Abstract:
Sensor circuitry including probabilistic switching devices, such as spin-transfer torque magnetic tunnel junctions (STT-MTJs), is configured to perform ultra-low power analog to digital conversion and compressive sensing. The analog to digital conversion and compressive sensing processes are performed simultaneously and in a manner that is native to the devices due to their probabilistic switching characteristics.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to data sensing and compression. More specifically, the present disclosure relates to analog to digital conversion and also to compressive sensing techniques. 
     BACKGROUND 
     Conventional analog-to-digital (ADC) circuitry such as Flash ADC circuitry, Sigma-delta ADC circuitry, and Successive Approximation Register (SAR) circuitry includes complicated sensing mechanisms, such as ladder comparators and additional digital-to-analog converter (DAC) circuitry. This complicated circuitry increase device cost in terms of area and power. In addition, because conventional ADC circuitry does not perform sampling rate compression natively, techniques such as random sampling or random convolution are used for sampling rate and data compression. This again adds to power and area cost. 
     SUMMARY 
     Aspects of the present disclosure include low power ADC circuitry using native compression techniques. The probabilistic switching nature of devices, such as spin-transfer torque magnetic tunnel junction (STT-MTJ) devices, may be exploited to perform analog to digital conversion (ADC) and compressive sensing. These processes may be performed simultaneously and in a manner that is native to the probabilistic switching characteristic of the devices without additional processing logic or conversion devices. The compressed signals may be easily and reliably reconstructed by receiver circuitry. 
     Aspects of the present disclosure provide ultra-low power compressive-sensing, which may be particularly advantageous for applications such as ambulatory electro-cardio gram (ECG) monitoring. In such applications, the ADC techniques provide reduced power consumption and native data compression techniques, and also provide reduced transmission rates, reduced radio power consumption, reduced radio congestion, reduced packet loss and/or reduced storage overheads. 
     A method of signal sensing according to an aspect of the disclosure includes presetting a bias value of a probabilistic sensor device, continuously sampling a signal relative to a bias until the probabilistic sensor device reaches a target state and recording an amount of time for the target state to be reached. According to an aspect of the present disclosure, the method also includes resetting the bias value to equal a current signal level and resetting the probabilistic sensor device. 
     A sensor device according to an aspect of the present disclosure includes means for continuously sampling a signal relative to a bias value until the sensor device reaches a target state relative to a bias level. The device also includes and means for recording an amount of time for the target state to be reached. According to an aspect of the present disclosure, the sensor device also includes means for resetting the bias value to equal a current signal level when the target state is reached and means for resetting the sensor device when the target state is reached. 
     A sensor apparatus according to an aspect of the present disclosure includes a cluster of spin torque transfer magnetic tunnel junctions (STT-MTJs) configured for continuously sampling a signal relative to a bias value until a sensor device reaches a target state relative to a bias level and counter circuitry coupled to the cluster of STT-MTJs. The counter circuitry is configured for recording an amount of time for the target state to be reached. According to an aspect of the disclosure, the sensor apparatus also includes precharge switching circuitry coupled to the cluster of STT-MTJs. The precharge switching circuitry is configured for resetting the bias value to equal a current signal level when the target state is reached. According to an aspect of the disclosure, reset circuitry is coupled to the cluster of STT-MTJs. The reset circuitry is configured for resetting the sensor device when the target state is reached. 
     This has outlined, rather broadly, the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described below. It should be appreciated by those skilled in the art that this disclosure may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the teachings of the disclosure as set forth in the appended claims. The novel features, which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages, will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, nature, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout. 
         FIG. 1  is a graph conceptually illustrating a general example of a relationship between probability of switching an STT-MTJ and the strength of a switching signal applied to the STT-MTJ according to aspects of the present disclosure. 
         FIG. 2  is a high level schematic diagram conceptually illustrating an apparatus for probabilistic sensing according to aspects of the present disclosure. 
         FIG. 3  is a graph conceptually illustrating sampling of an example input signal according to aspects of the present disclosure. 
         FIGS. 4A-4C  show a high level block diagrams conceptually illustrating the operation of STT-MTJs configured in a parallel multi-level cell (MLC) according to aspects of the present disclosure. 
         FIG. 5  is a high level schematic diagram conceptually illustrating an apparatus for adaptive slope quantization according to aspects of the present disclosure. 
         FIG. 6  is a process flow diagram illustrating a method for analog to digital conversion according to an aspect of the present disclosure. 
         FIG. 7  is a block diagram showing an exemplary wireless communication system in which an aspect of the disclosure may be advantageously employed. 
