JP3776795B2 - Inertial device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is mounted on an aircraft, rocket, or other moving body, and detects the angular velocity and acceleration of the moving body by an inertial sensor composed of a gyro and an accelerometer, and the posture angle, speed, and position of the moving body based on the angular velocity and acceleration. In particular, the present invention relates to an inertial device capable of receiving an external clock signal supplied from an external device such as an autopilot and outputting the inertial data to the external device in synchronization with the external clock signal.
[0002]
[Prior art]
In general, an inertial device is mounted on an aircraft, a rocket, or other moving body, and includes an inertial sensor and an arithmetic processing unit, and generates an attitude angle, a speed, and a position of the moving body. The posture angle, speed, and position of the moving body are referred to as inertial data of the moving body. The inertial sensor includes a gyro that detects the angular velocity of the moving body with respect to the three orthogonal axes of X, Y, and Z, and an accelerometer that detects the acceleration of the moving object with respect to the three orthogonal axes. Here, the angular velocity and acceleration detected by the inertial sensor are referred to as sensor output. The arithmetic processing unit calculates the attitude angle, speed, and position of the moving body, that is, inertia data based on the sensor output. The calculation for obtaining inertial data (attitude angle, speed and position) of the moving body based on the sensor output (angular velocity and acceleration) is called inertia calculation. Inertial calculation performs integral calculation on the sensor output to determine the angle and speed of the moving body with respect to three orthogonal axes of X, Y, and Z, and processes the data of the angle and speed to determine the attitude, speed, and position of the moving body. Is an operation for obtaining. Inertia data is provided to, for example, an autopilot device (automatic piloting device) of a mobile object and used as control system control data of the mobile object. FIG. 4 and FIG. 5 are diagrams respectively showing configurations of conventionally known inertia devices. The autopilot device is an example of an external device.
[0003]
4 includes an inertial sensor 2, an up / down counter 3, an arithmetic processing unit 44, and an external connection circuit 6. The inertial device 41 in FIG. Is supplied via the data bus 14. The inertial sensor 2 includes a gyro that detects angular acceleration and an accelerometer that detects acceleration. The inertial sensor 2 outputs angular velocity and acceleration about three orthogonal axes of X, Y, and Z as a sensor output 101. The inertial sensor 2 has an up terminal and a down terminal corresponding to each data (angular velocity or acceleration regarding three orthogonal axes of X, Y, and Z) in the sensor output 101, and when the data is a positive value The number of pulses representing the magnitude of the data is outputted from the up terminal, and when the data is a negative value, the number of pulses representing the magnitude of the data is outputted from the down terminal.
[0004]
The up / down counter 3 includes an up count input terminal and a down count input terminal corresponding to the up terminal and the down terminal of the inertial sensor 2, respectively. The up terminal of the inertial sensor 2 is connected to the upcount input terminal, and the down terminal of the inertial sensor 2 is connected to the downcount input terminal. The up / down counter 3 adds (counts up) the count value by the number of pulses input to the up-count input terminal at the timing of the clock signal 402 supplied from the arithmetic processing unit 44, and is input to the down-count input terminal. The count value is subtracted (counted down) by the number of pulses counted. This counting process in the up / down counter 3 is referred to as an up / down counting process. The up / down counting process is a kind of integration process because the sensor output 101 is counted in the cycle of the clock signal 402. The period of the clock signal 402 is a time of about 1 ms, for example. The up / down count processing output 103 of the up / down counter 3 is sent to the arithmetic processing unit 44. The arithmetic processing unit 44 performs inertia calculation for the up / down / count processing output 103 to generate inertia data 144. The inertia calculation in the calculation processing unit 44 includes integration calculation of the up / down / count processing output 103. The inertia data 144 is supplied to the external device 16 via the external connection circuit 6 and the data bus 14.
[0005]
In the conventional example of FIG. 4, the up / down counting process in the up / down counter 3 and the inertia calculation process in the arithmetic processing unit 44 are performed at an independent timing that is not related to the clock signal of the external device 16, that is, externally. The processing operation in the device 16 is performed asynchronously. Accordingly, it is unavoidable that the processing timing between the inertial device 41 and the external device 16 is shifted, and when the external device 16 receives the inertial data 144, the mobile object acquired by the inertial data 144 at any point in time. It is impossible for the external device 16 to know exactly whether the physical motion state is. Therefore, in the inertial device of FIG. 4, an error between the inertial data 144 and the physical motion state of the moving body is unavoidable.
