- Nano Express
- Open Access
Neuromorphic crossbar circuit with nanoscale filamentary-switching binary memristors for speech recognition
© Truong et al.; licensee Springer. 2014
- Received: 15 July 2014
- Accepted: 25 September 2014
- Published: 23 November 2014
In this paper, a neuromorphic crossbar circuit with binary memristors is proposed for speech recognition. The binary memristors which are based on filamentary-switching mechanism can be found more popularly and are easy to be fabricated than analog memristors that are rare in materials and need a more complicated fabrication process. Thus, we develop a neuromorphic crossbar circuit using filamentary-switching binary memristors not using interface-switching analog memristors. The proposed binary memristor crossbar can recognize five vowels with 4-bit 64 input channels. The proposed crossbar is tested by 2,500 speech samples and verified to be able to recognize 89.2% of the tested samples. From the statistical simulation, the recognition rate of the binary memristor crossbar is estimated to be degraded very little from 89.2% to 80%, though the percentage variation in memristance is increased very much from 0% to 15%. In contrast, the analog memristor crossbar loses its recognition rate significantly from 96% to 9% for the same percentage variation in memristance.
- Speech recognition
- Filamentary switching
- Binary memristors
The memristors that had been mathematically predicted by Leon O. Chua in 1971 as the fourth basic circuit element  were experimentally found in 2008 . Since the first prediction of memristors, they have been thought as a potential candidate for future neuromorphic computing systems. Among the many advantages of memristors, particularly, the nonlinear charge-flux relationship is important in mimicking synaptic plasticity of biological neuronal systems such as human brains [3–7].
In realizing memristor-based synaptic systems, a crossbar circuit that is made of only passive memristors can be thought of as the densest and simplest architecture among various synaptic circuits that have been developed previously. If a crossbar circuit is made of both memristors and selectors such as transistors and diodes, this kind of hybrid-type crossbar circuit is difficult to be stacked layer by layer. Thus, the pure crossbar circuit with only passive memristors can be a key element to implement the densest and simplest three-dimensional architecture of neuromorphic systems.
In addition to the advantage of popularity of filamentary-switching materials, binary memristors can be much more tolerant against statistical variations compared to analog memristors. This is due to the fact that HRS can still be much higher than LRS, in spite of the large amount of statistical variation in LRS and HRS.
In this paper, we propose a binary memristor crossbar circuit for recognizing five different vowels. The block diagram and the detailed circuit schematic are shown and explained in the following section. In addition, the circuit simulation and statistical simulation are performed, and the simulation results are discussed and finally summarized in this paper .
The weighted summation of Ia is calculated with 8Ia,3 + 4Ia,2 + 2Ia,1 + Ia,0, as explained just earlier. The circuit for performing the weighted summation is implemented by current mirror circuits as shown in Figure 4a. For example, to realize the weight of ‘1’, we use the current mirror circuit, which is composed of M7 and M8. Here, M7 and M8 should have the same size. By doing so, Ia,0 of M7 can be copied to M8. If the weight is 2, the size of M6 should be twice larger than M5. Thereby, the current of M6 can be twice larger than Ia,1. For the weight factor of 4, M4 should be four times larger than M3. For the weight factor of 8, M2 should be eight times larger than M1. The currents of M2, M4, M6, and M8 can be summated by Kirchhoff's current law. The capacitor Ca can be discharged by the weighted summation of Ia, which comes from M2, M4, M6, and M8. If the weighted summation of Ia is large, Ca can be discharged to GND very fast. Here, GND means the ground potential. If the weighted summation of Ia is small, it takes longer time to discharge Ca to GND. M9 is the precharge PMOS, which becomes on when the clock (CLK) signal is low. If M9 is on, the VCa node is precharged by VDD. When the CLK signal is high, M9 is off. At this time, VCa can be discharged by the weighted summation of Ia that comes from M2, M4, M6, and M8.
