Method for starting silicon resonance accelerometer based on pseudo-random number principle

Abstract

The invention relates to a method for starting a silicon resonance accelerometer based on a pseudo-random number principle, and the method comprises the steps: analyzing the oscillation starting condition of a resonance beam based on a mechanical model of the resonance beam in the silicon resonance accelerometer, and enabling the resonance beam to start oscillation when the frequency of an input signal is close to the resonance peak frequency of the resonance beam; a linear feedback shift register is adopted in a digital circuit to generate pseudo-random number sequence simulation white noise, and the white noise comprises resonance peak frequency; and exciting the resonant beam based on the pseudo-random number sequence, when the amplitude of the silicon resonant accelerometer reaches a preset amplitude, indicating that the resonant beam starts oscillation, and at the moment, switching to use a sinusoidal signal of the phase-locked loop to maintain oscillation of the resonant beam so as to realize self-adaptive starting of the silicon resonant accelerometer. According to the method, resonance beams with different resonance frequencies can be excited to start oscillation, the complexity of the starting process of the silicon resonance accelerometer digital circuit is reduced, the self-adaptive starting capability is improved, and the method has very high engineering application value.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of inertial navigation, in particular to a method for starting a silicon resonant accelerometer based on a pseudo-random number principle. BACKGROUND

[0002] The starting process of the digital circuit of the conventional silicon resonant accelerometer is as follows: a frequency sweeping range is set in the digital circuit, whether the output amplitude of the resonant beam at different frequencies reaches a threshold value is detected, the resonant frequency is found, and the resonant beam is maintained by keeping the resonant frequency; since the resonant frequency of each silicon resonant accelerometer produced is different, a suitable resonant frequency range needs to be manually debugged for each silicon resonant accelerometer, which greatly increases the complexity of the process and the time of manual operation. In addition, since the resonant frequency of the silicon resonant accelerometer is sensitive to temperature, the resonant frequency is different at different temperatures, so the resonant frequency may change with the change of temperature, and if the resonant frequency is not within the set frequency range, the silicon resonant accelerometer cannot be normally started, which seriously limits the engineering application of the digital circuit of the silicon resonant accelerometer and affects the further improvement of the performance of the silicon resonant accelerometer. At present, the digital circuit still lacks a simple and self-adaptive method for starting the silicon resonant accelerometer. SUMMARY

[0003] Therefore, it is necessary to provide a method for starting a silicon resonant accelerometer based on a pseudo-random number principle, comprising: S1: based on the mechanical model of the resonant beam in the silicon resonant accelerometer, the starting condition of the resonant beam is analyzed, and the starting condition is that the resonant beam starts when the input signal frequency is near the resonant peak frequency of the resonant beam; S2: a linear feedback shift register is used in the digital circuit to generate a pseudo-random number sequence to simulate white noise, and the white noise contains the resonant peak frequency; S3: the resonant beam is excited based on the pseudo-random number sequence, and when the amplitude of the silicon resonant accelerometer reaches a preset amplitude, it indicates that the resonant beam starts, at this time, a phase-locked loop is used to maintain the oscillation of the resonant beam, and the self-adaptive starting of the silicon resonant accelerometer is realized.

[0004] Preferably, S1 comprises: The resonant beam in the silicon resonant accelerometer is equivalent to a second-order spring-damping system, and the kinematic equation of the second-order spring-damping system is transformed by Laplace to obtain a transfer function; the modulus of the transfer function is calculated to construct an amplitude-frequency function; The derivative of the amplitude-frequency function is calculated and set to 0, and the resonant peak frequency is solved, and when the input signal frequency is near the resonant peak frequency of the resonant beam, it is considered that the starting condition of the resonant beam is met, and the resonant beam starts.

[0005] Preferably, the kinematic equation of the second-order spring-damping system is represented as: ; wherein, represents the equivalent mass of the resonant beam; represents the motion acceleration of the resonant beam; represents the motion damping of the resonant beam; represents the motion velocity of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion displacement of the resonant beam; represents the input signal of the resonant beam.