[0006]
An inertial device 51 (second conventional example) in FIG. 5 is a conventional example intended to eliminate this drawback, and each component is the same as that in the device in FIG. The inertial device 51 in FIG. 5 differs from the inertial device 41 in FIG. 4 in that an external clock signal 102 output from the external device 16 is introduced into the inertial device 51. The frequency of the external clock signal 102 is 1 kHz, for example.
[0007]
In FIG. 5, the sensor output 101 of the inertial sensor 2 is input to the up / down counter 3 and subjected to an up / down count process at a timing based on the external clock signal 102. The up / down count processing output 103 of the up / down counter 3 is sent to the arithmetic processing unit 44. The arithmetic processing unit 44 performs inertia calculation processing on the up / down count processing output 103 at the timing of the external clock signal 102 to generate inertia data 144. This inertia calculation includes an integral calculation similar to the calculation in FIG. In the inertial device 51 of FIG. 5, the start and end of the integration period of the integration calculation is the timing of the external clock signal 102. The inertial data 144 is supplied to the external device 16 via the external connection circuit 6 and the data bus 14 as in the case of the conventional example of FIG.
[0008]
As described for the up / down counter 3 in the inertial device 41 of FIG. 4, the counting process in the up / down counter 3 is a kind of integration process, and the calculation processing unit 44 also performs the integration process. The counting process in the counter 3 is a short time count of about 1 msec, and the integration process in the arithmetic processing unit 44 is performed by integrating the up / down count process output 103 for a longer time. The arithmetic processing unit 44 receives the external clock signal 102, generates an internal clock signal obtained by dividing the frequency of the external clock signal 102, and performs integration processing at the timing of the internal clock signal.
[0009]
In the conventional example of FIG. 5, the processing operations of the up / down counter 3 and the arithmetic processing unit 44 are synchronized with the external clock signal 102 of the external device 16, so that the inertial data 144 is Supplied to the device 16. Therefore, in the inertial device 51 (second conventional example) of FIG. 5, the disadvantages of the inertial device 41 (first conventional example) of FIG. 4 are avoided.
[0010]
[Problems to be solved by the invention]
In the above-described conventional example, in the case of the second conventional example proposed to compensate for the disadvantages of the first conventional example, the processing operations of the up / down counter 3 and the arithmetic processing unit 44 in the inertial device 51 are as follows. The time difference between the inertial data 144 output from the inertial device 51 and the data reception timing of the external device 16 is temporarily avoided, and the inertial data is generated. The external device 16 can accurately know at what point 144 represents the physical motion state of the mobile object acquired. However, the inertia calculation process in the calculation processing unit 44 includes an integral calculation, and the time interval from the start to the end of the integral calculation is defined by the external clock signal 102. Therefore, if the cycle of the external clock signal 102 varies, There is a defect that an error occurs in the integration time interval of the integration calculation, and the error in the integration time interval directly leads to deterioration in accuracy of the inertial data 144 (attitude angle, speed, and position) due to the inertia calculation in the calculation processing unit 44.
[0011]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an inertial apparatus that eliminates the above-mentioned drawbacks and can supply inertial data synchronized with an external clock signal to an external device with high accuracy even if the period of the external clock signal varies. There is.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides the following means.
[0013]
(1) An inertial sensor that detects the angular velocity and acceleration of a moving body, and a calculation unit that receives the output of the inertial sensor. The calculation unit receives an external clock signal output from an external device. In an inertial apparatus that obtains inertial data of the moving body by performing integral calculation processing on the output of the inertial sensor with respect to time based on a repetition period, and transmits the inertial data to the external device.
An external clock period measuring circuit for measuring a repetition period of the external clock signal and generating external clock period data representing the repetition period;
The arithmetic means receives the external clock cycle data, obtains a difference between the repetition cycle represented by the external clock cycle data and a reference value as an external clock cycle error, and based on the external clock cycle error, By correcting the cycle, a corrected external clock having a corrected repetition cycle is generated, and the inertial sensor output is subjected to an integral calculation process with respect to the time based on the repetition cycle of the corrected external clock, whereby the inertial data of the moving body is obtained. And inertia data is transmitted to the external device.