Figure 4b shows the winner-take-all circuit that can decide which capacitor becomes discharged the fastest among the five capacitors of Ca, Ci, Cu, Ce, and Co. The five capacitors of Ca, Ci, Cu, Ce, and Co are corresponding to the five vowels ‘a’, ‘i’, ‘u’, ‘e’, and ‘o’, respectively. Using the winner-take-all circuit, we can figure out that a certain vowel corresponding to the fastest-discharged capacitor is the best match with the input of a human voice. VCa, VCi, VCu, VCe, and VCo are the voltages on capacitors Ca, Ci, Cu, Ce, and Co, respectively. Here, I1, I2, I3, I4, and I5 are the comparators. In this case, I1 compares VCa with VREF. VREF is a reference voltage to the comparators. If VCa becomes lower than VREF, Da becomes high. Similarly, I2, I3, I4, and I5 compare VCi, VCu, VCe, and VCo with VREF. Di, Du, De, and Do become high when VCi, VCu, VCe, and VCo are lower than VREF. I6, I7, and I8 are the OR gates. I9 and I10 with the delay line of τ constitute a pulse generator circuit. FF1, FF2, FF3, FF4, and FF5 are D flip-flop circuits. Outputa, Outputi, Outputu, Outpute, and Outputo are the output signals of five D flip-flops from FF1 to FF5.
Now, we can consider Figure 5b, where the input voltages of Vi,0 and Vi,1 do not match with the stored memristance of M5, M6, M7, and M8. The current summation of Ib in Figure 5b can be expressed with Ib = I2,b + I3,b − I1,b − I4,b. Here, I2,b and I3,b are the forward currents through HRS. I1,b and I4,b are the reverse currents through LRS. If we compare the matched column's current of Ia in Figure 5a with the unmatched column's current of Ib, we can be sure that Ia is much larger than Ib. Thus, we can think that the reverse current does not degrade the recognition rate.
When the memristance variation is as low as 0%, the recognition rate of the analog memristor array is higher by 6.8% than the binary memristor array. This is due to the fact that the proposed binary memristor crossbar has a 4-bit resolution; thus, it loses some amount of accuracy compared to the analog memristor crossbar. As the percentage of variation in memristance is increased, the recognition rate of analog memristor crossbar becomes degraded very rapidly. For example, when the percentage variation in memristance becomes 5%, the recognition rate of the analog crossbar is decreased from 96% to 23%. On the contrary, the binary memristor crossbar can keep almost the same amount of recognition rate for five vowels. For a percentage variation as severe as 15%, the analog crossbar shows a recognition rate as low as 9%. However, the binary crossbar still keeps the recognition rate as high as 80%, indicating that it is only degraded by 9.2% compared to the percentage variation of 0%. This strong tolerance of the binary memristor crossbar is due to the fact that the accuracy of the information stored in binary memristors can be little affected by the percentage variation in memristance. Memristance of LRS can still be much smaller and cannot become larger than that of HRS, even though the percentage variation in LRS is very large. This is the reason why the binary memristor crossbar can maintain the recognition rate over 80% regardless of the percentage variation in memristance.
In this paper, the binary memristor crossbar circuit was proposed for neuromorphic application of speech recognition. Compared with analog memristors that are rare in available materials and need a complicated fabrication process, binary memristors which are based on the filamentary-switching mechanism are found more popularly and easy to be fabricated. Thus, we developed the neuromorphic crossbar circuit using filamentary-switching binary memristors instead of interface-switching analog memristors. The proposed binary memristor crossbar could recognize five vowels with 64 input channels and a 4-bit resolution. The proposed crossbar array was tested by 2,500 speech samples and verified to be able to recognize 89.2% of the total tested samples. Moreover, the recognition rate of the binary memristor crossbar is degraded very little only from 89.2% to 80%, even though the percentage statistical variation in memristance is increased from 0% to 15%. In contrast, the analog memristor crossbar is degraded significantly from 96% to 9% with the same percentage variation in memristance.
SNT and SJH are Ph.D. and M.S. students, respectively, who are studying in the School of Electrical Engineering, Kookmin University, Seoul, South Korea. KSM is a professor in the School of Electrical Engineering, Kookmin University, Seoul, South Korea.
The work was financially supported by NRF-2011-220-D00089, NRF-2011-0030228, NRF-2013K1A3A1A25038533, NRF-2013R1A1A2A10064812, and BK Plus with the Educational Research Team for Creative Engineers on Material-Device-Circuit Co-Design (Grant No: 22A20130000042), funded by the National Research Foundation of Korea (NRF), and by Global Scholarship Program for Foreign Graduate Students at Kookmin Univ. The CAD tools were supported by IC Design Education Center (IDEC), Daejeon, Korea.
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