate a composite code to modulate the carrier. After receiving the transmitted signal, the receiver first captures the precise phase of the transmitted pseudo code through the pseudo code synchronization capture circuit, and then generates a pseudo code phase that is completely consistent with the pseudo code phase of the transmitter as a local despread signal, so that the data information can be restored in time and the signal reception of the entire direct spread communication system is completed.
Frequency hopping: Frequency hopping technology is completely different from direct sequence spread spectrum technology and is another kind of spread spectrum. The carrier frequency of frequency hopping is controlled by a pseudo-random code. Within its working bandwidth, its frequency synthesizer continuously changes the frequency according to the random law of the PN code. At the receiving end, the receiver frequency synthesizer is controlled by the pseudo-random code and maintains the same change law as the transmitting end.
Frequency hopping is a spread spectrum in the sense that the carrier frequency continuously jumps within a certain range, rather than spreading the spectrum of the transmitted information, and will not obtain the processing gain of direct sequence spread spectrum. Frequency hopping is equivalent to an instantaneous narrowband communication system, which is basically equivalent to a conventional communication system. Since it cannot resist multipath and has low transmission efficiency, the effective transmission distance of a frequency hopping system with the same transmission power is smaller than that of a direct sequence spread spectrum system. The advantage of frequency hopping is anti-interference, and fixed frequency interference will only interfere with some frequencies. When used for the transmission of voice information, when fixed frequency interference only accounts for a part, it will not have a great impact on voice communication.
The hopping speed directly reflects the performance of the frequency hopping system. The higher the hopping speed, the better the anti-interference performance. The military frequency hopping system can reach tens of thousands of hopping per second. In fact, the mobile communication GSM system is also a frequency hopping system, and its specified hopping speed is 217 hopping per second. Due to cost considerations, the hopping speed of commercial frequency hopping systems is very slow, generally below 50 hopping/second. Since the slow hopping frequency hopping system can be easily implemented, low-speed wireless LAN products often use this technology.
2): Two methods for RF IC to achieve anti-interference in communication
Normally, frequency hopping in the strict sense is only used in military and high-end GSM and other wireless communication systems. For low-cost RF communication systems, because the transmission distance itself is short (within 10M), the mutual interference is small, so Tx can be allowed to transmit multiple times until Rx receives and returns the receiving flag. Another method is to use limited frequency hopping, that is, after the data is transmitted, it is determined whether Rx has received it. If not, the transmission frequency is changed (for example, increase 20MHz), and then a synchronization signal is sent to Rx, and then the transmission continues. This method is simple to implement, but the anti-interference performance is worse than the strict frequency hopping method.
3. How MCU communicates through RF IC
MCU can be divided into two types according to its memory type: the type without on-chip ROM and the type with on-chip ROM. For the chip without on-chip ROM, an external EPROM must be connected to it for application (typical chip is 8031). The chip with on-chip ROM is further divided into on-chip EPROM (typical chip is 87C51), MASK on-chip mask ROM (typical chip is 8051), on-chip FLASH (typical chip is 89C51), etc. Some companies have also launched chips with on-chip one-time programmable ROM (One Time Programming, OTP) (typical chip is 97C51). MASKROM MCU is cheap, but the program is solidified at the factory, which is suitable for applications where the program is fixed; FALSHROM MCU program can be repeatedly erased and written, which is very flexible, but the price is higher, which is suitable for applications that are not sensitive to price or for development purposes; OTPROM MCU price is between the first two, and at the same time has one-time programmable capability, which is suitable for applications that require both flexibility and low cost, especially electronic products with constantly updated functions and need to be mass-produced quickly.
After the two RF chips are paired, they can be used to transmit data. Rx first demodulates the received signal and then confirms it with the RF ID code stored in itself. After the judgment is consistent, it starts to store the received data and verify the data according to the corresponding FEC or CRC bit. After the reception is completed, the MCU reads the FIFO of Rx to get the transmitted data.
There are two modes for transmitting data: Direct mode and FIFO mode.
