Backup control circuit for emergency power-off of vehicle

Abstract

The invention provides a backup control circuit for emergency power-off of a vehicle, and the circuit comprises a one-key starting switch circuit which comprises a switch K; the first switch signal detection and turn-off circuit is connected with the one-key starting switch circuit and comprises an MCU (Microprogrammed Control Unit), and the MCU is used for outputting a vehicle power-off signal after identifying that the switch K is pressed for a long time; the second switch signal detection and turn-off circuit is connected with the one-key starting switch circuit and is used for outputting a vehicle power-off signal after identifying that the switch K is pressed for a long time; the MCU state monitoring and post-stage power supply control circuit is connected with the MCU and used for monitoring whether the state of the MCU is normal or not, if the state of the MCU is normal, a first control voltage is output, if the state of the MCU is abnormal, a second control voltage is output, the first control voltage is used for controlling a first switch signal detection and turn-off circuit to work, and the second control voltage is used for controlling a second switch signal detection and turn-off circuit to work; and the second control voltage is used for controlling the second switching signal detection and turn-off circuit to work. The method is used for ensuring the emergency power-off reliability of the vehicle.

Description

Technical Field

[0001] The present invention relates to the technical field of vehicle electronic control, and in particular to a backup control circuit for emergency power-off of a vehicle. Background Art

[0002] As an important part of PEPS (keyless entry and keyless start), the one-button start switch is used to control the normal power on and off of the vehicle. At the same time, to ensure driving safety, under some special working conditions, the driver can use the one-button start switch to emergency power off the vehicle.

[0003] The existing emergency power-off implementation method is that the driver long presses the one-button start switch, the MCU (single-chip microcomputer) of the BCM (body control unit) detects the valid switch signal, and the software executes the corresponding action to turn off the output circuit to power off the vehicle. Since this method needs to be executed by software, it is extremely dependent on the MCU. This emergency power-off method can only be implemented when the MCU is in normal status. However, in actual applications, the MCU may fail, such as abnormal power supply voltage, abnormal software operation, etc. In this case, the existing solution cannot achieve emergency power-off. Therefore, in order to ensure the safety of drivers and pedestrians, it is necessary to design a reliable circuit to ensure that the vehicle can be powered off in an emergency under various circumstances. Summary of the invention

[0004] The purpose of the present invention is to provide a backup control circuit for emergency power-off of a vehicle, which circuit can operate independently of the state of an MCU and ensure the reliability of emergency power-off of the vehicle.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0006] A backup control circuit for emergency power-off of a vehicle, comprising:

[0007] A one-button start switch circuit includes a switch K;

[0008] A first switch signal detection and shutdown circuit is connected to the one-key start switch circuit, and includes an MCU, wherein the MCU is used to output a vehicle power-off signal after identifying that the switch K is long pressed;

[0009] A second switch signal detection and shutdown circuit is connected to the one-key start switch circuit and is used to output a vehicle power-off signal after identifying that the switch K is long pressed;

[0010] The MCU status monitoring and post-stage power supply control circuit is connected to the MCU and is used to monitor whether the status of the MCU is normal. If it is normal, a first control voltage BATT_2 is output; if it is abnormal, a second control voltage BATT_3 is output. The first control voltage BATT_2 is used to control the operation of the first switch signal detection and shutdown circuit, and the second control voltage BATT_3 is used to control the operation of the second switch signal detection and shutdown circuit.

[0011] Optionally, the MCU status monitoring and post-stage power supply control circuit is used to monitor whether the software running status and power supply status of the MCU are normal.

[0012] Optionally, the MCU status monitoring and subsequent power supply control circuit includes:

[0013] A watchdog, whose input end is connected to the MCU and is used to receive a dog feeding signal sent regularly by the MCU;

[0014] A D flip-flop, whose input end is connected to the output end of the watchdog;

[0015] An AND gate, a first input end of which is connected to the output end of the D flip-flop, and a second input end of which receives a power supply monitoring signal of the MCU;

[0016] A transistor T1, whose base is connected to the output end of the AND gate, whose collector is connected to the voltage source KL30, and whose emitter is used to output the first control voltage BATT_2;

[0017] The transistor T2 has a base connected to the output end of the AND gate, an emitter connected to the voltage source KL15, and a collector used to output the second control voltage BATT_3.

