Foreword
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Car anti-theft, alarm functions and access control will become one of the essential functions of the car safety control system. Through the application of Maxim's transmitter MAXl472, receiver MAXl473 and microcontroller chip DS80C323, the principle and design scheme of a complete remote control door switch system is obtained. The system has significant advantages in terms of low power consumption , transmission and reception distance and reliability, and safety, which can double the effective control range of the RKE system.
The RKE system is important for improving the anti-theft and control of automobiles. Most RKE systems have car theft, alarm functions, and access control for cars and trunks, some of which also include remote start-ups and car-looking functions. The new generation of RKE is expected to adopt two-way communication. The remote keyless switch (PKE) installed in the car has been used as a standard configuration for the configuration of the car and has become an indispensable part of the car. The RKE system is important for improving the anti-theft and control of automobiles. Most RKE systems have car theft, alarm functions, and access control for cars and trunks, some of which also include remote start-ups and car-looking functions. The new generation of RKE is expected to use two-way communication, and the receiving device installed in the car will send data to provide drivers with information on fuel quantity and tire pressure.
The biggest challenge in RKE system design is to achieve low power consumption in RKE transmitters and receivers, while achieving long-distance communication and high reliability; especially as a switch access control system, security issues are also crucial. . This design proposes an implementation scheme of high security RKE system, and introduces the hardware principle circuit, software flow design and the principle of implementing code decoding.
1 overall system design
1.1 How the RKE system works
The RKE system consists of a keychain-like transmitter and a receiver mounted in the car. It usually operates in the IsM band of 315 to 450 MHz. The 868 MHz band was opened in Europe to meet the growing demand for remote door switch systems.
Figure 1 is a simple block diagram of the RKE system. As can be seen from the block diagram, the user presses the button switch on the key fob to trigger the system to work, wake up the CPU inside the RKE key fob, and the CPU sends the data stream to the radio frequency (RF) transmitter. The data stream is usually 64 to 128 bits long, including 1 preamble, 1 command code, and 1 rolling code, with a transmission rate of 2 to 20 kHz. The RKE RF receiver in the car captures the RF signal and demodulates it, transmits the data stream to cPu, decodes the data by cPu and sends the command to the command module. The modulation method is amplitude keying (AsK), the main purpose is to extend the battery life of the key fob.
1.2 RKE system design requirements
The key to RKE system design is to achieve a low cost system with stability, reliability and confidentiality at low current consumption. Therefore, the system's design requirements for power consumption, transmission and reception distance, reliability, and confidentiality are critical.
(1) Power management
For the transmitter, the battery takes 3 to 5 years of life; for the receiver, battery life is equally important. Because the receiver must remain active at all times, it listens for the transmission of user data. Typical specifications require that the average current does not exceed 1 mA. One way to solve this problem is to keep the receiver working for a significant period of time, ensuring that the time is long enough to determine if there is a legitimate transmission; the receiver sleeps for the rest of the time, and the receiver must Ability to quickly wake up to maximize the use of stored energy.
(2) Transceiver distance and reliability
RKE applications require good transmission and reception distances and reliable transmission. Increasing the sensitivity of the receiver and the power of the transmitter (the current consumption is not significantly increased) directly affects the transmission and reception distance and reliability. Obviously, low cost is a requirement because millions of such systems need to be installed.
(3) Security
The communication data of the RKE system should be confidential and not easily stolen by others. Early use of fixed cryptography is easy to crack; recent RKE systems are gradually implemented with integrated circuits with hopping coding, which greatly improves security.
2 hardware circuit design
The RKE system consists of a keychain launch module and an in-vehicle receiver module.
2.1 Keychain Launch Module
The key ring launching module consists of a push button switch, a CPU, a radio frequency transmitter and a button battery. The circuit principle is shown in Figure 2. The module is powered by a 3 V button battery. Table 1 shows the component parameter values ​​at different frequencies for a 50 Ω output, the value of which is affected by the PCB layout.
(1) button scan
The transmitting module is connected to three buttons, which are respectively used as locking, unlocking and searching functions, and are respectively connected with three external interrupts INTO, INT1 and INT3 of the microcontroller DS80C323. Pressing any button wakes up the DS80C323 and enters the appropriate interrupt handler. After processing, re-enter standby mode. The three LEDs display the status of the three buttons. When a button is pressed, the corresponding LED will be illuminated.
(2) Microcontroller DSSOC323
The DS80C323 is a low-power fast microcontroller from Maxim that is fully compatible with the 80C51 series in terms of external circuit connections and operating instructions. The DS80C323 has six external interrupts and features power supply fault management with an operating voltage range of 2.7 to 5.5 V.
The function of the DS80C323 is to scan the button with its external interrupt, and encrypt and encode the result of the scan, and send it to the data terminal DATA of the transmitter through P1.O. The DS1000C323's P1.1 controls the wake-up of the transmitter.
