Abstract:
A motor control apparatus to control a motor external to the motor control apparatus includes a microcontroller unit (MCU). The MCU includes mixed signal motor control circuitry adapted to perform back electromotive force (EMF) motor control in a first mode of operation. The mixed signal motor control circuitry is further adapted to perform field oriented control (FOC) in a second mode of operation.

Description:
TECHNICAL FIELD 
       [0001]    The disclosures relates generally to motor control apparatus and, more particularly, to apparatus for a motor control system using a microcontroller unit (MCU), and associated methods. 
       BACKGROUND 
       [0002]    Since their inception, electric motors have become more commonly used blocks in electrical, electronic, and electromechanical systems. Over time, different types of motors have been invented. Some motors have more specialized uses, while other motors, for example, the AC induction motor, have relatively widespread use in many areas. 
         [0003]    The applications of electric motors have also evolved over time. Consequently, electric motors are now used in many areas of scientific, consumer, industrial, and medical products. Although in some applications, for example, a typical consumer-grade cooling fan, the electric motor is powered on or off, other applications entail more sophisticated control of motors. For example, the speed, torque, direction of rotation, and perhaps other attributes of motors are controlled in various applications. 
         [0004]    To provide the capability to control various attributes of motors, motor controllers have been developed. The motor controllers usually include an electronic circuit that is used together with a power semiconductor drive circuit, such as an inverter. Different motor controllers can control different motors, such as alternating current (AC) motors or direct current (DC) motors, using techniques such as back electromotive force (back EMF) control, and field oriented control (FOC), direct space vector modulation (DSVM), pulse width modulation (PWM), etc., as persons of ordinary skill in the art understand. 
       SUMMARY 
       [0005]    A variety of motor control apparatus and related techniques are disclosed and contemplated. In one exemplary embodiment, a motor control apparatus to control a motor external to the motor control apparatus includes an MCU. The MCU includes mixed signal motor control circuitry adapted to perform back EMF motor control in a first mode of operation. The mixed signal motor control circuitry is further adapted to perform field oriented control in a second mode of operation. 
         [0006]    According to another exemplary embodiment, a motor control system includes a motor, and an inverter coupled to the motor to supply power to the motor. The motor control system further includes a single MCU. The MCU includes a mixed signal motor control circuit adapted to operate in first and second modes of operation. In the first mode of operation the mixed signal motor control circuit provides a first set of control signals to the inverter to control the motor using back EMF control. In the second mode of operation the mixed signal motor control circuit provides a second set of control signals to the inverter to control the motor using field oriented control. 
         [0007]    According to another exemplary embodiment, a method of controlling an electric motor, using an MCU having first and second modes of operation, includes selecting the first mode of operation or the second mode of operation. The method further includes configuring the MCU to operate in either the first mode of operation to control the motor using back EMF control, or in the second mode of operation to control the motor using field oriented control. The method further includes operating the MCU in the selected one of the first and second modes of operation to generate a set of motor control signal, and providing the set of motor control signals to an inverter adapted to control the motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The appended drawings illustrate only exemplary embodiments and therefore should not be considered as limiting its scope. Persons of ordinary skill in the art appreciate that the disclosed concepts lend themselves to other equally effective embodiments. In the drawings, the same numeral designators used in more than one drawing denote the same, similar, or equivalent functionality, components, or blocks. 
           [0009]      FIG. 1  illustrates a block diagram of a motor control system according to an exemplary embodiment. 
           [0010]      FIG. 2  depicts a circuit arrangement for controlling motors according to an exemplary embodiment. 
           [0011]      FIG. 3  shows a block diagram of a mixed signal motor control circuit according to an exemplary embodiment. 
           [0012]      FIG. 4  depicts a more detailed block diagram of a mixed signal motor control circuit according to an exemplary embodiment. 
           [0013]      FIG. 5  illustrates various signals and associated values used to implement different types of motor control schemes according to an exemplary embodiment. 
           [0014]      FIGS. 6A and 6B  show a partial block diagram of mixed signal motor control circuits according to exemplary embodiments. 
           [0015]      FIG. 7  depicts a circuit arrangement for extended or flexible multiplexing arrangements according to an exemplary embodiment. 
           [0016]      FIG. 8  illustrates a circuit arrangement to provide blanking time support according to an exemplary embodiment. 
           [0017]      FIG. 9  shows a circuit arrangement for using blanking time signals according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The disclosure relates generally to motor control apparatus and related techniques. More specifically, the disclosure relates to apparatus for a motor control system using an MCU, and associated methods. Motor control systems according to various embodiments provide a flexible, yet powerful, technique for controlling more than one type of motor, as described below in detail. 