         FIG. 8  is a block diagram illustrating a design workstation for circuit, layout, and logic design of a semiconductor component according to one aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     According to aspects of the present disclosure devices that exhibit a probabilistic switching characteristic, such as STT-MTJs, are employed for efficient sampling and analog to digital conversion of signals. Due in part to random thermal effects, devices such as STT-MTJs, may or may not switch states in response to an applied switching signal according to a probability of switching that is directly related to the strength of the switching signal. In STT-MTJs the two states are defined by parallel and anti-parallel magnetic alignments within the device. Switching from a parallel state to an anti-parallel state (P-AP) may occur at a different signal strength than switching from the anti-parallel state to the parallel state (AP-P). In other words, switching from the parallel state to the anti-parallel state (P-AP) may occur with a different probability than switching from the anti-parallel state to the parallel state (AP-P) for a given signal strength and pulse duration. The strength of the applied switching signal directly affects the probability that the device will switch states. 
       FIG. 1  illustrates an example of the relationship between the probability  102  of switching an STT-MTJ and the strength  104  of a switching signal applied to the STT-MTJ. The relationship shows that when a switching signal is applied at a given voltage and given pulse duration, the probability that the STT-MTJ will switch states is fixed. In this example, if the switching signal of 620 mV is applied for a predetermined pulse duration, the probability that the STT-MTJ will switch states (Psw) is 0.6. In other words, if a 620 mV pulse is applied for N=100,000 iterations to a single STT-MTJ having the illustrated switching probability characteristic, the STT-MTJ will switch states approximately 60,000 times. Similarly, if a pulse of 620 mV is applied for a single iteration to N=100,000 different STT-MTJs with the illustrated probabilistic characteristic, approximately 60,000 of the STT-MTJs will switch states. In this example the mean of the number of switching occurrences equals the number (N) of iterations or devices to which an iteration is applied times the switching probability (SW mean =N*Psw). The distribution of the number of switching occurrences is binomial around the mean with a large variance (σ=N*Psw*(1−Psw)). 
     According to aspects of the disclosure, the switching probability characteristic of an STT-MTJ may be exploited to sense the strength of the applied switching signal by counting a number of times an STT-MTJ switches or by counting a number of switched STT-MTJs within a population of STT-MTJs to which the switching signal is applied. 
     An apparatus for probabilistic sensing according to an aspect of the disclosure is described with reference to  FIG. 2 . Sensing circuitry  200  includes input amplifier circuitry  204  coupled to an input node  202 . Bias voltage supply circuitry  206  is coupled to the input amplifier circuitry  204 . STT-MTJ circuitry  208  including at least one STT-MTJ is coupled to the bias voltage supply circuitry  206 . Sense amplifier circuitry  210  is coupled to the STT-MTJ circuitry  208 . Counter circuitry  212  is coupled to the sense amplifier circuitry  210 . 
     In order to suitably span the working range of an STT-MTJ, a signal on the input node  202  of the sensing circuitry  200  is amplified by the amplifier circuitry  204  and biased by the bias voltage supply circuitry  206 .  FIG. 3  shows a voltage  302  versus time  304  graph  300  of the amplified and biased signal  306 . The amplified and biased signal  306  is iteratively applied to the STT-MTJ circuitry  208  at an appropriate sampling frequency. In the example shown in  FIG. 3 , the sampling frequency is 200 Hz which corresponds to a sampling interval  308  of 5 ms. During each sampling interval, the amplified and biased signal is applied to a single STT-MTJ for N=100,000 iterations  310  with a constant write time of 10 ns. 
     The sense amplifier circuitry  210  determines whether the STT-MTJ changed state for each iteration and the counter circuitry  212  counts how many state changes occurred for each sampling interval. According to aspects of the present disclosure, the number of state changes in each sampling interval is a digital value representing the analog input voltage for the interval. 
     In another aspect of the disclosure, the variance of the sensing circuitry  200  may be reduced by using a collection of STT-MTJs configured in parallel in the STT-MTJ circuitry  208 . This configuration of STT-MTJ circuitry  208  is referred to as a parallel multi-level cell (MLC). In a multi-level cell, errors introduced by some STT-MTJs are offset by opposite errors introduced by other STT-MTJs. Multi-level cell configurations according to aspects of the present disclosure provide improved tunability of frequency, resolution and power of the sensing circuitry  200 . 
     According to aspects of the present disclosure a multi-level cell configured with a number (N) of STT-MTJs in parallel can assume N+1 possible different states corresponding to different electrical resistance of the parallel STT-MTJs. The N+1 different states correspond to the number of switched STT-MTJs in the multi-level cell. Because switching of a single STT-MTJ is a Bernoulli Random variable ˜Ber(p), a combination of such STT-MTJs in a multi-level cell behaves like a Binomial random variable. The probability distribution of the multi-level cell is Binomial(N,p), where N is the number of STT-MTJs and p is the probability of switching for a single STT-MTJ. According to one aspect of the disclosure, a cluster of STT-MTJs may be configured in a multi-level cell to establish the pre-determined probability relationship. The cluster may include a number of substantially similar STT-MTJs or a number of substantially dissimilar STT-MTJs, for example. 