[0014]
(2) The inertial sensor includes a gyro that detects the angular velocity and an accelerometer that detects the acceleration,
The arithmetic means performs an up / down count process on the inertial sensor output at the timing of the external clock signal, and generates an up / down count process output; and an integral operation on the up / down count process output An arithmetic processing unit that generates the inertial data by performing processing,
The arithmetic processing unit supplies a control signal and a reference clock signal for controlling the operation of the external clock cycle measuring circuit to the external clock cycle measuring circuit,
The repetition frequency of the reference clock signal is greater than the repetition frequency of the external clock signal,
The external clock period measuring circuit includes an external clock signal repetition period detecting means for generating a pulse signal having a pulse width of a repetition period of the external clock signal, a counting means for counting the time of the pulse width with the reference clock signal, Taking out the count value output from the counting means, and sending the count value as the external clock cycle data to the arithmetic processing unit,
The external clock signal repetition period detecting means generates first and second pulse signals respectively representing first and second repetition periods that are continuous in the external clock signal;
The counting means includes first and second counters for counting the pulse widths in the first and second pulse signals with the reference clock signal, respectively.
The sending means includes first and second buffers that respectively extract count values of the first and second counters, and count values of the first and second counters that are extracted to the first and second buffers. A data bus that leads to the arithmetic processing unit as the external clock cycle data,
The inertia according to (1), wherein the control signal controls a counting period in the counting means and a timing of taking out the count values of the first and second counters by the first and second buffers. apparatus.
[0015]
(3) The external clock signal repetition period detecting means latches the external clock signal and generates a pulse signal that is synchronized with the external clock signal and alternately switches between a high level and a low level for each period of the external clock signal. The latch described as the first pulse signal and an inverter that generates the second pulse signal by inverting the polarity of the first pulse signal according to (2), Inertial device.
[0016]
(4) The correction external clock is a clock obtained by correcting a repetition period of an internal clock obtained by dividing the external clock based on the external clock period error, according to (1) to (3), Inertial device.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Next, the inertial device of the present invention will be described more specifically with reference to an embodiment of the present invention.
[0018]
FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. As shown in FIG. 1, the inertial device 1 of the present embodiment includes an inertial sensor 2, an up / down counter 3, an arithmetic processing unit 4, an external clock period measuring circuit 5, and an external connection circuit 6. Configured. FIG. 2 is a diagram showing an internal configuration of an embodiment of the external clock cycle measuring circuit 5, and FIGS. 3 (a), (b), (c), (d), (e), (f) , (G), (h), (i), (j), and (k) are timing charts of respective signals in the external clock period measuring circuit 5. As shown in FIG. 2, the external clock cycle measuring circuit 5 includes a latch 7, an inverter 8, counters 9 and 10, and buffers 11 and 12 corresponding to the arithmetic processing unit 4. .
[0019]
In FIG. 1, a sensor output 101 (corresponding to the aforementioned inertial sensor output) detected by the inertial sensor 2 is input to the up / down counter 3 and up / down at the timing of the external clock signal 102 supplied from the external device 16.・ Count processing is performed. The operation of the inertial sensor 2 and the up / down counter 3 is the same as described with reference to FIG. The up / down count processing output 103 output from the up / down counter 3 is input to the arithmetic processing unit 4. In the arithmetic processing unit 4, the inertia calculation processing is performed for the up / down / count processing output 103 as in the conventional inertial device of FIGS. 4 and 5. In the inertial device 1 of the present embodiment, the external clock cycle measuring circuit 5 measures the repetition cycle of the external clock signal 102, and uses the repetition cycle of the external clock signal 102 as external clock cycle data via the data bus 13. 4 is characterized in that it is supplied to 4.
[0020]
The arithmetic processing unit 4 uses the amount of deviation of the integration time interval indicated by the external clock cycle data from a predetermined external clock cycle (corresponding to the aforementioned reference value) as an error (corresponding to the aforementioned external clock cycle error), and The period of the internal clock obtained by dividing the clock signal 102 is corrected based on the error, and accurate inertia data excluding the influence of the frequency variation of the external clock signal 102 can be obtained.