[0018] Optionally, the voltage source KL30 is the normal power of a battery in the vehicle, and the voltage source KL15 is the power source generated after the vehicle is started.

[0019] Optionally, the one-button start switch circuit further includes a resistor R8 and a resistor R9, and the switch K is connected in parallel at both ends of the resistor R9.

[0020] Optionally, the second switch signal detection and shutdown circuit includes:

[0021] A reference level circuit, whose input terminal is connected to the second control voltage BATT_3, and whose output terminal is used to output the first level V1;

[0022] A resistor R4, a first end of which is connected to the second control voltage BATT_3, and a second end of which is connected to the one-key start switch circuit and outputs a second level V2; when the switch K is not pressed, V2>V1, and when the switch K is pressed, V2<V1;

[0023] A comparator U1, a positive end of which is connected to the second end of the resistor R4, and a negative end of which is connected to the output end of the reference level circuit;

[0024] A transistor T3, whose emitter is connected to the second control voltage BATT_3 and whose base is connected to the output end of the comparator U1;

[0025] The transistor T4 has a base connected to the collector of the transistor T3, an emitter connected to the ground, and a collector used to output the vehicle power-off signal.

[0026] Optionally, a diode D2 is connected between the resistor R4 and the one-key start switch circuit.

[0027] Optionally, the positive terminal of the comparator U1 is connected to a clamping diode D3.

[0028] Optionally, a resistor R5 is connected between the positive end of the comparator U1 and the resistor R4, a capacitor C4 is connected between the resistor R5 and the positive end of the comparator U1, and the capacitor C4 and the resistor R5 form an RC circuit.

[0029] Optionally, the first switch signal detection and shutdown circuit includes:

[0030] A resistor R1, a first end of which is connected to the first control voltage BATT_2, and a second end of which is connected to the one-key start switch circuit;

[0031] A resistor R2, a first end of which is connected to the second end of the resistor R1, and a second end of which is connected to the MCU;

[0032] The resistor R3 has a first end connected to the second end of the resistor R2 and a second end grounded.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] The present invention provides a one-button start switch emergency power-off control circuit that is independent of the normal operation of an MCU. Regardless of whether the MCU is in a normal state, the driver can achieve emergency power-off of the vehicle by long pressing the one-button start switch, thereby greatly improving driving safety in emergency situations. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for the description. Obviously, the drawings described below are an embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work:

[0036] Figure 1A circuit diagram of a backup control circuit for emergency power-off of a vehicle provided by one embodiment of the present invention;

[0037] Figure 2 This is the truth table for the control circuit of the present invention. DETAILED DESCRIPTION

[0038] The scheme proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer according to the following description. It should be noted that the accompanying drawings are in a very simplified form and are not in precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0039] like Figure 1 As shown, an embodiment of the present invention provides a backup control circuit for emergency power-off of a vehicle, including: a one-button start switch circuit 100, a first switch signal detection and shutdown circuit 200, a second switch signal detection and shutdown circuit 300, and an MCU status monitoring and post-stage power supply control circuit 400.

[0040] The one-button start switch circuit 100 includes a switch K. When the switch K is pressed for a long time, it indicates that the vehicle needs to be powered off. Generally, the one-button start switch circuit 100 is composed of two or more resistors connected in series. Figure 1 The figure schematically shows that resistors R8 and R9 are connected in series, and the switch K is connected in parallel at both ends of the resistor R9. Therefore, when the switch K is not pressed, the resistance value of the one-key start switch circuit 100 remains at R8+R9. After the switch K is pressed, the resistance value becomes R8.

[0041] The first switch signal detection and shutdown circuit 200 is connected to the one-button start switch circuit 100, and includes an MCU, and the MCU is used to output a vehicle power-off signal after identifying that the switch K is long pressed.