     According to another aspect of the present disclosure, the amplified and biased signal is applied to a multi-level cell for a sample time that is not fixed. Rather than counting the number of times an STT-MTJ changes state as described previously with reference to  FIG. 3 , the counter circuitry  212  counts the time for a multi-level cell to reach a particular state in which a certain number of STT-MTJs have reached a switched state in response to the amplified and biased signal. The time to reach a particular state is a digital signal representing the analog input voltage. When the target threshold is reached, the STT-MTJs within the multi-level cell are reset and the sensing process repeats. 
     An example of a multi-level cell according to an aspect of the present disclosure is described with reference to  FIGS. 4A-4C . In  FIG. 4A , a set of STT-MTJs arranged in parallel are in a preset state at time t=0 in which none of the STT-MTJs have been switched. In this example, the number of STT-MTJs N=5 and a target threshold of 3 switched STT-MTJs is predetermined. An input voltage is applied to the multi-level cell at time t=0 ns.  FIG. 4B  shows the multi-level cell at time t=6 ns.  FIG. 4C  shows the multi-level cell when the target threshold is reached at time t=9 ns. The time t=9 ns represents the magnitude of the analog input voltage. 
     Aspects of the present disclosure include a method for reducing the time and power for resetting individual STT-MTJs in a multi-level cell after an iteration of a sensing process. In one configuration, the sensing circuitry for the STT-MTJs may be configured with reset circuitry coupled to the STT-MTJs for bidirectional switching so that the input signal may be used for both setting and resetting an STT-MTJ. This configuration increases resolution of the sensing circuitry without using additional power. In another configuration of reset circuitry, the STT-MTJs in one multi-level cell may be reset while the STT-MTJs in another multi-level cell are being written to in the sensing/ADC process described above. These resetting methods may be combined in a third configuration of reset circuitry in which bidirectional STT-MTJs are configured in a pair of cooperating multi-level cells. One multi-level cell may be reset while the STT-MTJs in another multi-level cell are being written, for example. 
     According to an aspect of the present disclosure, sensing circuitry including a multi-level cell can be configured to perform data compression automatically during a sensing and ADC process. In this configuration, sensing circuitry is configured to quantize the slope of an input signal rather than the magnitude of the input signal as described above. Because slope is a measure of the change of information, sampling frequency may be reduced when the slope is relatively low in order to compress the quantized information. Circuitry for performing adaptive slope quantization using a multi-level cell configured to measures larger sloped portions of a signal more frequently than smaller sloped portions of the signal according to aspects of the present disclosure is described with reference to  FIG. 5 . 
     Referring to  FIG. 5 , adaptive slope quantization circuitry includes a multi-level cell  502  coupled to bias voltage supply circuitry  504 . First input amplifier circuitry  506  is coupled between the bias voltage supply circuitry  504  and an input node  508 . Second input amplifier circuitry  510  is coupled between the multi-level cell  502  and a precharge node  512 . A precharge capacitor  514  is coupled between the precharge node  512  and ground. Precharge switching circuitry  516  is coupled between the input node  508  and the precharge node  512 . 
     Adaptive slope quantization may be performed by closing the precharge switching circuitry  516  until the precharge capacitor  514  is charged to a present input voltage on the input node  508 . The precharge switching circuitry  516  is then opened to enable the multi-level cell  502  to sense a differential signal, which represents a change in the input signal voltage. Counter circuitry determines the time or number of cycles taken for the multi-level cell to reach a predetermined target state as described above with reference to  FIGS. 4A-4C , for example. When the target threshold is reached, the STT-MTJs within the multi-level cell are reset and the sensing process is repeated. 
     Because the multi-level cell is referenced to a precharge node, which is charged to a previous voltage, the time or number of cycles taken for the multi-level cell to reach a predetermined target state represents the change in voltage from the previous period and provides a quantized representation of the signal slope during the measurement period. This aspect of the disclosure automatically compresses the signal by acquiring less data for low slope portions of a signal because the multi-level cell takes longer to reach the target threshold when the input signal has a lower slope. Thus, the adaptive slope quantization circuitry adaptively adjusts the amount of information acquired in response to the signal slope. 
     A method of signal sensing according to aspects of the present disclosure is described with reference to  FIG. 6 . In block  602 , a bias value of a probabilistic sensor device is preset. In block  604 , a signal is continuously sampled relative to the bias value until the sensor device reaches a target state. In block  606 , an amount of time for the target state to be reached is recorded. In block  608 , the bias value is reset to equal a current signal level. In block  610 , the probabilistic sensor device is reset. 
