Abstract: First, briefly introduce each functional module and its characteristics of the system-level RF transceiver chip nRF24E1. Then, analyze the working principle of the cordless phone and introduce how to use nRF24E1 to realize the wireless reception and transmission of voice signals in the cordless phone. Finally, some experiences in practical application are given.
introduction
The nRF24E1 transceiver and nRF24E2 transmitter are two system-level chips introduced by Nordic VLSI, adopting advanced 0.18μs CMOS process, 6mm & TImes; 6mm 36-pin QFN package; based on the nRF2401 / 02 chip structure, the RF, 8051MCU , 9 input 12-bit ADC, 125 channels, UART, SPI, PWM, RTC, WDT are all integrated into a single chip; there are internal voltage high-speed (operating voltage 1.9 ~ 3.6V) and VDD voltage monitoring, usually switching time is less than 200μs, It has a data rate of 1Mbps and an output power of 0dBm; it does not require an external SAW filter. It is the world's first, world-wide 2.4GHz universal, completed low-cost radio frequency system-level chip. nRF24E1 / nRF24E2 is suitable for wireless mouse and keyboard, wireless handheld terminal, wireless frequency identification, digital video, remote control and automotive electronics and other short-range wireless high-speed applications.
1 nRF24E1 function introduction
The block diagram of nRF24E1 is shown in Figure 1.
(1) Microprocessor
The instruction system of the nRF24E1 microprocessor is compatible with the industry standard 8051 instruction system, but the instruction execution time of the two is slightly different. Generally, the execution time of each instruction of nRF24Ex is 4-20 clock cycles, while the execution time of each instruction of industry standard 8051 is 12-48 clock cycles. nRF24E1 increases ADC, SPI, RF receiver 1, RF receiver 2, 5 wake-up timer interrupt sources, and 3 timers same as 8051 than industry standard 8051. NRF24E1 contains a UART that is the same as 8051. Under the traditional asynchronous communication method, timer 1 and timer 2 can be used as the UART baud rate generator. In order to facilitate data transfer with the external RAM area, the nRF24E1 CPU also integrates 2 data pointers. The clock of nRF24E1 microcontroller comes directly from the crystal oscillator. 
There are 256 bytes of data RAM and 512 bytes of ROM in the microprocessor. After power-on reset or software reset, the processor automatically executes the code in the boot area of ​​the ROM. The user program is usually loaded from the EEPROM into a 4KB RAM under the guidance of the boot area. This 4KB RAM can also be used to store data. If the mask ROM (that is, the included ROM) is not used in the application, the program code must be loaded from the external nonvolatile memory. It is more common to extend the EEPROM of model 25320 through the SPI interface.
In order to control some functions not available in the standard 8051, nRF24E1 adds some special function registers, such as RADIO (P2), ADCCON, ADCDATAH, ADCDATAL, ADCSTATL, ADCSTATIC, PWMCON, PWMDUTY, etc. The P0 and P1 are also different from the standard 8051, and other special function gaps are the same as the standard 8051.
(3) PWM and SPI interface
nRF24E1 has a programmable PWM output. When in use, the function of DIO9 (ie P0.9) can be changed through the program, and the PWM can be programmable to work in 6, 7, or 8 bits.
The three ports of SPI (Serial Peripheral Interface) are reused with GPIO (DIN0, DION0 and DION1) and RF transceiver. The SPI hardware does not generate any chip select signals, and usually uses the GPIO bit (P0 port) as the chip select port of the external SPI device.
(3) RTC wake-up timer, WTD and RC oscillator
There is a low-power RC oscillator in nRF24E1. The oscillator cannot be disabled. When VDD ≥ 1.8V, it operates continuously. RTC wake-up timer and WTD (watchdog) are two 16 programmable timers, and their working clock is LP OSC of RC oscillator. The timing of wake-up timer and watchdog is about 300μs ～ 80ms, and the default value is 10ms.
(4) A / D converter
There are 9 channels of 10-bit ADC in nRF24E1, and the linear conversion time is 48 CPU instruction cycles per 10 bits. The 9 inputs of the A / D converter can be selected by software. Channels 0 to 7 can convert the voltage values ​​on the corresponding pins AIN0 to AIN7 to digital values, usually 8 is used to monitor the operating voltage of nRF24E1. The A / D converter works in 10-bit mode by default, and it can work in 6-bit, 8-bit or 12-bit mode through software.