[0042] The first switch signal detection and shutdown circuit 200 is a switch signal detection and shutdown circuit in the prior art, including resistors R1, R2 and R3, the first end of the resistor R1 is connected to the first control voltage BATT_2 (when the MCU is in a normal state, the first control voltage BATT_2 is powered, which will be described in detail below), the second end is connected to the one-key start switch circuit 100, the first end of the resistor R2 is connected to the second end of the resistor R1, and the second end is connected to the MCU, the first end of the resistor R3 is connected to the second end of the resistor R2, and the second end is grounded. Thus, resistors R1, R8 and R9 form a primary voltage divider circuit, and resistors R2 and R3 form a secondary voltage divider circuit. Since the voltage generated by the voltage division of resistors R1, R8 and R9 may exceed the range of the MCU, it needs to be divided again by resistors R2 and R3 before it can be connected to the MCU. When the first control voltage BATT_2 is energized, the voltage division ratio of the switch K in the one-button start switch circuit 100 is different when it is pressed and when it is not pressed, that is, the signal SIG3 input to the MCU is different. The MCU software controls the change of the signal SIG4 by identifying the signal change of SIG3, that is, outputs the vehicle power-off signal, turns off the output control chip, and realizes emergency power-off of the vehicle.

[0043] like Figure 1 As shown, capacitor C1 is an ESD capacitor, the first end of which is connected to the one-key start switch circuit 100, and the second end is grounded, which plays an electrostatic protection role for the first switch signal detection and shutdown circuit 200. Capacitor C2 is a signal filtering capacitor, the first end of which is connected to the MCU, and the second end is grounded, which is used for signal filtering. A diode D1 is connected between the resistor R1 and the one-key start switch circuit 100 to prevent reverse connection.

[0044] The second switch signal detection and shutdown circuit 300 is connected to the one-key start switch circuit 100, and is used to output a vehicle power-off signal after identifying that the switch K is long pressed.

[0045] The second switch signal detection and shutdown circuit 300 is a redundant switch signal detection and shutdown circuit provided in the present invention, including a reference level circuit 301, a resistor R4, a comparator U1, a transistor T3 and a transistor T4, specifically as follows:

[0046] The input end of the reference level circuit 301 is connected to the second control voltage BATT_3 (when the MCU state is abnormal, the second control voltage BATT_3 is powered, which will be described in detail below), and the output end is used to output the first level V1. Specifically, the reference level circuit includes resistors R6 and R7, and resistors R6 and R7 divide the second control voltage BATT_3 to output the first level V1.

[0047] The first end of the resistor R4 is connected to the second control voltage BATT_3, and the second end is connected to the one-key start switch circuit 100, and forms a voltage divider with the resistors R8 and R9 inside the one-key start switch circuit 100, so that the second end of the resistor R4 outputs the second level V2; and the second level V2 needs to meet the following conditions with the first level V1: when the switch K is not pressed, V2>V1, and when the switch K is pressed, V2<V1.

[0048] The positive end of the comparator U1 is connected to the second end of the resistor R4, and the negative end is connected to the output end of the reference level circuit 301, so that the voltage of the positive end of the comparator is V2, and the voltage of the negative end is maintained at V1. In addition, the comparator U1 is powered by the second control voltage BATT_3.

[0049] The emitter of the transistor T3 is connected to the second control voltage BATT_3, and the base is connected to the output end of the comparator U1. The base of the transistor T4 is connected to the collector of the transistor T3, the emitter is grounded, and the collector is used to output the vehicle power-off signal.

[0050] The working principle of the second switch signal detection and shutdown circuit 300 is: when the driver does not press the switch K, the resistors R4 and R8, R9 divide the voltage, at this time the voltage V2 is higher than V1, and the output signal SIG5 of the comparator U1 is high. Transistors T3 and T4 are in the off state, and the signal SIG4 is not affected and maintains the previous state. When the driver presses the switch K for a long time, the resistor R9 is bypassed, and the resistors R4 and R8 divide the voltage. At this time, the voltage V2 at the positive end of the comparator U1 is lower than the voltage V1 at the negative end, the signal SIG5 is pulled low, the transistors T3 and T4 are turned on, and the signal SIG4 is pulled low, that is, the vehicle power-off signal is output, and the output control chip is turned off to realize the emergency power-off of the vehicle. Therefore, the present invention identifies the valid signal of the one-button start switch circuit 100 by setting the threshold of the comparator U1, thereby realizing emergency power-off.

