An array of oscillators linked together by at least two hamiltonian paths

Abstract

The present disclosure relates to direct time-of-flight (DTOF) sensors and time to digital converters (TDCs) for a DTOF. The disclosure presents an oscillation circuit array for such a TDC. The oscillation circuit array comprises a plurality of oscillators designed identically to each other, and at least two wiring lines. Each wiring line is configured to connect the plurality of oscillators. A first wiring line is arranged to form a first closed path that passes through each of the plurality of oscillators once, and a second wiring line is arranged to form a second closed path that passes through each of the plurality of oscillators once and is different from the first closed path. Further, the plurality of oscillators is designed to oscillate at a same frequency and are synchronized in phase.

Description

[0001] AN ARRAY OF OSCILLATORS LINKED TOGETHER BY AT LEAST TWO HAMILTONIAN PATHS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to direct time-of-flight (DTOF) sensors, and time to digital converters (TDCs) for a DTOF. The disclosure presents an oscillation circuit array for such a TDC. The oscillators of the array, which may be ring oscillators, are linked together by at least two wiring lines following at least two closed paths through the array.

[0004] BACKGROUND

[0005] DTOF sensors - e.g., as illustrated in FIG. 1(a) - require numerous TDCs. A TDC is a circuit for recognizing events and for providing a digital representation of the time at which the events occurred. For example, a TDC may output the time of arrival for each of multiple incoming pulses. DTOF is a method for measuring a distance between the DTOF sensor and an object based on a time difference between an emission of a signal and its return to the DTOF sensor, after being reflected by the object.

[0006] For example, the numerous TDCs may be implemented in a TDC array, as illustrated in FIG. 1(b). The TDCs should have a low time step (typically in a range of lOps to lOOps). The lower this time step, the more accurate the DTOF sensor is at measuring distances. However, frame rate and accuracy requirements may lead to an increasing number of single photon avalanche diodes (SPADS) and TDCs connected to them, in the DTOF sensor.

[0007] Moreover, the TDCs need to be synchronized in frequency and phase for the DTOF sensor to function properly. Ideally, any error should be less than one least significant bit (LSB). However, due to the physical size of the DTOF sensor, it is not possible to distribute a central clock to all the TDCs with the required phase accuracy. Therefore, decentralized architectures have to be used, wherein the clock is generated locally inside each TDC.

[0008] The TDCs may use an architecture that relies on ring oscillators (ROs), due the RO’s relative simplicity, low area, and acceptable current consumption. A RO is an oscillator circuit constituted of delay elements connected in a ring, wherein an oscillating signal can go around the ring. A RO can generate a clock for the DTOF sensor. The clock may be fed to a counter, and a simple stop signal coming from front-end circuitry may send the value of the counter into local memory. As the frequency of the RO is known, the value of the counter leads to time information. To keep all the ROs inside the TDC array in phase, the ROs may be connected together.

[0009] A mathematical model was developed - the so-called Kuramoto model - for the behavior of such large sets of connected / coupled oscillators. The Kuramoto model shows (as can be intuitively understood) that the more connections there are between the ROs, the lower the resulting phase and frequency errors across the array will be. However, these connections are between sensitive nodes in the ROs, and negatively impact the performance of each RO (e.g., frequency, current consumption) mainly due to extra load. The Kuramoto model also shows that the closer the RO native frequencies are to one another, the lower the phase errors are, which are left in the resulting connected array. Thus, keeping any layout mismatch to a minimum is crucial, but is difficult due to the small area wanted for the sensor.

[0010] A trade-off between phase errors and (mainly) current consumption has to be made, and this leads to constraints in the electrical and physical implementation of the connections between the oscillators. SUMMARY

[0011] Various variants of connecting a RO array to keep their frequency / phase aligned are possible. For example, the ROs of the array may be connected in a matrix style. In another example, an array of ROs may be connected using a Hamiltonian path. A Hamiltonian path is a path in an undirected or directed graph that visits each vertex exactly once (in this case, each RO of the array corresponds to such a vertex).

