Patent Publication Number: US-5633630-A

Title: Watchdog circuit of vehicle tracking system

Description:
FIELD OF THE INVENTION 
     This invention relates to a watchdog circuit of a vehicle tracking system, and more particularly to a watchdog circuit of a vehicle tracking system for resetting a microcomputer by sensing a transmission time longer than a predetermined time in case of a malfunction occurring on a transmission device of a vehicle. 
     TECHNICAL BACKGROUND OF THE INVENTION 
     Generally, a vehicle tracking system comprises a central control station CCS, a plurality of base stations Sl-Sn, and equipment mounted on a vehicle as shown in FIG. 1. When a user requests the central control station CCS to inform him of a position of a vehicle, the central control station CCS transmits a call signal, including an identification number of the vehicle, through a call antenna. The equipment mounted on the vehicle transmits a signal showing its position to the central control station CCS after receiving the call signal including its identification number, and the central control station CCS informs the user of the position of the vehicle by determining the position after receiving the signal. The operation as described in the above statement is performed by a time sharing method as shown in FIG. 2, and every frame is allocated with an address of a vehicle subscriber. The equipment mounted on the vehicle shows its position when a predetermined time has elapsed after receiving a call signal. Because a transmission time is predetermined, the central control station CCS receives a transmission signal from the equipment mounted on the vehicle during a predetermined time for the vehicle. An emergency signal for notifying of an emergency situation of a vehicle is to be transmitted from a frame No. &#34;1&#34; as shown in FIG. 2. 
     Every frame of a plurality of vehicle subscribers is allocated with its exclusive address. The central control station CCS calls a series of addresses sequentially and receives a signal of position corresponding to a called address sequentially after a predetermined time has elapsed. Accordingly, if trouble occurs on the equipment mounted on the vehicle, the vehicle tracking system is not usable because the transmission time for a position signal is longer than a predetermined time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a watchdog circuit of a vehicle tracking system for resetting a microcomputer by sensing a transmission time longer than a predetermined time owing to malfunction of vehicle mounted equipment in the case of transmitting a position signal. 
     Another object of the present invention is to provide a watchdog circuit for preventing a malfunction of a vehicle tracking system. 
     According to the present invention, a watchdog circuit of a vehicle tracking system comprises a microcomputer 10, a code modulator and amplifier circuit 20, a high-frequency power amplifier 30, a first timer 40, an AND gate AN1, a driving circuit 50, a second timer 60 and a reset signal generating circuit 70. The microcomputer 10 controls the equipment mounted on a vehicle. The code modulator and amplifier circuit 20 modulates a PRN (Pseudo Random) code according to the control signal received from the microcomputer 10 and then amplifies the modulated signal. The high-frequency power amplifier 30 receives the amplified signal from the code modulator and amplifier circuit 20 and then amplifies the high-frequency component of the received signal, and transmits the high-frequency amplified signal through an antenna ANT1. The first timer 40 receives a power amplification control signal PAW outputted from the microcomputer 10 and counts the time (2T t ) for transmitting two frames, and then outputs the counted value. The AND gate AN1 receives the counted value outputted from the timer 40 and the power amplification control signal outputted from the microcomputer 10 and then generates a transmission interruption control signal. The driving circuit 50 receives the transmission interruption control signal from the AND gate AN1, thereby stopping an operation of the high frequency power amplifier 30. The second timer 60 receives the output signal of the first timer 40 and counts the time (T t ) for transmitting one frame, and then outputs the counted value. The reset signal generating circuit 70 receives the output signal of the second timer 60 and the power amplification control signal PAW outputted from the microcomputer 10 and then supplies a reset signal into the reset terminal of the microcomputer 10. 
     The present invention will now be described more specifically with reference to the drawings attached only by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a conventional vehicle tracking system; 
     FIG. 2 is a timing diagram of transmission and reception of a conventional vehicle tracking system; 
     FIG. 3 is a circuit diagram of the vehicle tracking system according to the present invention; and 
     FIGS. 4A through 4G are operation waveforms of the circuit of FIG. 3 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a microcomputer 10 controls equipment mounted on a vehicle, and a code modulator and amplifier 20 modulates and amplifies a pseudo-random code (PRN) according to a control signal of the microcomputer 10. A high-frequency power amplifier 30 amplifies a high frequency component of the PRN code signal modulated and amplified by the code modulator and amplifier 20 and transmits it through an antenna ANT 1. A first timer 40 transmits a signal by counting the time for transmitting two frames 2T t  after receiving a power amplification control signal PAW from the microcomputer 10. An AND gate AN1 generates a transmission interruption control signal for the high-frequency power amplifier 30 by receiving a counted signal from the first timer 40 and a power amplification control signal PAW from the microcomputer 10. A driving circuit 50 turns off an operation of the high-frequency power amplifier 30 by receiving a transmission interruption control signal from the AND gate AN1. A second timer 60 transmits a signal by counting for a frame transmission time T t  after receiving the output signal of the first timer. A reset signal generator 70 generates a reset signal for the microcomputer 10 by receiving the counted signal from the second timer 60 and the power amplification control signal PAW from the microcomputer 10. 
     Referring to FIGS. 3 and 4, an operation is described specifically as follows. The microcomputer 10 controls a transmission operation of the high-frequency power amplifier 30 for a frame transmission time T t  or an emergency signal transmission time. If the microcomputer 10 transmits a signal as shown in FIG. 4A owing to a fault caused by static electricity or any other cause, the first timer 40 transmits a high level signal as shown in FIG. 4B during the time for transmitting two frames 2T t  by counting. The AND gate AN1 transmits a signal as shown in FIG. 4C by receiving the signal from the first timer 40 as shown in FIG. 4B and the signal from the microcomputer 10 as shown in FIG. 4A. The driving circuit 50 drives the high-frequency power amplifier 30 to transmit a signal during the time for transmitting two frames 2T t  by receiving an output signal of the AND gate AN1. Therefore, a transmission is not performed over a time twice a frame transmission time 2T t  even if a false operation occurs in the microcomputer 10. 
     The second timer 60 generates a signal as shown in FIG. 4D by counting for a frame transmission time T t  after receiving the signal from the first timer 40 as shown in FIG. 4B. The AND gate AN2 generates a reset generation control signal by receiving the signal from the second timer 60 as shown in FIG. 4D and the signal from the microcomputer 10 as shown in FIG. 4A. One-shot circuit 71 generates a signal as shown in FIG. 4F so as to reset the microcomputer 10 by receiving the high level signal from the AND gate AN2. If the microcomputer 10 transmits a high level signal of power amplification control signal PAW to the first timer 40 continuously during the time for transmitting two frames 2T t  and continues to transmit a high level signal after a frame transmission time is elapsed in the second timer 60, the AND gate AN2 generates a reset generating signal so as to make the one-shot circuit 71 transmit a signal for resetting the microcomputer 10. Therefore, a transmission mode returns to a normal mode as shown in FIG. 4G when the one shot circuit 71 generates a signal as shown in FIG. 4F. 
     In conclusion, the watchdog circuit cuts off a transmission automatically by sensing a transmission time longer than a predetermined time of transmission owing to a false operation of a transmission device of a vehicle, and improves a function of a vehicle tracking system by resetting the microcomputer of the equipment mounted on the vehicle.