         [0019]    According to various embodiments, a motor control system may be used that supports both back EMF control and field oriented control. More specifically, in various embodiments, a motor control system uses an MCU that can support both back EMF control of a brushless DC (BLDC) motor, and field oriented control of a permanent magnet synchronous motor or AC induction motor. 
         [0020]    The control scheme may be flexibly programmed by the user of the motor control system. A variety of motors may be controlled using a single integrated MCU that includes mixed signal motor control circuitry. For example, in one application, the motor control circuitry may be programmed to control a BLDC motor in one mode of operation. In another application, the motor control circuitry may be programmed to control a permanent magnet synchronous motor or AC induction motor in another mode of operation. 
         [0021]    Conventional motor controllers address either sensorless control or field oriented control, but not both. Motor control systems according to various embodiments, however, provide the resources and flexibility to support both of these types of motor control in a single integrated MCU. The system supports back EMF control, using either an analog to digital converter (ADC) or comparators, in one mode of operation. It also supports field oriented control, using three, two, or one current sense resistors, in another mode of operation. 
         [0022]      FIG. 1  illustrates a block diagram of a motor control system according to an exemplary embodiment. The motor control system includes an MCU  15 , coupled to an external inverter and motor combination  30  via link  35 . The external inverter and motor combination  30  may include an inverter or other suitable circuitry to supply power or drive signals to a motor. 
         [0023]    In exemplary embodiments, the motor may be a BLDC, a permanent magnet synchronous motor, or an AC induction motor. As noted above, MCU  15  has multiple modes of operation, which allow support of the above types of motor. 
         [0024]    MCU  15  provides control signals to external inverter and motor combination  30  via link  35 , as described below in detail. Furthermore, via link  35 , external inverter and motor combination  30  may provide various data or information, for example, current signals or levels, to MCU  15 , for instance, to mixed signal motor control circuit  20 . 
         [0025]    In exemplary embodiments, link  35  may include one or more coupling mechanisms. The coupling mechanisms may include a variety of types of conductor, cable, printed circuit board (PCB) traces, etc. Generally, the type, number, and arrangement of the coupling mechanisms depends on the design and performance specifications for a given motor control system implementation, as persons of ordinary skill in the art understand. 
         [0026]    In the embodiment shown, MCU  15  includes mixed signal motor control circuit  20 , central processing unit (CPU)  25 , and motor control firmware circuit  30 . CPU  25  performs general control of MCU  15 , and may also provide a variety of data or information processing capabilities, as persons of ordinary skill in the art understand. 
         [0027]    Without limitation, CPU  25  may perform general programmable logic, arithmetic, control, and/or other tasks. CPU  25  may also perform various tasks related to motor control, as described below in detail. In particular, motor control firmware circuit  30  may include instructions or information that facilitates the performance of various tasks related to motor control by CPU  25 . 
         [0028]    In exemplary embodiments, motor control firmware circuit  30  may include a non-volatile memory (NVM), such as electrically programmable read only memory (EPROM), flash memory, and the like. Motor control firmware circuit  30  may be programmed in variety of ways, as persons of ordinary skill in the art understand, for example, by using links (not shown) that interface with circuitry external to MCU  15 . 
         [0029]    By programming motor control firmware circuit  30 , the system may be customized to perform a variety of motor control algorithms or techniques, the various parameters related to motor control may be modified, fine-tuned, updated, etc., as persons of ordinary skill in the art understand. In this manner, the motor control system provides a flexible platform for controlling several types of motor. 
         [0030]    Note that motor control firmware circuit  30  may be omitted in some embodiments, and its function implemented in hardware and/or combination of hardware or software, as desired. For example, if the flexibility of using firmware is not desired (or more flexibility is desired, for example, by using software), some or all of the functionality prescribed by the firmware may be implemented using hardware. The details of such hardware circuits depend on a particular implementation, as persons of ordinary skill in the art understand. 
         [0031]    In exemplary embodiments, mixed signal motor control circuit  20  operates in conjunction with CPU  25  and motor control firmware circuit  30  to control the motor (not shown explicitly), as described below in detail. Through link  35 , mixed signal motor control circuit  20  (or MCU  15 , generally) may provide control signals, data signals, or other types of information to external inverter and motor  30 , and receive data signals, status signals, or other types of information from external inverter and motor  30 , as described below in detail. 
         [0032]    Note that  FIG. 1  illustrates a simplified block diagram of MCU  15 . MCU  15  may include a variety of other circuits to provide desired features or functionality. Without loss of generality and limitation, MCU  15  may include one or more of other circuitry, such as a power-on reset (POR) circuit, power management unit (PMU), host interface circuitry, brownout detector, watchdog timer, and the like. In some embodiments, one or more of the above circuits may be included in MCU  15 , as desired. 
