Expandable analog current sorter based on magnitude

[0002] 1. Field of the Invention

[0003] The present invention relates to a sorter for sorting a plurality of currents, more particular, to an expandable current magnitude sorter for sorting a plurality of currents.

[0004] 2. Description of Related Art

[0005] Sorting is the operation of arranging non-sequential data into sequential data. Such sorting operations have been widely used in data processing system in many fields. Currently, there are several types of sorting processes available, such as bubble sorting, shell sorting, fast sorting, etc. However, those processes are difficult to perform in integrated circuitry. The sorting operation is implemented essentially by utilizing software in computers. Therefore, the operational speed, real-time processing and application for the sorting operation are seriously limited.

[0006] The implementation of the sorting operation in hardware has been gradually developed. However, the existing sorting circuits are almost always digital sorting circuits. The structure of a digital sorting circuit is very complicated and the required sorting time is very long. Although the digital sorting circuit can also be used for analog signals, A/D and D/A converters are required, so that the structure of the circuit is even more complicated. In addition, errors or transformations may occur in the conversion between digital signals and analog signals.

[0007] Accordingly, an analog current sorting circuit is desired, thus, the present invention is designed for this purpose.

[0008] One object of the present invention is to provide a current sorter which is an analog current sorter with simple structure.

[0009] Another object of the present invention is to provide a current sorter such that the number of currents to-be-sorted can be significantly increased by cascading a plurality of the current sorters. In accordance with one aspect of the present invention, the current sorter comprises an input circuit unit for receiving a plurality of input currents to-be-sorted, a winner-take-all (WTA) circuit unit for finding the maximum current, a feedback control and voltage output circuit unit for generating feedback control signals and output voltages indicating the maximum current, and an output circuit unit for outputting sorted currents. A plurality of input currents are simultaneously input to the input circuit unit and the sorted results are output in a time-shared manner on the output circuit unit.

[0010] Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

[0011] FIG. 1A and FIG. 1B are circuit diagrams of a current sorter in accordance with the present invention.

[0012] FIG. 2 shows the pin configuration of the current sorter for operating in a single mode.

[0013] FIG. 3 is a timing diagram for the current sorter operating in the single mode.

[0014] FIG. 4 shows the pin configuration of the current sorter for operating in an expanded mode.

[0015] FIG. 5 is a timing diagram for the current sorter operating in the expanded mode.

[0016] FIG. 6 illustrates a simulation result of the pulse shape of Vout of the first case in Table 1 which gives PSPICE simulation results for three cases in the single mode.

[0017] FIG. 7 illustrates a simulation result of the pulse shape of Vout of the first case in Table 2 which gives PSPICE simulation results for three cases in the expanded mode.

[0018] Referring to FIG. 1A, an embodiment of an expandable current sorter in accordance with the present invention is illustrated by taking three input currents for example, which comprises four circuit units: an input circuit unit 10, a winner-take-all (hereinafter abbreviated as WTA) circuit unit 20, a feedback control and voltage output circuit unit 30, and an output circuit unit 40.

[0019] The input circuit unit 10 comprises three identical input units where Iin_i(0≦i≦2) designate three input currents to-be-sorted. For simplicity, only the input unit on the left-most side is described. In this input unit, mirror transistors M₁₁and M₁₂constitute a current mirror and mirror transistors M₁₄and M₁₅also constitutes another current mirror. A switch transistor M₁₃controls the magnitude of an output current I₀. When the switch transistor M₁₃is on, I₀equals Iin₀, and when the switch transistor M₁₃is off, I₀is zero.