[0051] Further, a diode D2 is connected between the resistor R4 and the one-key start switch circuit 100 to prevent reverse connection. The positive end of the comparator U1 is connected to a clamping diode D3 to protect the positive end of the comparator U1 from overvoltage. A resistor R5 is connected between the positive end of the comparator U1 and the resistor R4 to limit current to protect the comparator U1. A capacitor C4 is connected between the resistor R5 and the positive end of the comparator U1, and the capacitor C4 and the resistor R5 form an RC circuit.

[0052] like Figure 1As shown, capacitor C3 is an ESD capacitor, with a first end connected to the one-key start switch circuit 100 and a second end grounded, which plays an electrostatic protection role for the second switch signal detection and shutdown circuit 300. Capacitors C5 and C6 are power supply filter capacitors, with a first end connected to the second control voltage BATT_3 and a second end grounded, which are used for filtering on the power line.

[0053] The first control voltage BATT_2 and the second control voltage BATT_3 mentioned above are provided by the MCU state monitoring and post-stage power supply control circuit 400, so that when the MCU state is normal, the first switch signal detection and shutdown circuit 200 controls the emergency power-off of the vehicle, and when the MCU state is abnormal, the second switch signal detection and shutdown circuit 300 controls the emergency power-off of the vehicle, so that the emergency power-off operation of the vehicle can be operated independently of the state of the MCU, thereby ensuring the reliability of the emergency power-off. At the same time, due to the control of the MCU state monitoring and post-stage power supply control circuit 400, the first control voltage BATT_2 and the second control voltage BATT_3 will not be powered on at the same time, thereby avoiding the problem of the first switch signal detection and shutdown circuit 200 and the second switch signal detection and shutdown circuit 300 affecting each other and causing the emergency power-off failure.

[0054] In this embodiment, the MCU status monitoring and post-stage power supply control circuit 400 is connected to the MCU to monitor whether the status of the MCU is normal. If it is normal, a first control voltage BATT_2 is output; if it is abnormal, a second control voltage BATT_3 is output. The first control voltage BATT_2 is used to control the operation of the first switch signal detection and shutdown circuit 200, and the second control voltage BATT_3 is used to control the operation of the second switch signal detection and shutdown circuit 300.

[0055] Specifically, the MCU status monitoring and subsequent power supply control circuit 400 is used to monitor the status of the MCU and control the on and off of the power supply of the subsequent circuit according to the MCU status. The MCU status monitoring and subsequent power supply control circuit 400 is used to monitor whether the software running status and power supply status of the MCU are normal. The specific truth table is as follows: Figure 2 As shown, only when the MCU power supply state and the software running state are normal, the first control voltage BATT_2 is output to make the first switch signal detection and shutdown circuit 200 work, while in other states (including the MCU power supply state is normal but the software running state is abnormal, the MCU power supply state is abnormal but the software running state is normal, and the MCU power supply state is abnormal and the software running state is abnormal), the second control voltage BATT_3 is output to make the second switch signal detection and shutdown circuit 300 work.

[0056] The MCU status monitoring and post-stage power supply control circuit 400 includes: a watchdog, a D flip-flop, an AND gate, a transistor T1 and a transistor T2, specifically as follows: the input end of the watchdog is connected to the MCU, for receiving a dog feeding signal sent periodically by the MCU; the input end of the D flip-flop is connected to the output end of the watchdog; the first input end of the AND gate is connected to the output end of the D flip-flop, and the second input end receives the power monitoring signal of the MCU; the base of the transistor T1 is connected to the output end of the AND gate, the collector is connected to the voltage source KL30, and the emitter is used to output the first control voltage BATT_2; the base of the transistor T2 is connected to the output end of the AND gate, the emitter is connected to the voltage source KL15, and the collector is used to output the second control voltage BATT_3.