[0006] Preferably, the transfer function is represented as: , ; wherein, represents the Laplace transform of the output displacement of the resonant beam; represents the Laplace transform of the input signal of the resonant beam; represents the equivalent mass of the resonant beam; represents the motion damping of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the complex frequency variable; represents the imaginary unit; represents the input signal frequency.

[0007] Preferably, the amplitude frequency function is represented as: ; wherein, represents the amplitude frequency function; represents the input signal frequency; represents the equivalent mass of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion damping of the resonant beam.

[0008] Preferably, the process of solving the resonant peak frequency comprises: rewriting the amplitude frequency function, the rewritten amplitude frequency function is represented as: ; ; ; ; ; wherein, represents the amplitude frequency function; represents the input signal frequency; represents the equivalent mass of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion damping of the resonant beam. represents the natural frequency of the resonant beam; represents the damping ratio; derivative rewriting the amplitude-frequency function and setting its derivative to 0, the first relationship between the input signal frequency and the natural frequency of the resonant beam is solved when the amplitude is maximum, and the first relationship is represented as: ; a simple transformation is performed on the first relationship to obtain the calculation formula of the resonant peak frequency, which is represented as: ; wherein, represents the resonant peak frequency.

[0009] Preferably, S2 comprises: Step 1: arranging a linear feedback shift register on a digital circuit, and setting the bit number of the linear feedback shift register and the primitive polynomial; Step 2: inputting an initial sequence to the linear feedback shift register, extracting the state at the corresponding position in the initial sequence based on the taps set in the primitive polynomial, and performing XOR operation based on all the extracted states to obtain a feedback bit; The bit number of the initial sequence corresponds to the bit number of the linear feedback shift register, and each state in the initial sequence includes 1 or 0; the leftmost side of the initial sequence is the highest bit, and decreases sequentially to the lowest bit; Step 3: moving each state in the initial sequence one bit to the right, removing the state at the lowest bit in the initial sequence, and placing the feedback bit in the highest bit to obtain an updated sequence; the removed state is placed in the pseudo-random number sequence; Step 4: replacing the pre-updated sequence with the updated sequence, repeating steps 2-4 until the maximum cycle period is completed, to obtain a pseudo-random number sequence, and the states in the pseudo-random number sequence are arranged in the order of placement; When the bit number is greater than or equal to 12 bits, the power spectral density characteristics of the pseudo-random number sequence tend to approach the power spectral density characteristics of white noise, achieving white noise simulation.

[0010] Preferably, the bit number of the linear feedback shift register comprises 12 bits.

[0011] Preferably, the primitive polynomial comprises: ; wherein, represents the primitive polynomial of the 12-bit linear feedback shift register; represents the highest bit of the 12-bit initial sequence; represents the 6th bit from right to left in the 12-bit initial sequence; represents the 4th bit from right to left in the 12-bit initial sequence; indicates the lowest bit of the tap in the 12-bit initial sequence.

[0012] Preferably, the maximum cycle period of the linear feedback shift register is 4095 cycles when the bit number is 12 bits.

[0013] Beneficial effects: The method is based on the mechanical model of the resonant beam in the silicon resonant accelerometer, analyzes the starting condition of the resonant beam, and based on the principle of pseudo-random number, uses the linear feedback shift register to generate a pseudo-random number sequence to simulate white noise, which can stimulate the resonant beam with different resonant frequencies to start, reduces the complexity of the digital circuit starting process of the silicon resonant accelerometer, improves the adaptive starting ability, and has strong engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0015] Figure 1 The flow chart of the method for starting the silicon resonant accelerometer based on the principle of pseudo-random number in the embodiments of the present application.

[0016] Figure 2 The Bode diagram of the resonant beam in the embodiments of the present application.