[0020] The WTA circuit unit 20 having three inputs is provided to find the maximum current among the input currents. The WTA circuit unit 20 includes three identical sections and only the left-most section is described for convenience. In the WTA circuit unit 20, the dimensions of all the NMOS transistors corresponding to the transistors M₂₁, M₂₂, M₂₃and M₂₄are identical. The width to length ratio (W/L) of M₂₈is twice as that of M₂₁. VO, IO terminals and control terminal C (NC is an inverse signal of C) are adapted for chip expansion which will be described later on. The WTA circuit unit 20 is a high-precision and high-speed interconnected network where the number of transistors therein is linearly related to that of the inputs thereof. To analyze the operation of the WTA circuit unit, the control terminal C is first set to a high voltage level which cause the gate and drain of the NMOS transistor M₂₈short-connected whereby the transistor M₂₈works in the saturation region and behaves as a diode. When the WTA circuit unit 20 is in operation, voltages V₀, V₁and V₂are established respectively by the input current I₀, I₁and I₂from input circuit unit 10. For the sake of convenience, assuming I₀=max (I₀, I₁, I₂), we have V₀=max (V₀, V₁,V₂). Transistors M₂₃, M₂₄and corresponding NMOS transistors constitute a differential circuit, and voltages V₀, V₁and V₂are input voltages to the differential circuit. When |V₀−V_i>(2I_y/β)^½ for i=1,2 is satisfied, where β=[μC_ox/2](W/L) and W/L is the width to length ratio of M₂₈, I_Yflows through a differential transistor having the maximum input voltage; that is, the drain currents of M₂₃and M₂₄are I_Y/2 and the drain currents of the other corresponding differential transistors are zero. On the other hand, transistors M₂₁, M₂₂, M₂₃, M₂₄and M₂₈constitute a Willson current mirror resulting in I_Y=2I₀and the drain currents of M₂₃and M₂₄being I₀. Therefore, we have I_wout=I₀=max (I₀, I₁, I₂) and the maximum current has been obtained. In addition, PMOS mirror transistors M₂₆and M₂₇and NMOS transistors M₂₅constitute a non-linear current-to-voltage transform circuit. This current-to-voltage transform circuit transforms the drain current of M₂₃to a low voltage level for outputting from VSout₀if the drain current is smaller than a predetermined threshold value, and transforms the drain current of M₂₃to a high voltage level for outputting from VSout₀if the drain current is larger than the threshold value. The threshold value is adjusted by an external bias voltage VP. The other corresponding non-linear circuit-to-voltage transform circuits in WTA circuit unit 20 are similar to aforesaid transform circuit.

[0021] The feedback control and voltage output circuit unit 30 comprises three identical transfer units 31,32,33. The circuit diagram for each transfer unit 31,32,33 is shown in FIG. 1B where the SW unit therein is a CMOS switch and the NCK is an inverse signal of CK. The feedback control and voltage output circuit unit 30 generates feedback control signals CT_i(0≦i≦2) according to VSout_i(0≦i≦2), which are output from WTA circuit unit 20, to control the output currents of the input circuit unit 10. In addition, the feedback control and voltage output circuit unit 30 is able to convert the low-to-high voltage level from VSout to a high voltage pulse for outputting from Vout. This high voltage pulse is used to determine the corresponding input terminal with respect to the sorted output current for processing the sorted currents.

[0022] The output circuit unit 40 is designed for outputting sorted currents. NMOS transistors M₃₂, M₃₃and M₃₄are three switch transistors respectively controlled by non-overlapped clock signals CK₀, CK₁, and CK₂. PMOS transistors M₃₅, M₃₆, and M₃₇are mirror transistors, each is identical to M₃₁in size. The control terminal C (NC is an inverse signal of C) is used for chip expansion. In operating the current sorter, the IO terminal is floating and the control terminal C is set to high whereby the gate and drain of the PMOS transistor M₃₁is short-connected. Under the control of clock CK_i(0≦i≦2), the output current I_woutfrom the WTA circuit unit 20 can be mirror-mapped to output terminals in a time shared way to generate Iout₀, Iout₁and Iout₂. The Iout₀, Iout₁and Iout₂are the sorted currents of input currents Iin_i(0≦i≦2).

[0023] Two operation modes are provided for the current sorter; they are single mode and expanded mode. In the single mode, there is only one chip with N inputs used to sort N input currents. In the expanded mode, there are M chips, each has N inputs, used to sort M×N input currents.

[0024] Taking N=3 and M=2 for example, FIG. 2 and FIG. 3 show the pin configuration and timing diagrams for a chip in the single mode. To operate in the single mode, the control terminal C is set to high, the IO and VO terminals are floating, and the gates and drains of M₂₈and M₃₁are short-connected respectively. The operation of sorting is started by first asserting a high voltage level of reset signal which results in the Vout_i(0≦i≦2) being low voltage levels and CT_i(0≦i≦2) being high voltage levels whereby I_i(0≦i≦2)=Iin_i(0≦i≦2) in the input circuit unit 10. In addition, the VSout of the WTA circuit unit 20 is sampled by the feedback control and voltage output circuit unit 30 due to the high voltage level of the CT. Assuming Iin₀=max (Iin₀, Iin₁, Iin₂), the maximum current I_wout=Iin₀=max (Iin₀, Iin₁, Iin₂) is obtained from the WTA circuit unit 20. Meanwhile, VSout₀is high and VSout₁and VSout₂are low. At the instance of T1, clock signals CK and CK₀become high. In the output circuit unit 40, the high CK₀drives the switch transistor M₂₃on and the maximum current I_woutis mirror-mapped to the drain of M₃₅to generate Iout₀, that is, Iout₀=I_wout=Iin₀. In the feedback control and voltage output unit 30, the high VSout₀causes Vout₀to be high while Vout₁and Vout₂remain low due to the low voltage levels of VSout₁and VSout₂. At the instance of T2, clock signals CK and CK₀become low. In the output circuit unit 40, the switch transistor M₂₃is off and Iout₀is still the maximum current Iin₀due to the sampling/holding effect of the switch current mirror. In the feedback control and voltage output unit 30, the low CK causes Vout₀and CT₀to be low while Vout₁and Vout₂remain low and CT₁and CT₂remain high. Thus, a high voltage pulse is generated on the Vout₀terminal. On the other hand, the low CT₀isolates a portion of the feedback control and voltage output unit 30, which is corresponding to Iin₀, from unit 2 whereby Vout₀and CT₀always remain low until the next reset signal is inserted. In the input circuit unit 10, the low CT₀makes M₁₃off resulting in I₀being zero, whereby I₀will not influence the sequential operations. Similarly, the second maximum current is determined by the process described above and held on Iout₁terminal. A high voltage pulse is also generated on the corresponding Vout terminal. In this manner, all of the input currents to-be-sorted are presented on Iout_i(0≦i≦2) in an order of magnitude under the control of the clock signals. Meanwhile, high voltage pulses are sequentially generated on the corresponding Vout terminals for determining the input terminals with respect to the sorted currents.

