Application of Single Chip Microcomputer Technology in Cordless Phones

　　introduction

　　Cordless telephone (CT) is a low-power duplex wireless telephone connected to the user end of the local exchange network. Generally, cordless telephones have all the functions of ordinary telephones, such as redial, pause, hang up, hands-free, memory, etc. However, since the cordless telephone is wirelessly connected between the mobile phone and the landline, it has some features that ordinary telephones do not have, such as channel switching and identity recognition.

　　Major developed countries abroad have invested a lot of manpower and material resources in the development and research of cordless phones, and quickly put them into commercial use. In just over a decade, they have developed from CT-0 to CT-3; from analog to digital; from indoor to outdoor public; from single-zone to multi-zone, and can perform cross-zone switching and roaming communications. Multi-channel scanning cordless phones (CT-1) have been commercialized in developed countries abroad since the mid-1980s and have developed rapidly.

　　1 System Block Diagram and Principle

　　The CT-1 system usually consists of a landline and a mobile phone. The landline and the mobile phone use a multi-channel connection to establish a wireless signal connection, and the working mode is heterodyne duplex.

　　The current internationally specified transmission frequency of cordless phones is 48/45 MHz, with a total of 10 channels. Due to external electromagnetic interference, some channels may not communicate well. Users can select the best channel by switching channels during use. The identity recognition function is very important in cordless phones. Early cordless phones often cause phone theft because they did not use single-chip microcomputer control. In the single-chip microcomputer system, this problem can be solved by setting passwords on the handset. This system uses a 16-bit password, and the number of passwords reaches 64K, which greatly improves the security of the system.

　　By comparing the landline system block diagram (Figure 1), we can see that the main functions of the landline MCU are: (1) Receiving control code. The control code is sent by the mobile phone MCU, modulated and transmitted by the mobile phone, and then received by the landline and filtered and shaped before being input into the landline MCU. (2) Sending control code. The control code is output by the landline MCU, modulated and transmitted by the landline to the mobile phone. (3) Dual phase-locked loop (DPLL) control. Initialize the phase-locked loop, transmit phase-locked data (determine the system RF frequency), and realize the RF channel selection function. (4) Dual audio generator. In fact, it is a DTMF (dual tone multi-frequency) subroutine in the MCU. As a civilian product, it requires large quantities, low cost, and high quality, and various measures need to be taken to reduce costs. In fact, there are mature single-chip microcomputer products with hardware DTMF generators, but the price is relatively high. If software is used to generate DTMF signals, the same function can be achieved with an ordinary general-purpose MCU. (5) Ring detection. Detect whether there is a ring current coming from the local telephone network. (6) Relay control. Used to control line off-hook and pulse dialing. (7) Other signal generation and signal control. Such as channel number display control, transmitting circuit power control, transmitting circuit and receiving circuit audio blocking control, pulse/dual tone multi-frequency dialing mode conversion control, etc.

　　The system block diagram of a mobile phone is similar to that of a landline phone, except that the mobile phone has some unique modules. In addition to the functions of 1, 2, and 3 of the landline phone, the main functions of the MCU include mobile phone power control, keyboard scanning control, mobile phone ring signal generation, various LED controls (call indication, low voltage indication), charging detection, battery low voltage detection, etc.

　　Figure 1 Block diagram of landline phone

　　This article briefly describes two modules that both handsets and landlines have. One is the voice processing network, which consists of an audio filter and a compander. The former is usually composed of an op amp (such as LM324) to form a second-order low-pass network, and the latter uses the TA3 1101 integrated block. The other is a DPLL dual phase-locked loop. This article uses the MC145162 integrated block, which is a general-purpose programmable dual phase-locked loop and is mainly used in CT-1 products.

　　2 Introduction to the MCU COP840C

　　Unlike general industrial control systems, cordless telephone systems require low cost as civilian products, and low power consumption and low operating voltage as mobile communication products. The author uses the COP840C microcontroller from National Semiconductor (NS), which has the characteristics of low cost (suitable for civilian products) and low power consumption (suitable for handheld devices). The following is a brief introduction to it.