[0017] Figure 3 The structure diagram of the n-bit linear feedback shift register in the embodiments of the present application.

[0018] Figure 4 The waveform diagram of the pseudo-random number sequence in the embodiments of the present application.

[0019] Figure 5 The power spectral density diagram of the pseudo-random number sequence in the embodiments of the present application.

[0020] Figure 6 The simulation whole-process result diagram in the embodiments of the present application.

[0021] Figure 7 The simulation result diagram of the resonant beam stimulated by the simulated white noise in the embodiments of the present application.

[0022] Figure 8 The phase-locked loop stabilization process diagram in the embodiments of the present application. DETAILED DESCRIPTION

[0023] In order to make the above objectives, characteristics and advantages of the present application more apparent, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. It is apparent, however, to one skilled in the art, that the present application can be practiced without using these specific details in other ways, and one skilled in the art can make similar improvements without departing from the scope of the present application, and therefore the present application is not limited to the specific embodiments disclosed below.

[0024] In addition, the terms "first", "second", etc. are used only for descriptive purposes and should not be construed as indicating or implying relative importance or an indicated number of technical features. Therefore, the features defined as "first", "second", etc. can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise specifically limited.

[0025] As shown in Figure 1 The present embodiment provides a method for starting a silicon resonant accelerometer based on the principle of pseudo-random numbers, comprising: S1: Based on the mechanical model of the resonant beam in the silicon resonant accelerometer, the starting condition of the resonant beam is analyzed, and the starting condition is that when the input signal frequency is near the resonant peak frequency of the resonant beam, the resonant beam starts to vibrate.

[0026] Specifically, this step includes: The resonant beam in the silicon resonant accelerometer is equivalent to a second-order spring-damping system, and the kinematic equation of the second-order spring-damping system is Laplace transformed to obtain a transfer function; The kinematic equation of the second-order spring-damping system is represented as: ; Wherein, represents the equivalent mass of the resonant beam; represents the motion acceleration of the resonant beam; represents the motion damping of the resonant beam; represents the motion velocity of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion displacement of the resonant beam; represents the input signal of the resonant beam, which is the electrostatic driving force applied to the resonant beam by the driving comb teeth.

[0027] The transfer function is represented as: , ; Wherein, represents the Laplace transform of the output displacement of the resonant beam; represents the Laplace transform of the input signal of the resonant beam; representing the equivalent mass of the resonant beam; representing the motion damping of the resonant beam; representing the equivalent stiffness of the resonant beam; representing a complex frequency variable; representing a unit of imaginary part; representing an input signal frequency.

[0028] Due to the production and manufacturing process specification of the silicon resonant accelerometer, it is vacuum packaged, so the kinematics equation of the second-order spring-damping system is close to the undamped system, and the module construction amplitude-frequency function is calculated by the transfer function; the amplitude-frequency function is represented as: ; wherein, representing the amplitude-frequency function; representing an input signal frequency; representing the equivalent mass of the resonant beam; representing the equivalent stiffness of the resonant beam; representing the motion damping of the resonant beam.

[0029] Deriving the amplitude-frequency function and setting its derivative to 0, the resonant peak frequency is solved, and when the input signal frequency is near the resonant peak frequency of the resonant beam, it is considered to satisfy the starting condition of the resonant beam, and the resonant beam starts; Specifically, the process of solving the resonant peak frequency includes: Rewriting the amplitude-frequency function, the rewritten amplitude-frequency function is represented as: ; ; ; ; ; wherein, representing the amplitude-frequency function; representing an input signal frequency; representing the equivalent mass of the resonant beam; representing the equivalent stiffness of the resonant beam; representing the motion damping of the resonant beam; representing the natural frequency of the resonant beam; representing a damping ratio.