[0025] In the expanded mode, it is able to sort M×N currents by expanding M chips, each having N current inputs. In this mode, the reset terminals, the CK terminals, the VO terminals and the IO terminals of the M chips are connected together respectively. Meanwhile, one of the control terminal C is set to high and the others are set to low. For convenience, taking N=3 and M=2 for example, FIG. 4 and FIG. 5 give the pin configuration and timing diagrams respectively. Assuming that C₁is set to high and C₂is set to low, referring to FIG. 1 again, the gate voltage of the NMOS transistor M₂₈in chip 2 is low and the gate voltage of the PMOS transistor M₃₁is high; that is, the M₂₈and M₃₁do not have any effect on sorting operations. Thus, the M₂₈and M₃₁in chip 1 are shared by two chips. Obviously, the two chips, each having three inputs, have been merged to one chip having six current inputs. The operation of this sixinput chip is similar to that of the three-input chip as described above; that is, all of the input currents to-be-sorted are presented on Iout_i(0≦i≦5) in an order of magnitude under the control of the clock signals and high pulses are sequential generated on the corresponding Vout terminals.

[0026] It is appreciated that, no matter whether in the single mode or in the expanded mode, the sorting time is linear relative to the number of input currents N; that is, the time complexity of sorting is O(N). In terms of the circuit structure, due to its simple structure, the chip area is linear relative to the number of input currents; that is, the area complexity is also O(N). Moreover, the sorter is ideally suited for various applications. Because the current outputs lout and voltages Vout are generated individually, it is possible to select the desired pins as required. The manner for outputting current can be controlled by adjusting the clock CK_iin the output circuit unit 40. For example, when only CK₀is asserted, the sorter is simplified to be a circuit for finding the maximum current, and when only CK₂is asserted, it is simplified to be a circuit for finding the minimum current

[0027] PSPICE simulations are made to the current sorter for several typical cases as shown in FIG.6 and FIG. 7. In the single mode, a simulation is made by taking M=1 and N=3 for example. For a first case, Iin_i(i=0,1,2) are 135 μA, 140 μA and 130 μA respectively. For a second case, Iin_i(i=0,1,2) are 90 μA, 95 μA and 100 μA respectively. For a third case, Iin_i(i=0, 1,2) are 50 μA, 40 μA and 45 μA respectively. Referring to FIG. 6, a simulation output waveform of Vout_i(i=0,1,2) is given for the first case. Table 1 gives the output values of lout_i(i=0,1,2) for the three cases and the maximum errors ε_maxbetween the input currents and the corresponding output currents. In the expanded mode, a simulation is made by taking M=2 and N=3 for example. For a first case, Iin_i(i=0,1,2,3,4,5) are 135 μA, 125 μA ,130 μA, 115 μA, 140 μA and 120 μA respectively. For a second case, Iin_i(i=0,1,2,3,4,5) are 85 μA, 100 μA, 90 μA, 80 μA ,75 μA and 95 μA respectively. For a third case, Iin_i(i=0,1,2,3,4,5) are 45 μA, 25 μA, 40 μA, 35 μA, 30 μA and 50 μA respectively. Referring to FIG. 7, a simulation output waveform of Vout_i(i=0,1,2,4,5) is given for the first case. Table 2 gives the output values of Iout_i(i=0,1,2,3,4,5) for the three cases and the maximum errors ε_maxbetween the input currents and the corresponding output currents. According to the PSPICE simulation results, it is known that the offset between an input current and its corresponding output current is small. The maximum offset is smaller than 5 μA and thus the current sorter in accordance with the present invention has an advantage in providing high distinguishing capability.

[0028] Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.