[0012] Notably, also plenty RO architectures exist. Simple ROs may be constituted of three inverters. As shown in FIG. 1(c), each inverter may thereby provide a phase (as an output) of the RO, and for each RO either one phase (left illustration) or three phases (right illustration) out of three may be connected, as an example.

[0013] In the above-mentioned matrix array approach, the loadings between ROs are different. Comer ROs of the array are only connected to two other ROs, while edge ROs are connected to three other ROs, and central ROs are even connected to four other ROs. These different loadings lead to different oscillating frequencies (in line with the Kuramoto model), and to an increase in residual phase / offset error. Having up to four ROs connected together also increases the load and current consumption.

[0014] The Hamiltonian path approach guarantees that each RO sees a balanced load (only two connections to neighbors per RO), however, the approach increases the path length between extreme (positioned) ROs. The approach also introduces some asymmetry in the connectivity path, which is not good for layout matching, especially if there is an electrical gradient in the RO due to manufacturing limitations.

[0015] In view of the above, an objective of this disclosure is to provide an improved oscillation circuit array. The array should be intrinsically balanced in loading and at low load value, while being immune to an external gradient influence. A particular objective is this to keep a number of loads identical for each individual oscillator of the array. Another particular objective is to compensate for a gradient of an initial / nominal oscillator frequency. Overall, a better trade-off than in conventional arrays and solutions is desired. An aim is to enable a TDC using oscillators that are connected together in the array to keep them synchronized.

[0016] These and other objectives are achieved by the solution of this disclosure as described in the independent claims. Advantageous implementations are further described in the dependent claims.

[0017] A first aspect of this disclosure provides an oscillation circuit array for a TDC, the oscillation circuit array comprising: a plurality of oscillators being designed identically to each other; and at least two wiring lines, each of the at least two wiring lines being configured to connect the plurality of oscillators; wherein a first wiring line of the at least two wiring lines is arranged to form a first closed path that passes through each of the plurality of oscillators once; wherein a second wiring line of the at least two wiring lines is arranged to form a second closed path that passes through each of the plurality of oscillators once and is different from the first closed path; and wherein the plurality of oscillators are designed to oscillate at a same frequency and wherein the plurality of oscillators are synchronized in phase.

[0018] The oscillation circuit array of the first aspect allows reducing residual errors that may be left in the oscillators, so that the array has the same frequency and same phase for each oscillator. The use of the at least two closed paths, particularly the introduction of the second closed path, may compensate for any physical asymmetry in the first closed path, and may cancel any gradient effect of the oscillators in the array. For example, a rotated version of the first closed path may be selected for the second closed path, to create a symmetry. Notably, more than two closed paths, i.e., one or more extra wiring lines corresponding to one or more extra paths, could be used in addition to the first and second closed path. This may depend on how much compensation and symmetry is wanted. Each closed path can be a composite of various sub-paths.

[0019] The oscillation circuit array is intrinsically balanced in loading and at low load value, while it is immune to external gradient influence. A number of loads can be kept identical for each individual oscillator, since the paths are connected to each oscillator once.

[0020] Notably, in the oscillation circuit array, the plurality of oscillators may be arranged in columns (multiple columns along the x- axis) and rows (multiple rows along the y-axis).

[0021] In an implementation of the first aspect, each respective oscillator comprises a plurality of inverters as delay elements; an output of a first inverter of the plurality of inverters of the respective oscillator is connected to the first wiring line; and an output of a second inverter of the plurality of inverters of the respective oscillator is connected to the second wiring line.

[0022] In an implementation of the first aspect, the output of the first inverter is a first phase of the respective oscillator, and the output of the second inverter is a second phase of the respective oscillator.

[0023] The second closed path may be used on a different phase of the oscillators than the first closed path, in order to compensate for physical asymmetry in the first closed path.

[0024] In an implementation of the first aspect, each of the plurality of oscillators is directly connected to a same number of neighboring oscillators of the plurality of oscillators with the at least two wiring lines.