[0030] Deriving the rewritten amplitude-frequency function and setting its derivative to 0, the first relationship between the input signal frequency and the natural frequency of the resonant beam when the amplitude is maximum is solved, and the first relationship is represented as: ; After simple transformation of the first relationship, the calculation formula of the resonant peak frequency is obtained, which is represented as: ; in, This indicates the resonant peak frequency. Because silicon resonant accelerometers have a very good quality factor (typically in the tens of thousands), the resonant beam has excellent frequency selectivity. The resonant beam only has a large amplitude response near the resonant peak frequency. Therefore, when the input signal frequency is near the resonant peak frequency of the resonant beam, the resonant beam will oscillate.

[0031] In this embodiment, the specific parameter values ​​of the resonant beam of the silicon resonant accelerometer are shown in Table 1. Table 1 Summary of specific parameters of the resonant beam

[0032] From the above parameters, we can obtain Figure 2 The Bode plot of the resonant beam shown in the figure shows that the resonant beam has a large amplification gain only near the resonant peak frequency. Therefore, the frequency of the electrostatic driving force applied to the resonant beam by the driving comb must be near the resonant peak frequency in order to satisfy the oscillation condition of the resonant beam.

[0033] S2: In digital circuits, a pseudo-random number sequence is generated using a linear feedback shift register to simulate white noise. Since the power spectral density of white noise is constant at all frequencies, the resonant peak frequency is also necessarily contained in the white noise. Therefore, a resonant beam can be started by simulating white noise.

[0034] Specifically, the steps include: Step 1: Arrange the linear feedback shift register on the digital circuit, and set the number of bits and the primitive polynomial of the linear feedback shift register; Step 2: Input the initial sequence into the linear feedback shift register, extract the state at the corresponding position in the initial sequence based on the taps set in the primitive polynomial, and perform an XOR operation on all extracted states to obtain the feedback bit; The number of bits in the initial sequence corresponds to the number of bits in the linear feedback shift register, and each state in the initial sequence includes either 1 or 0; the leftmost bit of the initial sequence is the most significant bit, and the sequence decreases sequentially to the least significant bit. Step 3: Shift each state in the initial sequence one bit to the right, remove the lowest bit of the initial sequence, and put the feedback bit into the highest bit to obtain the updated sequence; put the removed state into the pseudo-random number sequence; Step 4: Replace the previous sequence with the updated sequence, and repeat steps 2-4 until the maximum cycle period is completed to obtain a pseudo-random number sequence. The states in the pseudo-random number sequence are arranged in the order they were put in. When the number of bits is greater than or equal to 12, the power spectral density characteristics of the pseudo-random number sequence approach those of white noise, thus realizing white noise simulation.

[0035] In the embodiment, the structure of the linear feedback shift register is as shown in Figure 3 The input of the highest level register is related to the state of each subsequent register, which can be expressed as: ; Wherein, 、 、 respectively represent the coefficients before each register; represents the XOR operation; 、 、 respectively represent the input of the lowest level register, the second lowest level register, and the second highest level register.

[0036] Based on the above state relationship, the linear feedback shift register can be corresponded to a polynomial, which is expressed as: ; Wherein, represents the polynomial; represents the highest bit of the tap in the initial sequence; represents the second highest bit of the tap in the initial sequence; represents the second lowest bit of the tap in the initial sequence; represents the lowest bit of the tap in the initial sequence; From the above formula, when the initial sequence is set, the linear feedback shift register will perform a cycle, and a linear feedback shift register of n bits can only traverse 2 n -1 kinds of states at most. In order to ensure the maximum cycle period (cycle 2 n -1 times), the polynomial is set to a primitive polynomial, and the sequence generated by the polynomial is a pseudo-random number sequence.

[0037] In the embodiment, the number of bits of the linear feedback shift register includes 12 bits, and when the number of bits is 12, the maximum cycle period of the linear feedback shift register is 4095 cycles.