(3) RF transmitter MAX1472
The MAXl472 is a VHF/UHF phase-locked loop-based ASK/00K transmitter that operates in the 300-450 MHz band and supports data rates up to 100 kbps. When operating at a voltage of 2.1 V, it leaves the single-cell Li-Ion battery and consumes only 100 nA in standby mode. When matched to a 50-Ω system, the MAXl472's power amplifier provides an output level of +10 dBm and remains above 43. %s efficiency. The MAXl472 transmitter is ideal for applications where low cost, high capacity, and volume are key factors.
Once the enable pin level of the MAX1472 is active, the MAXl472 requires only 250μs for the PLL and crystal to stabilize and transmit data. The MAX1472 uses a crystal-based PLL that avoids many of the common problems with LC-based filtering or SAW transmitters. The inherent inherent crystal frequency accuracy requires a narrower receiver IF bandwidth to improve system sensitivity. With the MAXl473, the IF bandwidth can be reduced from 600kHz to 50 kHz, resulting in a 9 dB sensitivity improvement. The improved sensitivity means that the RKE system can achieve longer distance transmission and higher transmission reliability.
2.2 In-vehicle receiving module
The in-vehicle receiving module is composed of a radio frequency receiver, a microcontroller and an automobile command execution mechanism. The RF receiver demodulates the received 00K modulated data into original data; the microcontroller decodes and decrypts the original data to obtain valid instruction information, and sends it to the instruction executing unit, and the instruction executing unit completes the corresponding action.
Considering that the in-vehicle receiver module is always in operation, the microcontroller still uses the fast low-power DS80C323, and the DS80C323 controls the receiver to be in an intermittent sleep state with active/sleep alternates.
For the RF receiver, use the MAXl473 paired with the transmitter MAXl472. The MAXl473 is a fully integrated, low-power , CMOS super-heterodyne ASK receiver operating in the 300-450 MHz band with a high sensitivity of 114 dBm and image carrier rejection above 50 dB. The chip consumes less than 1.5μA in shutdown mode and 5.2 mA in receive mode. The MAXl473 accepts data rates up to 100 kbps with a transition time from shutdown mode to valid data output of less than 250 μs. The MAXl473 includes a Level 1 Automatic Gain Control (AGC) circuit that reduces the gain of the low noise amplifier (LNA) by 35dB when the RF input signal level is greater than a 57dBrm. The receiver uses an on-chip phase-locked loop (PLL) with a received signal strength indication (RSS 10.7 MHz IF filter with integrated voltage-controlled oscillator VCD) and a baseband data recovery circuit. The MAXl473 requires very few external components. The RF front end of the RKE system receiving module is shown in Figure 3.
3 system software process design
The software process of the transmitter is shown in Figure 4.
The receiving module is divided into three working states according to the tasks to be processed: the listening state of the wake-up and the sleep alternately, the state of receiving the data, and the state of processing the received data. The receiving system should always be in an intermittent sleep state to listen to incoming information. When a valid communication command is received, the system is triggered to receive data. After the data is received, the data is processed in a related manner, and the executing mechanism completes the corresponding operation, and the receiving system re-enters the listening state.
4 system code decoding design
In general, a one-way RKE system consists of a control terminal (keychain transmitter module) and an actuator (in-vehicle receiver module). The control terminal transmits and modulates the control information, and the execution end receives, demodulates, decodes and performs corresponding operations according to the control information. The key to the safety of a one-way RKE system is coding. Early use of fixed passwords is vulnerable to "wireless interception" and is easily cracked. Here, the use of hopping cryptography technology can effectively avoid "wireless interception" and improve security. The following describes the system codec design principles.
The system implements the codec process through the microcontroller DSC80C323 software programming.
The encoding process is shown in Figure 6(a). The encoder detects the key input, wakes up the system from the power saving state, adds the synchronization count to l, and encrypts the key with the serial number to form the ciphertext data, and sends the data with the key value and the like. Since the sync count value is different each time it is sent, even if the same button is pressed multiple times. The sync count is automatically scrolled forward and the transmitted codewords will not occur again. The scrolling range is 216 count values. When a new key is pressed during the transfer, the current transmission is terminated and a new transfer is initiated; otherwise, whether or not the button has been released, the transmission is completed and goes to sleep.
The decoding process is shown in Figure 6(b). After receiving the data packet, the decoding circuit separates the key value from the ciphertext, and decrypts the ciphertext with the key to restore the serial number and the synchronization counter value, and after driving the serial number and the synchronization counter value, drives the corresponding value according to the key value. Executive agency.
Conclusion
     Car anti-theft, alarm functions and access control will become one of the essential functions of the car safety control system. Through the application of Maxim's transmitter Max1472, receiver Max1473 and microcontroller chip DS80C323, the principle and design scheme of a complete remote door switch system is obtained. The system has obvious advantages in terms of low consumption, transmission and reception distance and reliability, and safety, which can double the effective control range of the RKE system. At present, we are continuing our efforts in this regard and strive to open up a broader application space.
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