         [0033]    Furthermore, rather than using an MCU, one may use other types of circuits and/or firmware or software to implement motor control systems according to various embodiments. For example, one may use microprocessors, finite state machines, programmable logic (e.g., field programmable gate arrays), and the like, by making appropriate modifications to the circuitry shown in  FIG. 1 . The choice of circuitry and associated firmware/software depends on factors such as design and performance specifications for a given motor control system implementation, available technology, cost, etc., as persons of ordinary skill in the art understand. 
         [0034]      FIG. 2  depicts a circuit arrangement for controlling motors according to an exemplary embodiment. More specifically,  FIG. 2  shows a block diagram of external inverter and motor  30 . 
         [0035]    Generally,  FIG. 2  shows a three phase inverter coupled to a motor  60 . In addition,  FIG. 2  shows a set of resistor dividers to scale various voltages related to the inverter/and or motor  60 , as described below in detail. Furthermore,  FIG. 2  shows current sensing resistors  51 A- 51 C and  54 , as described below in detail. 
         [0036]    In the embodiment shown, the inverter is a three phase inverter and drives a three phase motor  60 . As persons of ordinary skill in the art understand, however, other arrangements are possible, and contemplated, and may be implemented by making appropriate modifications. 
         [0037]    For example, in some embodiments, the inverter may be a single phase inverter and may drive a single phase motor. In such a situation, two of the three inverter legs shown in  FIG. 2  are used to drive the single phase motor. In some embodiments using this approach, the motor may be a brushed DC motor. Generally, the topology of the inverter and type of motor depend on the design and performance specifications for a given motor control system implementation, as persons of ordinary skill in the art understand. 
         [0038]    Referring to the exemplary embodiment shown in  FIG. 2 , the inverter includes three legs or circuit branches. Each leg includes an upper transistor, and a lower transistor.  FIG. 2  includes upper transistors  45 A- 45 C and lower transistors  48 A- 48 C, which correspond to the three phases, respectively. 
         [0039]    Transistors  45 A- 45 C and  48 A- 48 C act as switches to provide power from a link or supply, with a voltage V HV , to motor  60 , in a manner known to persons of ordinary skill in the art. Note that, although  FIG. 2  shows power metal oxide semiconductor field effect transistors (MOSFETs), other types of switch or device may be used, as persons of ordinary skill in the art understand. 
         [0040]    Without limitation, bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), etc., may be used, as desired. The choice of switch or device selection depends on a variety of factors, such as power and/or voltage level, size of motor  60 , switching frequency of the devices, cost, available technology, etc., as persons of ordinary skill in the art understand. 
         [0041]    A set of buffers or drivers  40  drive upper transistors  45 A- 45 C and lower transistors  48 A- 48 C. Drivers  40  may provide appropriate drive signals to cause the switching of upper transistors  45 A- 45 C (n-channel MOSFETs in the embodiment shown) and lower transistors  48 A- 48 C (n-channel MOSFETs in the embodiment shown) in response to control signals from MCU  15 . Note that, in some embodiments, upper transistors  45 A- 45 C may be p-channel MOSFETs, depending on factors such as voltage an power levels, as persons of ordinary skill in the art understand. 
         [0042]    More specifically, a set of control signals AH-CH serve as input signals to drivers  40  for upper transistors  45 A- 45 C, respectively. Another set of control signals, AL-CL, serve as input signals to drivers  40  for lower transistors  48 A- 48 C, respectively. By controlling signals AH-CH and AL-CL, MCU  15  may control upper transistors  45 A- 45 C and lower transistors  48 A- 48 C, thus controlling the supply of power to the corresponding phases of motor  60 . 
         [0043]    More specifically, node  57 A of upper transistor  45 A and lower transistor  48 A drives the first phase of motor  60 . Node  57 B of upper transistor  45 B and lower transistor  48 B drives the second phase of motor  60 . Finally, node  57 C of upper transistor  45 C and lower transistor  48 C drives the third phase of motor  60 . 
         [0044]    A set of current sense resistors,  51 A- 51 C, sense the current flowing in each leg or branch of the inverter, by generating a set of voltages that are supplied to MCU  15 . More specifically, resistor  51 A senses the current flowing in the first branch of the inverter, giving rise to voltages +I A  and −I A . The difference between voltages +I A  and −I A  is proportional to the current through the first branch of the inverter. Thus, resistor  51 A provides a differential signal to MCU  15  that is proportional to and indicates the level of current in the first branch of the inverter. 