[0021]
An external clock signal 102 is input to the external clock cycle measuring circuit 5, and a reference clock signal 104, a counter (9) clear signal 107, a counter (10) clear signal 108, a buffer are received from the arithmetic processing unit 4. (11) An enable signal 111 and a buffer (12) enable signal 112 are input. Signals 107, 108, 111 and 112 correspond to the aforementioned control signals. From the external clock cycle measurement circuit 5, external clock cycle data (data configured by alternately selecting the counter (9) count data 109 and the counter (10) count data 110) representing the repetition cycle of the external clock signal 102 is data. The data is output to the arithmetic processing unit 4 via the bus 13.
[0022]
In the arithmetic processing unit 4, as described above, the repetition cycle of the external clock signal 102 is known from the external clock cycle data, and the actual repetition cycle of the external clock signal 102 is shifted from the reference repetition cycle of the external clock signal 102. Is calculated as an external clock signal (102) repetition period error. Then, the arithmetic processing unit 4 generates a corrected external clock by correcting the repetition period of the internal clock obtained by dividing the external clock signal 102 based on the external clock signal (102) repetition period error, and repeats the correction external clock. Inertia data is generated by time integration of the up / down count processing output 103 with the time based on the period. The inertial data obtained by time integration of the up / down count processing output 103 is almost the same as the inertial data obtained by integrating with the repetition period of the reference value when the repetition period of the external clock signal 102 is the reference value. It is accuracy. This inertial data is input to the external connection circuit 6 via the data bus 14 and further transmitted to the external device 16 via the data bus 15.
[0023]
As described above, in the inertia calculation in the calculation processing unit 4, it is possible to obtain the error of the repetition period of the external clock signal 102 and improve the calculation accuracy of the inertia data. In the following, with reference to FIG. 1, FIG. 2 and FIG. 3, the present embodiment will be described focusing on the operation contents of one example of the external clock cycle measuring circuit 5. FIG.
[0024]
As shown in FIG. 1, the external clock signal 102 (see FIG. 3B) output from the external device 16 is input to the up / down counter 3 as described above, and the arithmetic processing unit 4 and It is also input to the external clock cycle measuring circuit 5. A reference clock signal 104 (see FIG. 3A) is supplied from the arithmetic processing unit 4 to the external clock cycle measuring circuit 5, and a counter (9) clear signal 107 (FIG. 3G) is used as a control signal. The counter (10) clear signal 108 (see FIG. 3 (h)), the buffer (11) enable signal 111 (see FIG. 3 (i)) and the buffer (12) enable signal 112 (see FIG. 3 (j)). Is supplied from the arithmetic processing unit 4. The frequency of the external clock signal 102 is 1 kHz. The reference clock signal 104 is generated by a transmitter in the arithmetic processing unit 4 and is a signal having a stable frequency of 10 MHz, and is asynchronous with the external clock signal 102.
[0025]
In FIG. 2, an external clock signal 102 from the external device 16 is input to the latch 7, from which a counter enable signal 105 (corresponding to the first pulse signal described above, see FIG. 3C). The signal is output and input to the counter 9 and is inverted by the inverter 8 to be input to the counter 10 as an inverted counter enable signal 106 (corresponding to the second pulse signal described above, see FIG. 3D). The
[0026]
The counter 9 (corresponding to the above-mentioned first counter) receives the counter enable signal 105 and the reference clock signal 104, and, as shown in FIG. 3 (e), in the period from the timing T1 to T2, The clock signal 104 is counted. The count value in the counter 9 is held as counter (9) count data 109 in a time zone after the timing T2. The counter (9) count data 109 represents the cycle of the external clock signal 102. Similarly, the counter 10 (corresponding to the above-mentioned second counter) receives the inverted counter enable signal 106 and the reference clock signal 104 and receives the timing T2 to T5 as shown in FIG. Until the reference clock signal 104 is counted. The count value in the counter 10 is held as counter (10) count data 110 in a time zone after the timing T5. Similarly to the counter (9) count data 109, the counter (10) count data 110 represents the cycle of the external clock signal 102. The counter (9) count data 109 and the counter (10) count data 110 represent alternating repetition periods in the external clock signal 102, respectively.