[0038] In the embodiment, the primitive polynomial is: ; Wherein, represents the primitive polynomial of the 12-bit linear feedback shift register; represents the highest bit of the tap in the 12-bit initial sequence; represents the 6th bit from right to left in the 12-bit initial sequence; represents the 4th bit from right to left in the 12-bit initial sequence; represents the lowest bit of the tap in the 12-bit initial sequence. According to actual application conditions, other forms of primitive polynomials can be selected.​

[0039] In the embodiment, when the initial sequence is 010100011010, the waveform of the generated pseudo-random number sequence is as shown in Figure 4 The power spectral density obtained from the pseudo-random number sequence is as shown in Figure 5 It can be seen from Figure 5 that the pseudo-random number sequence is substantially uniformly distributed in different frequency bands, and the power spectral density characteristic is similar to white noise. Therefore, the digital circuit can generate the pseudo-random number sequence through the linear feedback shift register to simulate the white noise to excite the resonant beam.

[0040] S3: based on the pseudo-random number sequence to excite the resonant beam, when the amplitude of the silicon resonant accelerometer reaches the preset amplitude, it indicates that the resonant beam is started, at this time, the sine signal of the phase-locked loop is switched to maintain the oscillation of the resonant beam, and the adaptive start of the silicon resonant accelerometer is realized.

[0041] In the embodiment, the pseudo-random number sequence generated by the 12-bit linear feedback shift register is used to simulate the resonance, and the simulation results of the whole process are as shown in Figure 6 , the simulation results of the resonant beam excited by the analog white noise are as shown in Figure 7 , and the stable process of the phase-locked loop is as shown in Figure 8 . It can be seen from the simulation results that after the pseudo-random number sequence is input into the resonant beam at the beginning, due to the frequency selection characteristic of the resonant beam, the resonant beam only outputs the signal of the resonant peak frequency range, and finally reaches the preset amplitude with the accumulation of energy. Switch to the phase-locked loop to maintain the oscillation of the resonant beam, and complete the start of the silicon resonant accelerometer. The simulation results show that the pseudo-random number sequence can realize the starting of the resonant beam, and prove the feasibility and correctness of the starting method provided in the embodiment.

[0042] The method provided in the embodiment for starting the silicon resonant accelerometer based on the pseudo-random number principle has the following beneficial effects: The method is based on the mechanical model of the resonant beam in the silicon resonant accelerometer, analyzes the starting condition of the resonant beam, and based on the principle of the pseudo-random number, uses the linear feedback shift register to generate the pseudo-random number sequence to simulate the white noise, which can excite the resonant beam with different resonant frequencies to start. It has good applicability to the adaptive start of the silicon resonant accelerometer based on the digital circuit, and can be expanded and applied to other resonant sensor schemes, and has universality and versatility.

[0043] The technical features of the above-described embodiments can be combined arbitrarily. In order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the present disclosure.

[0044] The above embodiments only express several implementation ways of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation to the patent scope of the application. It should be pointed out that for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers, characterized by, Comprising: S1: based on the mechanical model of the resonant beam in the silicon resonant accelerometer, the resonant beam is analyzed to meet the starting condition, and the resonant beam is started when the input signal frequency is near the resonant peak frequency of the resonant beam; S2: a linear feedback shift register is used in the digital circuit to generate a pseudo-random number sequence to simulate white noise, and the white noise contains the resonant peak frequency; S3: based on the pseudo-random number sequence, the resonant beam is excited, and when the amplitude of the silicon resonant accelerometer reaches a preset amplitude, it indicates that the resonant beam is started, at this time, the phase-locked loop is switched to use the sine signal to maintain the oscillation of the resonant beam, and the adaptive start of the silicon resonant accelerometer is realized.

2. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 1, characterized in that, S1 comprises: The resonant beam in the silicon resonant accelerometer is equivalent to a second-order spring-damping system, and the kinematics equation of the second-order spring-damping system is Laplace transformed to obtain the transfer function; the modulus of the transfer function is calculated to construct the amplitude-frequency function; The derivative of the amplitude-frequency function is taken and set to 0, and the resonant peak frequency is solved, and when the input signal frequency is near the resonant peak frequency of the resonant beam, it is considered to meet the starting condition of the resonant beam, and the resonant beam is started.

3. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 2, characterized in that, The kinematics equation of the second-order spring-damping system is represented as: ； wherein, represents an equivalent mass of the resonant beam; represents an acceleration of motion of the resonant beam; represents a damping of motion of the resonant beam; represents a velocity of motion of the resonant beam; represents an equivalent stiffness of the resonant beam; represents a displacement of motion of the resonant beam; represents an input signal of the resonant beam.

4. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 2, characterized in that, The transfer function is represented as: ， ； wherein, represents the Laplace transform of the output displacement of the resonant beam; represents the Laplace transform of the input signal of the resonant beam; represents the equivalent mass of the resonant beam; represents the motion damping of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the complex frequency variable; represents the imaginary unit; represents the input signal frequency.

5. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 2, characterized in that, The amplitude-frequency function is represented as: ； wherein, represents the amplitude-frequency function; represents the input signal frequency; represents the equivalent mass of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion damping of the resonant beam.

6. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 5, characterized in that, The process of solving the resonant peak frequency comprises: The amplitude-frequency function is rewritten, and the rewritten amplitude-frequency function is represented as: ； ； ； ； ； wherein, represents the amplitude-frequency function; represents the input signal frequency; represents the equivalent mass of the resonant beam; represents the equivalent stiffness of the resonant beam; represents the motion damping of the resonant beam; represents the natural frequency of the resonant beam; represents the damping ratio; The derivative of the rewritten amplitude-frequency function is taken and set to 0, and the first relationship between the input signal frequency and the natural frequency of the resonant beam when the amplitude is maximum is solved, and the first relationship is represented as: ； The first relationship is simply transformed to obtain the calculation formula of the resonant peak frequency, which is represented as: ； wherein represents the resonant peak frequency.

7. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 1, characterized in that S2 Comprising: Step 1: arranging a linear feedback shift register on a digital circuit, and setting the number of bits and the primitive polynomial of the linear feedback shift register; Step 2: inputting an initial sequence to the linear feedback shift register, extracting the states at the corresponding positions in the initial sequence based on the taps set in the primitive polynomial, and performing XOR operation based on all the extracted states to obtain a feedback bit; The number of bits of the initial sequence corresponds to the number of bits of the linear feedback shift register, and each state in the initial sequence includes 1 or 0; the leftmost side of the initial sequence is the highest bit, and it decreases to the lowest bit in turn; Step 3: moving each state in the initial sequence one bit to the right, removing the lowest state in the initial sequence, and placing the feedback bit in the highest bit to obtain an updated sequence; the removed state is placed in the pseudo-random number sequence; Step 4: replacing the updated sequence with the updated sequence, repeating steps 2-4 until the maximum cycle period is completed, obtaining the pseudo-random number sequence, and arranging the states in the pseudo-random number sequence in the order of placement; When the number of bits is greater than or equal to 12 bits, the power spectral density characteristics of the pseudo-random number sequence tend to approach the power spectral density characteristics of white noise, achieving white noise simulation.

8. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 7, characterized in that, The number of bits of the linear feedback shift register comprises 12 bits.

9. The method of starting a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 8, characterized in that, The primitive polynomial comprises: ； wherein denotes the primitive polynomial of a 12-bit linear feedback shift register; denotes the highest bit of the tap in the 12-bit initial sequence; denotes the 6th bit from the right in the 12-bit initial sequence; denotes the 4th bit from the right in the 12-bit initial sequence; denotes the lowest bit of the tap in the 12-bit initial sequence.

10. The method for starting up a silicon resonant accelerometer based on the principle of pseudo-random numbers according to claim 1, characterized in that, When the number of bits is 12 bits, the maximum cycle period of the linear feedback shift register is 4095 cycles.

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