         [0045]    Similarly, resistor  51 B senses the current flowing in the second branch of the inverter, giving rise to voltages +I B  and −I B , which is provided to MCU  15  as a differential signal. Finally, resistor  51 C senses the current flowing in the third branch of the inverter, giving rise to voltages +I C  and −I C , which is provided to MCU  15  as a differential signal. 
         [0046]    Note that in some embodiments, the current may be sensed by using two sense resistors, for example,  51 A- 51 B. In this situation, voltages +I A  and −I A  and +I B  and −I B  are provided to MCU  15  as differential signals. In other embodiments, current may be sensed by one resistor. 
         [0047]    Specifically, resistor  54  may be used to sense the current flowing through the branches of the inverter (more specifically, through lower transistors  48 A- 48 C), to develop voltages +I DC  and −I DC . Voltages +I DC  and −I DC  may be provided to MCU  15  as a differential signal. 
         [0048]    As noted, in some embodiments, some of the current sense resistors might not be used. In such situations, the unused resistors may be replaced with short circuits (e.g., a length of wire, PCB trace, etc.) to decrease or eliminate the power that would otherwise be dissipated in the unused sense resistor(s). 
         [0049]    As noted, a set of resistor dividers are used to scale various voltages in the circuit shown in  FIG. 2 . The resistor dividers scale the motor or link voltages down to levels that are safe, compatible, or comparable to the supply and/or input voltages of MCU  15 . 
         [0050]    Thus, resistor dividers scale the three phase voltages and provide the resulting scaled phase voltages as V A , V B , and V C , respectively. Similarly, a resistor divider is used to scale the supply or link voltage, V HV , down to a voltage V M . An additional resistor divider provides a scaled virtual Y sum of the three phase voltages as V. 
         [0051]    Note that, depending on the mode of operation (e.g., back EMF versus FOC), some of the resistor dividers may not be used. For example, the resistor divider that generates V M  is used for field oriented control. Thus, for field oriented control, the other voltage dividers may not be used. 
         [0052]    The various voltages from the resistor dividers (e.g., V A -V C , V M , V Y ) are provided to MCU  15  via link  35 . MCU  15  uses those voltages to perform back EMF or field oriented control in various modes of operation. 
         [0053]    Note that in some embodiments, the various voltages present in the circuit in  FIG. 2  (e.g., V A -V C , V Y ) may be comparable to, or safe to apply to, MCU  15 . In such situations, the applicable resistor dividers may be omitted, as desired, and the corresponding voltages may be coupled to MCU  15 . 
         [0054]      FIG. 3  shows a block diagram of a mixed signal motor control circuit  20  according to an exemplary embodiment. More specifically,  FIG. 3  illustrates a block diagram of the general architecture of mixed signal motor control circuit  20  (a more detailed circuit arrangement is presented in  FIG. 4 ). 
         [0055]    As noted above, the circuit supports both back EMF control of a BLDC motor and field oriented control of a permanent magnet synchronous motor or an AC induction motor. When used for field oriented control, the circuit supports the use of three, two, or one sense resistor to measure the motor currents. 
         [0056]    As noted above, mixed signal motor control circuit  20  operates in conjunction with other parts of MCU  15  (see  FIG. 1 ). Accordingly, the blocks shown in  FIG. 2  cooperate with other MCU circuitry, such as CPU  25 , motor control firmware circuit  30 , etc. 
         [0057]    Referring to  FIG. 2 , mixed signal motor control circuit  20  includes programmable gain amplifier (PGA)  65 , signal sector circuit  68 , ADC  70 , comparator  72 , center-aligned PWM circuit  74 , DSVM circuit  76 , and signal selector circuit  78 . Mixed signal motor control circuit  20  receives a variety of signals, such as all or a subset of V A , V B , V C , V Y , V M , +I A , −I A , +I B , −I B , +I C , −I C , +I DC , and −I DC . Mixed signal motor control circuit  20  uses the input signals to generate control signals (e.g., AH-CH and AL-CL) for the inverter (not shown). 
         [0058]    Referring to  FIG. 2 , mixed signal motor control circuit  20  uses PGA  65  to amplify one or more input signals, such as +I A , −I A , +I B , −I B , +I C , −I C , +I DC , and −I DC . In some embodiments, PGA  65  may use several amplifiers that correspond to the number of input signals. 
         [0059]    Signal selector circuit  68  receives outputs of PGA  65  and other inputs to mixed signal motor control circuit  20 , such as V A , V B , V C , V Y , V M . Signal selector circuit  68  selectively provides the input signals to ADC  70  and comparator  72 . ADC converts the signals provided to it to digital signals, which it provides to other parts of MCU  15 , such as CPU  25  (not shown). 