　　COP840C is an 8-bit microcontroller produced by NS. Its main features are: 8-bit CMOS processor; low power consumption, fully static; I/O and registers are mapped to data memory address space; 2 KROM, 128 Byte RAM and 23 I/O ports; variable, software-resettable I/O; 16-bit universal clock with an associated 16-bit auto-reload/capture register; three clock working modes: ①PWM; ②external event counting; ③independent capture register; three interrupt sources: external interrupt, clock interrupt and software trap interrupt. For detailed performance, please refer to the relevant manual of NS.

　　3 Interface Circuit

　　As a civilian communication product, the cordless phone is very different from the general industrial control system in terms of both the interface circuit and the entire system design. The differences are mainly reflected in: (1) The interface circuit is not complex and is not allowed to be complex. Especially as a mobile communication phone, its size, weight, power consumption, etc. have strict requirements, so the interface design strives to be simple. (2) Since the entire circuit system is a mixed analog and digital system, in some occasions, the signal processing such as signal shaping and filtering requirements are particularly strict.

　　3.1 Mobile Phone Circuit

　　Receiver power control. Usually, mobile phones use scanning reception to save power, that is, the power is intermittently turned on and off, and the power on-off ratio is usually 1:3-1:5. For example, if it is turned on for 200 ms and then cut off for 600 ms, this reception method can greatly increase the standby time, and the mobile phone can be kept on for 3 to 5 days (rechargeable battery pack at 270 mA). Since it is just a simple switch function, ordinary PNP transistors can be used as switch tubes. This simple control is used a lot, such as controlling various LED indicators, controlling the sending circuit, shutting off the receiving circuit (sound-blocking diode), and controlling the power supply of the transmitting circuit.

　　The battery voltage is low, so as to warn the user to charge in time. The circuit uses a dedicated integrated circuit KIA7834. When the power supply voltage is lower than 3.4 V, the 3rd pin of KIA7834 outputs a low level, which notifies the CPU and controls the low voltage indicator light at the same time.

　　Receive signal processing circuit. As mentioned above, in this system, the signal shaping and filtering circuits are particularly important. This article uses the operational amplifier contained in the frequency discrimination integrated circuit itself. Usually, the LM324 operational amplifier is also used to make a second-order filter to meet the requirements of sub-audio signaling of cordless phones.

　　3.2 Landline Circuit

　　Relatively speaking, the landline interface circuit is more complicated. First, at the receiving signal input end, the signal output from the frequency discrimination integrated block passes through the second-order infinite gain multi-feedback active low-pass filter composed of LM324, and its cut-off frequency is about 400 Hz. Then it enters the shaping circuit composed of LM324, which is actually a comparator, and its output is input to the CPU through the limiting circuit. During the debugging process, the filter parameters should be adjusted accordingly with the different signal baud rates to achieve satisfactory communication effects.

　　The CPU controls the 8-segment LED display by controlling the 74L S164 to generate the current channel number for mobile phone and landline communication. The 28th pin of the MCU is the relay control line. After the control signal is inverted by the transistor, it controls the on and off of the relay to achieve the line off-hook, landline use indicator light and pulse dialing. In wired telephones, the off-hook function is controlled by a dedicated switch (reed), and the pulse dialing function is generated by a dedicated integrated block.

　　The dual-tone multi-frequency signal generated by the CPU software method is a digital signal, and the discrete component D/A conversion method can be used. After D/A conversion, it should be connected to a second-order active filter and then transmitted to the local telephone line through a transformer. The filter must also be carefully debugged in the prototype stage so that the various indicators of the DTMF signal meet the national standard requirements. For incoming call ring current detection, an optocoupler device is used as a transmission element. When a ring current enters the line, the CPU detects a low level.