     A sensor device according to aspects of the present disclosure may include means for continuously sampling a signal relative to a bias value until the sensor device reaches a target state relative to a bias level and means for recording an amount of time for the target state to be reached. The sensor device may further include means for resetting the bias value to equal the current signal level when the target state is reached; and means for resetting the sensor device when the target state is reached. The means for continuously sampling a signal may include circuitry such as STT-MTJ circuitry  208 , which is described with reference to  FIG. 2 , for example. The means for recording an amount of time for the target state to be reached may include circuitry such as counter circuitry  212 , which is described with reference to  FIG. 2 , for example. The means for resetting the bias value may include circuitry such as pre-charge switching circuitry  516 , which is described with reference to  FIG. 5 , for example. The means for resetting the sensor device may include the previously described reset circuitry, for example. In another configuration, the aforementioned means may be any module or any apparatus configured to perform the functions recited by the aforementioned means. Although specific means have been set forth, it will be appreciated by those skilled in the art that not all of the disclosed means are required to practice the disclosed configurations. Moreover, certain well known means have not been described, to maintain focus on the disclosure. 
       FIG. 7  shows an exemplary wireless communication system  700  in which a configuration of the disclosed signal sensing methods may be advantageously employed. For purposes of illustration,  FIG. 7  shows three remote units  720 ,  730 , and  750  and two base stations  740 . It will be recognized that wireless communication systems may have many more remote units and base stations. Remote units  720 ,  730 , and  750  include the sensing circuitry  725 A,  725 B, and  725 C, respectively.  FIG. 7  shows forward link signals  780  from the base stations  740  and the remote units  720 ,  730 , and  750  and reverse link signals  790  from the remote units  720 ,  730 , and  750  to base stations  740 . 
     In  FIG. 7 , the remote unit  720  is shown as a mobile telephone, remote unit  730  is shown as a portable computer, and remote unit  750  is shown as a fixed location remote unit in a wireless local loop system. For example, the remote units may be cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment. Although  FIG. 7  illustrates remote units, which may employ sensing circuitry according to the teachings of the disclosure, the disclosure is not limited to these exemplary illustrated units. For instance, sensing circuitry according to configurations of the present disclosure may be suitably employed in any device. 
     Although specific circuitry has been set forth, it will be appreciated by those skilled in the art that not all of the disclosed circuitry is required to practice the disclosed configurations. Moreover, certain well known circuits have not been described, to maintain focus on the disclosure. 
       FIG. 8  is a block diagram illustrating a design workstation used for circuit, layout, and logic design of a semiconductor component, such as the sensing circuitry disclosed above. A design workstation  800  includes a hard disk  801  containing operating system software, support files, and design software such as Cadence or OrCAD. The design workstation  800  also includes a display  802  to facilitate design of a circuit  810  or a semiconductor component  812  such as the sensing circuitry. A storage medium  804  is provided for tangibly storing the circuit design  810  or the semiconductor component  812 . The circuit design  810  or the semiconductor component  812  may be stored on the storage medium  804  in a file format such as GDSII or GERBER. The storage medium  804  may be a CD-ROM, DVD, hard disk, flash memory, or other appropriate device. Furthermore, the design workstation  800  includes a drive apparatus  803  for accepting input from or writing output to the storage medium  804 . 
     Data recorded on the storage medium  804  may specify logic circuit configurations, pattern data for photolithography masks, or mask pattern data for serial write tools such as electron beam lithography. The data may further include logic verification data such as timing diagrams or net circuits associated with logic simulations. Providing data on the storage medium  804  facilitates the design of the circuit design  810  or the semiconductor component  812  by decreasing the number of processes for designing semiconductor wafers. 
     For a firmware and/or software implementation, the methodologies may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. A machine-readable medium tangibly embodying instructions may be used in implementing the methodologies described herein. For example, software codes may be stored in a memory and executed by a processor unit. Memory may be implemented within the processor unit or external to the processor unit. As used herein the term “memory” refers to types of long term, short term, volatile, nonvolatile, or other memory and is not to be limited to a particular type of memory or number of memories, or type of media upon which memory is stored. 
     If implemented in firmware and/or software, the functions may be stored as one or more instructions or code on a computer-readable medium. Examples include computer-readable media encoded with a data structure and computer-readable media encoded with a computer program. Computer-readable media includes physical computer storage media. A storage medium may be an available medium that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer; disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     In addition to storage on computer readable medium, instructions and/or data may be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus may include a transceiver having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular configurations of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding configurations described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.