[0025] This achieves an intrinsically balanced oscillation circuit array.

[0026] In an implementation of the first aspect, the first closed path and the second closed path are respectively designed to minimize a maximum number of connections between any two oscillators of the plurality of oscillators, wherein a connection is a part of the first or the second wiring line, which is arrange between two adjacent oscillators respectively on the first closed path or the second closed path.

[0027] Notably, finding a closed path, for instance a Hamiltonian path, with such a minimized distance (minimized number of connections) can be solved in general terms with an algorithm and / or methodology.

[0028] In an implementation of the first aspect, the first closed path and the second closed path are designed to reduce at least one of: an electrical gradient along at least one direction of the oscillation circuit array, a phase gradient along at least one direction of the oscillation circuit array, and a frequency gradient along at least one direction of the oscillation circuit array.

[0029] In this way, the performance of a TDC array that is implemented by the oscillation circuit array can be improved.

[0030] In an implementation of the first aspect, a shape of the first closed path and a shape of the second closed path are symmetrical in at least one direction of the oscillation circuit array.

[0031] For example, the second closed path comprises a shape that is equal to a shape of the first closed path, but rotated by 180°.

[0032] In an implementation of the first aspect, the first closed path and the second closed path form a common centroid structure. Common-centroid layouts are widely used in analog design to make circuits resilient to variations. Ideally, the centroids of the common centroid structure may exactly coincide. The oscillation circuit array may be symmetric about both the x-axis and the y-axis.

[0033] In an implementation of the first aspect, the first closed path and the second closed path are Hamiltonian paths connecting the oscillators.

[0034] In an implementation of the first aspect, the oscillators are ring oscillators.

[0035] A second aspect of this disclosure provides a method for an oscillation circuit array, the method comprising: connecting a plurality of oscillators with at least two wiring lines, wherein the oscillators are designed identically to each other; wherein a first wiring line of the at least two wiring lines is arranged to form a first closed path that passes through each of the plurality of oscillators once; wherein a second wiring line of the at least two wiring lines is arranged to form a second closed path that passes through each of the plurality of oscillators once and is different from the first closed path; and wherein the plurality of oscillators are designed to oscillate at a same frequency and wherein the plurality of oscillators are synchronized in phase.

[0036] More than two closed paths can be formed, if deemed necessary, wherein these closed paths are beneficially Hamiltonian path as well. For example, four closed paths could be formed in total in the array using four wiring lines. In this case, the first closed path and the second closed path could be based on a symmetry “north / south” (or “top / down”), e.g. along the columns of the array, so as to reduce an impact of a gradient in that y-direction. A third and a fourth closed path could additionally be based on a symmetry “left / righf ’, e.g. along the rows of the array, so as to reduce an impact of a gradient in that x-direction.

[0037] A third aspect of this disclosure provides a computer program comprising instructions which, when the program is executed by a processor, cause the processor to carry out the method according to the second aspect.

[0038] In summary of the above, the proposed solution aims to achieve a good connectivity of a grid connection of oscillators in an array, and to achieve its relative immunity to external gradient influences with the intrinsic balance connection of Hamiltonian path. A first Hamiltonian path may be used on a first phase of the oscillators, and a different second closed path can be used on a different phase of the oscillators to compensate for physical asymmetry in the first closed path, and to cancel any gradient effect in the array. Usually, a rotated version of the first closed path is a good option to create symmetry. One or more extra Hamiltonian path can be used depending how much compensation and symmetry is wanted. The first and second closed paths can be a composite of various paths, wherein the second closed path still compensates for asymmetry of the first closed path.

[0039] It has to be noted that all entities, elements, units and means described in the present application could be implemented by software or hardware elements or any kind of combination thereof. All steps performed by the various entities described in the present application, as well as the functionalities described to be performed by the various entities, are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity, which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented by respective software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF DRAWINGS

[0040] The above described aspects and implementation forms are explained in the following description in relation to the enclosed drawings, in which:

[0041] FIG. 1 shows (a) an example of a DTOF sensor, (b) an example of a TDC array, and (c) examples of ring oscillators comprising three inverters.