[0027]
As shown in FIG. 3 (i), the buffer 11 (corresponding to the first buffer described above) receives the buffer (11) enable signal 111 output from the arithmetic processing unit 4 at timing T3, and receives timing T2 Thereafter, the counter (9) count data 109 held in the counter 9 is fetched. The counter (9) count data 109 is transmitted to the arithmetic processing unit 4 via the buffer 11 and the data bus 13. At the timing T4 after the transfer of the counter (9) count data 109, the counter (9) clear signal 107 is input from the arithmetic processing unit 4 to the counter 9, and the counter (9) count held in the counter 9 is counted. Data 109 is cleared.
[0028]
The series of linkage operations relating to the counter 9 and the buffer 11 is exactly the same for the counter 10 and the buffer 12 (corresponding to the second buffer described above). As shown in FIG. 3 (j), the buffer 12 receives the buffer (12) enable signal 112 output from the arithmetic processing unit 4 at timing T6, and the data is held in the counter 10 after timing T5. The counter (10) count data 110 is taken in, and the counter (10) count data 110 is transmitted to the arithmetic processing unit 4 via the buffer 12 and the data bus 13. At timing T7 after the transfer of the counter (10) count data 110, the counter (10) clear signal 108 is input from the arithmetic processing unit 4 to the counter 10, and the counter (10) count data held in the counter 10 is received. 110 is cleared.
[0029]
FIG. 3 (k) shows a usage time zone of the data bus 13 when the counter (9) count data 109 and the counter (10) count data 110 are transmitted to the external device 16 via the data bus 13, respectively. FIG. In the transmission time zone 13 a corresponding to the buffer (11) enable signal 111 and the buffer (12) enable signal 112, the counter count data 109 and 110 are sequentially transmitted to the arithmetic processing unit 4 via the data bus 13. The counter (9) count data 109 and the counter (10) count data 110 are alternately output to the arithmetic processing unit 4 via the data bus 13 in synchronization with the external clock signal 102. As described above, when the counter count data 109 and 110 are alternately supplied to the arithmetic processing unit 4 via the data bus 13, the arithmetic processing unit 4 uses the data 109 and 110 as integral data as external clock cycle data. recognize. 3K indicates a time zone in which the data bus 13 is in a high impedance state, and a transmission time that can be used for transmission of data signals other than the counter count data 109 and 110. The band is shown.
[0030]
Thus, the external clock cycle measuring circuit 5 in FIG. 2 supplies the external clock cycle data composed of the counter count data 109 and 110 to the arithmetic processing unit 4, and the arithmetic processing unit 4 generates the external clock signal based on the external clock cycle data. An error of 102 repetition periods (external clock period error) is calculated, the repetition period of the external clock signal 102 is corrected based on the external clock period error, a corrected external clock is generated, and based on the repetition period of the corrected external clock Time integration of the up / down count processing output 103 is performed with time. Since the time data in the time integration is accurate, the arithmetic processing unit 4 can calculate accurate inertia data excluding the influence of the error of the repetition period of the external clock signal 102. However, since the reference clock signal 104 is asynchronous with the external clock signal 102, the repetition period of the external clock signal 102 measured by the external clock period measurement circuit 5 has an error of two cycles of the reference clock signal 104 at the maximum. There is. However, in this embodiment, since the reference clock signal 104 is 10 MHz, the error for these two periods is 200 nanoseconds (ns), which is a variation of several tens of microseconds that can occur in the repetition period of the external clock signal 102. Compared to the negligible size.
[0031]
In addition, even if the error of the repetition period of the external clock signal 102 is due to jitter of the repetition period of the external clock signal 102 and the jitter is a relatively large value, in this embodiment, the error of the external clock signal 102 Since the repetition period is measured for each period, even if there is an error based on the jitter in the inertial data obtained by calculation according to the repetition period of the external clock signal 102, accurate inertial data in which the error is corrected Can be output. If the external clock period measurement circuit measures the average value of a plurality of repetition periods in the external clock signal 102, the error of the integral value based on the individual jitter cannot be corrected, but in this embodiment, as described above, Since the repetition period of the external clock signal 102 is measured for each period, an error based on jitter for each repetition period can also be corrected.