[0057] When the software of the MCU runs normally, the MCU will send the dog feeding signal WDI to the watchdog regularly. At this time, the output signal WDO of the watchdog is in a high state. After passing through the D flip-flop, the signal SIG1 is locked to high. If the software of the MCU runs abnormally, that is, the dog feeding signal WDI is not sent regularly, the state of the signal WDO becomes low, and the signal SIG1 is also low. Figure 1 The signal PG in the middle is the power monitoring signal of the MCU. When the output voltage of the power chip of the MCU is normal, the signal PG is high, and when the voltage is abnormal (overvoltage or undervoltage), the signal PG is low. When the MCU state is normal (power supply is normal, software runs normally), the signal SIG1 and the signal PG are both high, and the signal SIG2 is also high. At this time, the transistor T1 is turned on, the transistor T2 is turned off, BATT_2 is powered on, and BATT_3 is not powered, thereby outputting the first control voltage BATT_2, so that the first switch signal detection and shutdown circuit 200 works, and the second switch signal detection and shutdown circuit 300 does not work. When the MCU power supply is overvoltage or undervoltage, or the MCU software runs abnormally, one or both of the signals SIG1 and the signal PG are low, then the signal SIG2 is low, at this time the transistor T2 is turned on, the diode T1 is turned off, BATT_3 is powered on, and BATT_2 is not powered, thereby outputting the second control voltage BATT_3, so that the second switch signal detection and shutdown circuit 300 works, and the first switch signal detection and shutdown circuit 200 does not work.

[0058] In this embodiment, the voltage source KL30 is the normal power of the battery in the vehicle, that is, the normal power of the battery in the vehicle is used to power the first switch signal detection and shutdown circuit 200, and the voltage source KL15 is the power source formed after the vehicle is started, that is, the power source only available after the vehicle is started is used to power the second switch signal detection and shutdown circuit 300. Therefore, the redundant second switch signal detection and shutdown circuit 200 is powered from KL15 (that is, the power source formed after the vehicle is started), so that in the low power consumption mode, the redundant second switch signal detection and shutdown circuit 300 is always without power, thereby reducing the current consumption of the BCM in the low power consumption mode.

[0059] In addition, the working principle of the switch anti-mistouch function in this embodiment is as follows: when the first switch signal detection and shutdown circuit 200 is working, by configuring the debounce (anti-jitter) time of the MCU software, only when the software recognizes a valid switch signal for a long time (such as 2 seconds), that is, the driver presses the switch K for a long time, the MCU output will perform the corresponding operation, pull down the signal SIG4, and realize the vehicle power-off.

[0060] When the second switch signal detection and shutdown circuit 300 is working, the delay capacitor C4 and the current limiting resistor R5 form an RC circuit. When the switch K is pressed, due to the existence of RC, the voltage V2 at the positive end of the comparator U1 will slowly decrease. Before V2 drops to V1, if the switch K is released, the V2 level will rise again and the vehicle will not be powered off. By adjusting the values ​​of R5 and C4, the time for pressing the switch K can be changed.

[0061] In summary, the backup control circuit for emergency power-off of a vehicle provided by the present invention, firstly, makes a redundant design for the switch signal detection and shutdown circuit (including the existing normal circuit and redundant circuit) to ensure that emergency power-off can be achieved when the MCU is normal or abnormal. Secondly, the normal circuit and the redundant circuit will not work at the same time to prevent the emergency power-off failure caused by mutual interference between the two circuits. Thirdly, the redundant circuit does not work in low power consumption mode, reducing the static current consumption of the BCM. Finally, both the normal circuit and the redundant circuit have the function of preventing accidental touch of the switch.