[0042] FIG. 2 shows an oscillation circuit array according to this disclosure, wherein the oscillators of the array are linked together by at least two closed paths rotated against another.

[0043] FIG. 3 shows an exemplary connection of two closed paths to the outputs of different inverters of oscillators of the oscillation circuit array.

[0044] FIG. 4 shows another oscillation circuit array according to this disclosure, wherein the oscillators of the array are linked together by at least two closed paths forming a common centroid structure.

[0045] DETAILED DESCRIPTION OF EMBODIMENTS

[0046] In the following, the solution of this disclosure will be explained by illustrating and describing exemplary oscillation circuit arrays, wherein the oscillators may be ROs. Plenty of RO architectures are possible for the solution of this disclosure. Only to simplify explanations and illustrations of the solution of this disclosure, oscillation circuit arrays are illustrated having exemplarily 20 oscillator circuits, particularly ROs, which are constituted of three inverters each. It is moreover assumed that the connections between the ROs are ideal wires. However, the solution of this disclosure is not limited to any of the above. Changing any of those parameters is possible, and does not impact fundamentally the idea underlying the solution. Similarly, the number of phases inside any oscillator in the oscillation circuit arrays of this disclosure is a design parameter. For instance, at least two phases may be used for implementing the solution, however, the solution can be expended to more than two phases.

[0047] FIG. 2 shows an exemplary implementation of the oscillation circuit array 200 according to this disclosure. The oscillation circuit array 200 can be used for a TDC, for instance in a DTOF sensor for performing DTOF measurements. The oscillation circuit array 200 comprises a plurality of oscillators 203, which are designed identically to each other. The plurality of oscillators 203 are designed to oscillate at the same frequency. Notably, fabrication errors are possible and may lead to deviations between the identically designed oscillators 203, for example, to slightly different frequencies if oscillators 203 were individually used. The oscillators 203 may be arranged in columns and rows of the array 200, for example, in 4 columns and 5 rows, as shown in FIG. 2.

[0048] The oscillation circuit array 200 further comprises at least two wiring lines 201, 202. Ideal wiring lines or wires may be assumed. Each of the at least two wiring lines 201, 202 is configured to connect the plurality of oscillators 203 together. Thereby, a first wiring line 201 of the at least two wiring lines 201, 202 is arranged to form a first closed path Hl (solid line), which passes through each of the plurality of oscillators 203 once. The first closed path Hl may thus be referred to as a first Hamiltonian path. A second wiring line 202 of the at least two wiring lines 201, 202 is arranged to form a second closed path H2 (dotted line), which passes through each of the plurality of oscillators 203 once and is different from the first closed path Hl . The second closed path H2 may thus be referred to as a second Hamiltonian path. The wiring lines 201, 203 form connections between any two oscillators 203, which are adjacent on a closed path Hl , H2. The plurality of oscillators 203 are synchronized in phase and have the same frequency in the array 200. This may be a consequence of the two closed paths Hl, H2, formed by the two wiring lines 201, 202, which connect the oscillators 203 of the array 200 with another.

[0049] The first closed path Hl and the second closed path H2 may respectively be designed to reduce at least one of: an electrical gradient along at least one direction of the oscillation circuit array 200, a phase gradient along at least one direction of the oscillation circuit array 200, and a frequency gradient along at least one direction of the oscillation circuit array 200.

[0050] For example, a unidirectional gradient of the oscillators’ 203 frequencies may be assumed as an example, as indicated by the arrow along the y-axis in FIG. 2 (it may be referred to as a “north / south” gradient). This gradient in particularly the RO frequencies may be caused by a supply Ir drop, for example. The ROs 203 of the array 200 are exemplarily denoted RO1 to R020, wherein the numbering is increased along each row and row-by-row, respectively, as shown.