[0032]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an inertial device that can supply inertial data synchronized with an external clock signal to an external device with high accuracy even if the cycle of the external clock signal varies.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
2 is a configuration diagram showing an example of an external clock period measurement circuit in the embodiment of FIG. 1; FIG.
FIG. 3 is a timing diagram showing signals at various parts in the external clock period measurement circuit of FIG. 2;
FIG. 4 is a block diagram showing a conventional example.
FIG. 5 is a block diagram showing another conventional example.
[Explanation of symbols]
1, 41, 51 Inertial device 2 Inertial sensor 3 Up / down counter 4, 44 Arithmetic processing unit 5 External clock cycle measuring circuit 6 External connection circuit 7 Latch 8 Inverter 9, 10 Counter 11, 12 Buffer 13, 14, 15 Data bus 16 External device 101 Sensor output 102 External clock signal 103 Up / down counter 3 up / down count processing output 104 Reference clock signal 105 Counter enable signal 106 Inverted counter enable signal 107 Counter (9) Clear signal 108 Counter (10) Clear signal 109 Counter (9) Count data 110 Counter (10) Count data 111 Buffer (11) Enable signal 112 Buffer (12) Enable signal 144 Inertial data 402 Clock signal

Claims (4)

An inertial sensor for detecting the angular velocity and acceleration of the moving body; and an arithmetic means for receiving an output of the inertial sensor; the arithmetic means receives an external clock signal output from an external device; In an inertial apparatus that obtains inertial data of the moving body by performing integral calculation processing on the output of the inertial sensor with respect to time based on the time, and transmits the inertial data to the external device.
An external clock period measuring circuit for measuring a repetition period of the external clock signal and generating external clock period data representing the repetition period;
The arithmetic means receives the external clock cycle data, obtains a difference between the repetition cycle represented by the external clock cycle data and a reference value as an external clock cycle error, and based on the external clock cycle error, By correcting the cycle, a corrected external clock having a corrected repetition cycle is generated, and the inertial sensor output is subjected to an integral calculation process with respect to the time based on the repetition cycle of the corrected external clock, whereby the inertial data of the moving body is obtained. And inertia data is transmitted to the external device. The inertial sensor includes a gyro that detects the angular velocity and an accelerometer that detects the acceleration,
The arithmetic means performs an up / down count process on the inertial sensor output at the timing of the external clock signal, and generates an up / down count process output; and an integral operation on the up / down count process output An arithmetic processing unit that generates the inertial data by performing processing,
The arithmetic processing unit supplies a control signal and a reference clock signal for controlling the operation of the external clock cycle measuring circuit to the external clock cycle measuring circuit,
The repetition frequency of the reference clock signal is greater than the repetition frequency of the external clock signal,
The external clock period measuring circuit includes an external clock signal repetition period detecting means for generating a pulse signal having a pulse width of a repetition period of the external clock signal, a counting means for counting the time of the pulse width with the reference clock signal, Taking out the count value output from the counting means, and sending the count value as the external clock cycle data to the arithmetic processing unit,
The external clock signal repetition period detecting means generates first and second pulse signals respectively representing first and second repetition periods that are continuous in the external clock signal;
The counting means includes first and second counters for counting the pulse widths in the first and second pulse signals with the reference clock signal, respectively.
The sending means includes first and second buffers that respectively extract count values of the first and second counters, and count values of the first and second counters that are extracted to the first and second buffers. A data bus that leads to the arithmetic processing unit as the external clock cycle data,
2. The inertial device according to claim 1, wherein the control signal controls a counting period in the counting unit and a timing of taking out count values of the first and second counters by the first and second buffers. 3. . The external clock signal repetition period detecting means latches the external clock signal, synchronizes with the external clock signal, and generates a pulse signal that alternately switches between a high level and a low level for each period of the external clock signal. The inertial device according to claim 2, comprising: a latch that generates the first pulse signal, and an inverter that generates the second pulse signal by inverting the polarity of the first pulse signal. 4. The inertial device according to claim 1, wherein the corrected external clock is a clock obtained by correcting a repetition cycle of an internal clock obtained by dividing the external clock based on the external clock cycle error.

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