(5) Wireless transceiver
The nRF24E1 transceiver communicates with other modules through an internal parallel port or an internal SPI port, and has the same functions as the monolithic RF transceiver nRF2401. The data preparation signal output by the DuoCeiver receiver can be made an interruption of the microprocessor through the program or passed to the CPU through the GPIO port. NRF240x works in the global open 2.4-2.5GHz frequency band. The transceiver consists of a complete frequency synthesizer, a power amplifier, an adjuster and 2 receivers. The output power, channel and other RF parameters can be controlled by programming the special function RADIO (0xA0). In transmit mode, the RF current consumption is only 10.5mA, and in receive mode it is 18mA. In order to save energy, the transceiver can be turned on / off through a program.
2 Cordless phone
The cordless telephone system works based on two analog audio channels. Therefore, the wireless transceiver system of the cordless telephone must be able to simulate these two analog audio channels with high fidelity.
The audio frequency of the voice is generally between 300 and 3400 kHz, which is similar to the working frequency of the fixed telephone. When digitizing voice, at least an 8-bit processor is required for processing. In general, the sampling frequency of voice is 6.8kHz. In actual use, the sampling frequency is often set to 8kHz, so the working rate of each audio channel is at least 64kb / s. The increase in sampling frequency is conducive to improving the sound quality and reducing the requirements for preprocessing, but at the same time, the requirements for the data processing rate are correspondingly higher.
According to the radio frequency communication protocol, the audio information is divided and packaged according to the address at the sending end, and the data is checked and integrated at the receiving end, which can prevent eavesdropping and improve stability and security. To regenerate the audio signal from the digital signal, the simplest method is to filter the digital signal and process it with pulse width modulation. Regenerating 8-bit audio signals requires an 8-bit pulse width modulator. The higher the carrier frequency of the pulse width modulator, the lower the filtering requirements after reception. 
3 Application of nRF24E1 in cordless telephone
(1) Basic principles
nRF24E1 meets the performance requirements required for cordless phones:
â‘ Built-in 10-bit A / D converter can be used for audio sampling and battery monitoring;
â‘¡2.4GHz RF transceiver, unique ShockBurst communication method;
â‘¢ Provide 8-bit PWM output for D / A.
Audio pre-filtering, post-filtering and amplification must be performed off-chip. The basic principle of data transmission with cordless telephone using nRF24E1 is shown in Figure 2.
(2) A / D conversion
The sampling signal of the on-chip ADC of nRF24E1 is stored in the microcontroller 8051 before there is not enough RF data packet. After the sampled data packet is full, the 8051 stores the next new data packet while converting the full data packet to the RF front end. In the ShockBurst communication mode, each packet is an audio sampling signal of 24 bytes or 3ms. When there is no audio signal input, you can reduce the output or only output very short status information. In this way, not only reduce the task of data transmission, but also reduce system power consumption. In system design, this energy-saving method must be implemented at the sending end.
(3) Radio frequency communication
The radio frequency connection must be able to guarantee a data rate of 64 kb / s in both directions, and requires that the connection is full-duplex, that is, both transceivers can work simultaneously. Due to the ShockBurst feature, all protocol-related operations are handled by hardware. although
nRF24E1 uses a low-speed 8051 microcontroller, but the wireless transmission speed can reach 1Mb / s. After initial configuration, nRF24E1 can control the RF transceiver.
The time slot is determined by the sampling frequency (8kHz = 125μs). Each time slot, A / D must be read once, and the PWM value is also updated once. The master-slave transceiver must be synchronized (handshake) before data transmission. The data packet used by RF can be set to 248 bits (8-bit pilot signal + 16-bit CRC + 32-bit address + 24 bytes of useful data), therefore, each data contains 24 sampling signals. In order to meet the real-time requirements, it must be sent once every 3ms.
(4) D / A conversion
When the RF front end receives a valid data packet and the microcontroller receives an RF reception interrupt, the valid data part of the received data packet can be separated by the I / O gap of the RF front end, and then the separation The valid data part is stored as the output signal of PWM. The PWM output is driven by an 8-bit PWM engine and does not require a microcontroller to share processing tasks. The maximum carrier frequency of the PWM modulator in nRF24E1 is 64kHz, which is easy to filter after data reception.
4 Conclusion
Proven by use, nRF24E1 is very suitable for use as a transceiver module for cordless phones, it has the following advantages:
* nRF24E1 is embedded with 8051, which is easier to reduce the volume;
* No need to write code to compress audio signal;
* It is easier to achieve high-quality, safe communication;
* The 2.4GHz transmission and reception frequency band is a global open frequency band;
* Battery supervision is more convenient and power consumption is low. With 120mAh battery, it can reach 13 hours of talk time or 1200 hours of standby time;
* The GPIO of nRF24E1 makes it easier to expand other functions, such as volume control and LED indication.
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