[0051] The first closed path Hl may be chosen to strongly connect the ROs 203 in the y-axis. However, it can be seen that the RO14 and RO18, although they are arranged close to one another in the array 200, are 9 connections away on the first closed path Hl . Notably, a connection is in this respect defined as a part of the first wiring line 201 that is arranged between any two adjacent oscillators 203 along the first closed path Hl . Moreover, the maximum distance between two ROs on the first closed path Hl is 10 connections (RO1 to RO8, for example)

[0052] The second closed path H2 may therefore be selected to be symmetrical to the first closed path Hl in the at least one direction, here corresponding to the y-direction of the gradient, of the oscillation circuit array 200. For instance, the second closed path H2 may be a 180° rotation of the first closed path Hl . Considering both closed paths Hl and H2 together, the maximum distance between two ROs is only 7 connections. In the y-axis, it is only 4 connections at maximum, and a lot of connections are doubled increasing the coupling factor of the oscillators 203 even more.

[0053] As shown in FIG. 3, each respective oscillator 203, particularly RO, may comprise a plurality of inverters 301, 302, wherein the inverters 301, 302 function as delay elements. An output (e.g., a first phase) of a first inverter 301 of the respective oscillator 203 is connected to the first wiring line 201, and an output (e.g., a second phase) of a second inverter 302 of the respective oscillator 203 is connected to the second wiring line 202. This is the same for each oscillator 203, and thus the first phase of each oscillator 203 is connected to the first wiring line 201, and the second phase of each oscillator 203 is connected to the second wiring line 202.

[0054] FIG. 4 shows another exemplary implementation of the oscillation circuit array 200 according to this disclosure. The oscillating array 200 of FIG. 4 may be designed for a multiple frequency gradient, e.g., a RO frequency gradient along more than one direction of the oscillation circuit array 200. Notably, in FIG. 2 this gradient was assumed to be only along one direction (y- direction), but it could now be along both the y- and the x-direction.

[0055] To address such a multiple RO frequency gradient, a common centroid structure may be formed with the two closed paths Hl and H2. That is, the first closed path Hl and the second closed path H2 may form a common centroid structure, as illustrated. A common centroid structure may comprise paired components - here the two paths Hl and H2 - which are symmetrically arranged around a central point - here the center of the array 200. This can reduce variations from these gradients, for instance, caused by manufacturing differences.

[0056] In summary, an oscillation circuit array 200 including a connectivity network of oscillators 203, for instance ROs, is created consisting of at least two closed (Hamiltonian) paths Hl and H2, which may be connected on different phases of the oscillators 203. The two closed paths Hl and H2 combined together create particularly a symmetric connectivity network in one or more directions (axes) of the array, in a way to reduce any existing gradient e.g. in the oscillator frequency. This gradient could be due to intrinsic manufacturing limitation or any other layout effect, such as Ir drop or temperature.

[0057] The solution of this disclosure, i.e. the oscillation circuit array 200, is applicable to any array of a TDC, which uses oscillators 203 like ROs that are connected together, so as to make sure their timings are aligned precisely. Thus, the solution is usable in any DTOF sensors (e.g. for light detection and ranging (LIDAR)) that relies on connected TDCs to keep track of time. LIDAR is a remote sensing method that uses light in the form of a pulsed laser to measure ranges. The bigger the size of the DTOF sensor, the bigger this invention is beneficial as layout gradient effects becomes more and more prevalent as area increases.

[0058] The solution of this disclosure provides various advantages, which are explained in the following. For example, the solution is able to restore the symmetry of the connectivity structure / network of the oscillators 203 of the array 200. This can reduce any layout and / or electrical gradient impact on the array 200. Moreover, each oscillator 203 may have the same number of connections to its neighboring oscillators 203. This may lead to a better frequency homogeneity across the array 200. Further, the choice of Hamilton paths allows minimal use of connections between oscillators 203. This may lead to a higher frequency (or lower current consumption) for each oscillator 203. Further, the use of multiple Hamilton paths Hl, H2 leads to a reduction of the maximum connectivity length between extreme positioned oscillators 203. This may result in a tighter phase / frequency control of the array 200.