[0062] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0063] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A backup control circuit for emergency power-off of a vehicle, characterized in that: include: A one-button start switch circuit comprises a switch K; A first switch signal detection and shutdown circuit is connected to the one-key start switch circuit, and includes an MCU, wherein the MCU is used to output a vehicle power-off signal after identifying that the switch K is long pressed; A second switch signal detection and shutdown circuit is connected to the one-key start switch circuit and is used to output a vehicle power-off signal after identifying that the switch K is long pressed; The MCU status monitoring and post-stage power supply control circuit is connected to the MCU and is used to monitor whether the status of the MCU is normal. If it is normal, a first control voltage BATT_2 is output; if it is abnormal, a second control voltage BATT_3 is output. The first control voltage BATT_2 is used to control the operation of the first switch signal detection and shutdown circuit, and the second control voltage BATT_3 is used to control the operation of the second switch signal detection and shutdown circuit.

2. The backup control circuit for emergency power-off of a vehicle as claimed in claim 1, characterized in that: The MCU status monitoring and post-stage power supply control circuit is used to monitor whether the software running status and power supply status of the MCU are normal.

3. The backup control circuit for emergency power-off of a vehicle as claimed in claim 2, characterized in that: The MCU status monitoring and subsequent power supply control circuit includes: A watchdog, whose input end is connected to the MCU and is used to receive a dog feeding signal sent regularly by the MCU; A D flip-flop, whose input end is connected to the output end of the watchdog; An AND gate, a first input end of which is connected to the output end of the D flip-flop, and a second input end of which receives a power supply monitoring signal of the MCU; A transistor T1, whose base is connected to the output end of the AND gate, whose collector is connected to the voltage source KL30, and whose emitter is used to output the first control voltage BATT_2; The transistor T2 has a base connected to the output end of the AND gate, an emitter connected to the voltage source KL15, and a collector used to output the second control voltage BATT_3.

4. The backup control circuit for emergency power-off of a vehicle as claimed in claim 3, characterized in that: The voltage source KL30 is the normal power of the battery in the vehicle, and the voltage source KL15 is the power source generated after the vehicle is started.

5. The backup control circuit for emergency power-off of a vehicle as claimed in claim 1, characterized in that: The one-button start switch circuit further includes a resistor R8 and a resistor R9, and the switch K is connected in parallel at both ends of the resistor R9.

6. The backup control circuit for emergency power-off of a vehicle as claimed in claim 1, characterized in that: The second switch signal detection and shutdown circuit comprises: A reference level circuit, whose input terminal is connected to the second control voltage BATT_3, and whose output terminal is used to output the first level V1; A resistor R4, a first end of which is connected to the second control voltage BATT_3, and a second end of which is connected to the one-key start switch circuit and outputs a second level V2; when the switch K is not pressed, V2>V1, and when the switch K is pressed, V2<V1; A comparator U1, a positive end of which is connected to the second end of the resistor R4, and a negative end of which is connected to the output end of the reference level circuit; A transistor T3, whose emitter is connected to the second control voltage BATT_3 and whose base is connected to the output end of the comparator U1; The transistor T4 has a base connected to the collector of the transistor T3, an emitter connected to the ground, and a collector used to output the vehicle power-off signal.

7. The backup control circuit for emergency power-off of a vehicle as claimed in claim 6, characterized in that: A diode D2 is connected between the resistor R4 and the one-key start switch circuit.

8. The backup control circuit for emergency power-off of a vehicle as claimed in claim 6, characterized in that: The positive terminal of the comparator U1 is connected to a clamping diode D3.

9. The backup control circuit for emergency power-off of a vehicle as claimed in claim 6, characterized in that: A resistor R5 is connected between the positive terminal of the comparator U1 and the resistor R4 , and a capacitor C4 is connected between the resistor R5 and the positive terminal of the comparator U1 . The capacitor C4 and the resistor R5 form an RC circuit.

10. The backup control circuit for emergency power-off of a vehicle as claimed in claim 1, characterized in that: The first switch signal detection and shutdown circuit comprises: A resistor R1, a first end of which is connected to the first control voltage BATT_3, and a second end of which is connected to the one-key start switch circuit; A resistor R2, a first end of which is connected to the second end of the resistor R1, and a second end of which is connected to the MCU; The resistor R3 has a first end connected to the second end of the resistor R2 and a second end grounded.

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