　　In this system, anti-interference ability is very important. There are high-frequency, low-frequency, analog and digital signals in the system, and various interferences will affect the operation of the system. The following measures are mainly taken: (1) High-frequency isolation, add a shielding cover to the high-frequency part, especially the transmitting part. (2) The power supply part is graded and filtered by modules to minimize the crosstalk between different functional blocks. (3) Pay attention to the layout of the circuit board design, especially the isolation of high-frequency and low-frequency as well as the isolation of analog and digital. [page]

　　4 Software Design

　　The main flow of the landline program is shown in Figure 2. The mobile phone program is similar and will not be described in detail. Due to space constraints, it is impossible to list the detailed flow of each module. The following mainly introduces the design concept and program of the landline DTMF module. The flow chart of this module is shown in Figure 3.

         Figure 2 Main flow of the landline program

         Figure 3 DTMF module flow chart

　　DTMF Introduction: It uses a pair of audio signals to mark one data. The pair of audio signals are sent and received from the high-frequency audio group and the low-frequency audio group respectively. The international regulations on the corresponding relationship between audio and data are shown in Table 1.

　　Table 1 Audio-data correspondence

　　The design idea of ​​the DTMF module is mainly: set up a data table in the CPU ROM to simulate various audio signals (including the frequency signals of the low-frequency group and the high-frequency group). When the program determines that a DTMF signal is to be output, first find the data corresponding to the high and low audio frequencies of the DTMF signal, and then combine them arithmetically. The synthesized value is output from the L0~L5 pins of the L port, and the DTMF signal is obtained after passing through a D/A impedance network. The DTMF signal output duration is 100 ms, and the synthesized value changes every 118 μs. The CPU timer is used to count the 100 ms signal width, and an interrupt is generated at 100 ms to terminate the DTMF signal output.

　　The following is a detailed introduction to the principle and process of the DTMF module. This module is divided into two submodules: KBDEC and DTMFLP. The former is used for DTMF keyboard decoding, and the latter is used for dual audio generation.

　　The KBDEC submodule decodes the low-true DTMF keyboard input and converts it into a corresponding DTMF code Code 1, whose binary form is 0000 RRCC, where RR and CC represent four row values ​​and four column values ​​respectively.

　　DTMFLP module. First, use Code 1 to check the ROM data table Table 1 to get four values ​​related to the DTMF key. These four values ​​represent the size and related starting address of the high and low audio ROM data table Table 2. Use these four values ​​to take out the high and low audio sine wave frequency values ​​from the ROM data table Table 2 every 117.33 μs, and output them to the L0-L5 pins of the L port after arithmetic combination. After passing through the external impedance ladder network, the corresponding DTMF signal is obtained. The high and low audio have different reference values ​​corresponding to the data in Table 2. The high frequency reference value is 16, and the low frequency reference value is 13. The difference 16-13=3 is necessary to meet the requirements of the DTMF signal: the high frequency group is 2 dB higher than the low frequency group to compensate for the transmission loss.

　　The generation of the data table in Table 2. Multiples of 117.33 μs can achieve a good approximation for the eight frequency components in the DTMF signal. Take 1 336 Hz as an example: there are 19 samples of three cycles in Table 2, so the period of its synthetic signal is [19×117.33 μs]/3 1 345.73 Hz. Compared with 1 336 Hz, the relative difference is +0.73%, which is within the national standard DTMF error range. As for how to determine the number of cycles and samples of each audio in Table 2, due to the limitation of ROM capacity, it is impossible to put too much data in the ROM, so the number of cycles is set to no more than 10, and the maximum error is set to 0.8%, for example, then a simple high-level language program can be compiled using the exhaustive method. According to the number of cycles and samples of each audio in Table 2, the data in Table 2 can be calculated. In fact, in order to obtain the best approximation during calculation, there is a so-called "sine wave area criterion".

　　5 Conclusion

　　This single-chip system uses COP840C to realize various control and communication functions of cordless phones, and uses software to generate DTMF signals, which reduces system costs. It also has the characteristics of good working reliability, low power consumption, and long standby time (up to 72 hours). Cordless phone products using this single-chip system have been put into mass production.