[0059] The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

CLAIMS1. An oscillation circuit array (200) for a time to digital converter, TDC, (100) the oscillation circuit array (200) comprising: a plurality of oscillators (203) being designed identically to each other; and at least two wiring lines (201, 202), each of the at least two wiring lines (201, 202) being configured to connect the plurality of oscillators (203); wherein a first wiring line (201) of the at least two wiring lines (201, 202) is arranged to form a first closed path (Hl) that passes through each of the plurality of oscillators (203) once; wherein a second wiring line (202) of the at least two wiring lines (201, 202) is arranged to form a second closed path (H2) that passes through each of the plurality of oscillators (203) once and is different from the first closed path (Hl ); and wherein the plurality of oscillators (203) are designed to oscillate at a same frequency and wherein the plurality of oscillators (203) are synchronized in phase.

2. The oscillation circuit array (200) according to claim 1 , wherein each respective oscillator (203) comprises a plurality of inverters (301, 302); an output of a first inverter (301) of the plurality of inverters (301, 302) of the respective oscillator (203) is connected to the first wiring line (201); and an output of a second inverter (302) of the plurality of inverters (301, 302) of the respective oscillator (203) is connected to the second wiring line (202).

3. The oscillation circuit (200) according to claim 2, wherein the output of the first inverter (301) is a first phase of the respective oscillator (203), and the output of the second inverter (302) is a second phase of the respective oscillator (203).

4. The oscillation circuit array (200) according to one of the claims 1 to 3, wherein each of the plurality of oscillators (203) is directly connected to a same number of neighboring oscillators (203) of the plurality of oscillators (203) with the at least two wiring lines (201, 202).

5. The oscillation circuit array (200) according to one of the claims 1 to 4, wherein the first closed path (Hl) and the second closed path (H2) are respectively designed to minimize a maximum number of connections between any two oscillators (203) of the plurality of oscillators (203), wherein a connection is a part of the first or the second wiring line (201, 202), which is arranged between two adjacent oscillators (203) respectively on the first or the second closed path (Hl, H2).

6. The oscillation circuit array (200) according to one of the claims 1 to 5, wherein the first closed path (Hl) and the second closed path (H2) are designed to reduce at least one of: an electrical gradient along at least one direction of the oscillationcircuit array (200), a phase gradient along at least one direction of the oscillation circuit array (200), and a frequency gradient along at least one direction of the oscillation circuit array (200).

7. The oscillation circuit array (200) according to one of the claims 1 to 6, wherein a shape of the first closed path (Hl) and a shape of the second closed path (H2) are symmetrical in at least one direction of the oscillation circuit array (200).

8. The oscillation circuit array (200) according to one of the claims 1 to 7, wherein the first closed path (Hl) and the second closed path (H2) form a common centroid structure.

9. The oscillation circuit array (200) according to one of the claims 1 to 8, wherein the first closed path (Hl) and the second closed path (H2) are Hamiltonian paths connecting the oscillators (203).

10. The oscillation circuit array (200) according to one of the claims 1 to 9, wherein the oscillators (203) are ring oscillators.

11. A method for an oscillation circuit array (200), the method comprising: connecting a plurality of oscillators (203) with at least two wiring lines (201, 202), wherein the oscillators (203) are designed identically to each other; wherein a first wiring line (201) of the at least two wiring lines (201, 202) is arranged to form a first closed path (Hl) that passes through each of the plurality of oscillators (203) once; wherein a second wiring line (202) of the at least two wiring lines (201, 202) is arranged to form a second closed path (H2) that passes through each of the plurality of oscillators (203) once and is different from the first closed path (Hl); and wherein the plurality of oscillators (203) are designed to oscillate at a same frequency and wherein the plurality of oscillators is synchronized in phase.

12. A computer program comprising instructions which, when the program is executed by a processor, cause the processor to carry out the method according to claim 11.

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