6. Working status and mutual conversion of RF IC (A7105)
A7105 RF CHIP has 6 main states, sleep state, STB state, WPLL state, TX state, CAL state.
1. SLEEP state: When entering the sleep state, the chip's internal reference voltage source (band gap) and crystal oscillation circuit will be turned off.
2. STB state: STB state includes IDLE mode, Standby mode and PLL mode. RF IC enters any mode according to strobe command.
1) In IDLE mode, the IC internal reference voltage source is turned on, while the crystal and PLL are turned off.
2) Standby mode: When power is on or reset, the RF IC will enter standby mode. At this time, the regulator, reference power supply and crystal are turned on.
3) PLL mode: IC internal reference voltage source, oscillation and PLL are both turned on.
3. WPLL (waiting PLL) state: When any state enters the TX/TX state, it will enter this state or directly bypass the state and enter the Tx/Rx state based on the current state or whether the setting value of the control register PLL I, PLLII, PLL III, PLL IV changes.
4. TX/RX state: RF IC will decide to enter TX or RX state according to the strobe command.
When using RFO mode and working in TX STATE, the RF chip will automatically transmit the TX packet (Preamble+ID+TX FIFO payload). When the transmission is completed, the RF chip will return to the original state.
When using FIFO mode to work in RX state, the RF chip will enter RX state to wait for Tx data. If the Tx end does have transmitted data, when the ID code is correct and the set data length is received, the RF chip will automatically return to the original state.
5. CAL state: In the CAL state, there are three independent calibration items. In the STB state, when the corresponding registers are enabled, the IF filter calibration, VCO band, and VCO current calibration procedures will be entered. After completing the calibration procedure, the corresponding flag bits are cleared and the system returns to the STB state.
7: Matters needing attention in the application of RF IC (A7105)
1. A7105 operating current and operating voltage, and how to enter power saving mode
When the IC works in Rx state, the maximum current consumption is 16mA; when working in Tx state, the maximum current consumption is 19mA;
RF output maximum power: 0dbm;
After transmitting/receiving data, if the system wants to save power, it can enter SLEEP mode with a current consumption of <1uA.
After the RF IC completes its work, the MCU can directly send a Strobe command to put it into the SLEEP state. However, it should be noted that before entering the SLEEP state, TWOR or WOR must be set first.
2. Working process of power saving mode (Introduction to TWOR and WOR)
TWOR (Wake up On Radio using Timer): The RF IC has a built-in timer and a low-speed RC OSC. When the system wants to save power, it can put the system into the SLEEP state. After that, at regular intervals, a signal is sent to wake up the MCU to resume working status and process RF events. This is the TWOR function.
WOR (Wake up On Radio): It also uses the built-in timer or low-speed RC OSC of RF IC. When the time is up, it only wakes up the RF IC to enter the working state. The MCU will not be woken up to receive data unless a signal is received.
3. Data transfer rate:
The minimum transmission rate is 10KBps and the maximum is 500KBps. However, when transmitting data, the closer to the upper limit of the transmission rate, the lower the receiving sensitivity of the Rx end. That is, when the minimum transmission rate is met, appropriately reducing the transmission rate can improve the Rx receiving sensitivity and expand the receiving distance.
4. Operating frequency and number of operating channels of RF IC (A7105)
The operating frequency of A7105 is 2400~2483MHz, that is, when initializing Tx/Rx, the operating frequency must be set within this range.
The bandwidth occupied by each channel is 500KHz, that is, there are about 160 channels in the frequency range of 2400~2483.
5. When setting the Tx and Rx operating frequencies and switching between them during operation, you should always ensure that the frequency of Rx is one intermediate frequency (500KHz for A7105) different from that of Tx, which is the same as other RF systems.
6. Application scope of A7105
As a low-cost RF transmission/reception solution, A7105 is used in applications where the amount of data transmitted is relatively small, the cost is low, and the standby current is small, such as wireless remote control, wireless mouse, wireless keyboard, and wireless joystick.
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