         [0060]    Comparator  72  compares input signals (described below in detail in connection with  FIG. 4 ) and provides a plurality of inhibit or kill signals used to implement cycle by cycle current limiting. One or more inhibit or kill signals are provided to center-aligned PWM circuit  74  and DSVM circuit  76 . 
         [0061]    Depending on the mode of operation or type of control desired, center-aligned PWM circuit  74  performs pulse width modulation such that the centers of the control signals AH-CH and AL-CL are aligned. Similarly, depending on the mode of operation or type of control desired, DSVM circuit  76  performs discrete space vector modulation. 
         [0062]    When used for back EMF control, the circuit supports using either comparator  72  or ADC  70  to measure the back EMF. DSVM circuit specifically addresses the single resistor current sensing. When comparator  72  is not being used for back EMF control, it may be used for a cycle-by-cycle or persistent current limit by providing inhibit or kill signals to disable or inhibit the PWM or DSVM outputs. 
         [0063]    The output signals of center-aligned PWM circuit  74  and DSVM circuit  76  are provided to signal selector circuit  78 . Depending on the mode of operation or type of control desired, signal selector circuit  78  provides as its output signals either the output signals of center-aligned PWM circuit  74  or the output signals of DSVM circuit  76 . 
         [0064]    The output signals of signal selector circuit  78  are provided to external inverter and motor  30  (see  FIG. 1 ). More specifically, the output signals of signal selector circuit  78  constitute the drive or control signals AH-CH and AL-CL for upper transistors  45 A- 45 C and  48 A- 48 C, respectively (see  FIG. 2 ). 
         [0065]      FIG. 4  depicts a more detailed block diagram of the mixed signal motor control circuit  20  of  FIG. 3 . Referring to  FIG. 4 , the PGA includes three programmable gain amplifiers  65 A- 65 C coupled to receive the signals from the sense resistors (see  FIG. 2 ). 
         [0066]    Referring back to the embodiment shown in  FIG. 4 , PGAs  65 A- 65 C can provide a programmable gain from 1 to about 100. The gain programmability and range provides compatibility with a relatively wide range of motor sizes and sense resistors. As persons of ordinary skill in the art understand, however, other gain values may be used, depending on design and performance specifications for a given motor control system implementation. 
         [0067]    In some embodiments, PGAs  65 A- 65 C may provide level shifting of the input voltages that correspond to sensed currents. For example, the input voltages may be on the order of ± 0 . 1  volt with respect to ground. PGAs  65 A- 65 C may shift that level to about ½V DD , where V DD  represents the supply voltage of mixed signal motor control circuit  20  or MCU  15 . The level shifting facilitates signal processing by other circuitry in mixed signal motor control circuit  20 , such as ADC  70  and/or comparator  72 . 
         [0068]    Sense resistors are readily available from as much as 1 ohm to about 200 μΩ. Control of higher current motors will generally use a sense resistor with a lower resistance value and higher power dissipation. The upper gain setting of PGAs  65 A- 65 C will accommodate a sense resistor with a full-scale output voltage of about 10 mV. Thus, up to 100 Amperes of current may be sensed using a 1-Watt sense resistor. 
         [0069]    Note that in some embodiments, voltage gain may be provided in ADC  70 , rather than via PGAs  65 A- 65 C. This arrangement may be used, for example, in situations where the relatively wide gain range and level shifting of PGAs  65 A- 65 C, described above, are not desired or used. In still other embodiment, a combination of gain in PGAs  65 A- 65 C and ADC  70 , as desired. 
         [0070]    The sense voltages from the sense resistors used for field oriented control (i.e., +I A , −I A , +I B , −I B , +I C , −I C ) are differential bipolar (i.e., with both positive and negative swings) signals. The motor phase current is negative for 180° of an electrical cycle, and likewise the sense resistor current is negative for 180°. In exemplary embodiments, the typical differential input signal range is about ±10 mV to about ±100 mV. 
         [0071]    The negative terminal voltages are typically at about the motor ground potential. Nevertheless, stray inductance (e.g., from wiring, PCB traces, etc.) may cause voltage spikes or swings around the ground potential (as prescribed by the familiar equation, V L =L d i /d t ). In exemplary embodiments, a common mode range of about ±1 V may be used to accommodate a stray inductance of about 10 nH (the value of L in the equation above) and a current switching rate of 100 Amperes per microsecond (the value of d i /d t  in the equation above). 
         [0072]    Referring to  FIG. 4 , in the embodiment shown, mixed signal motor control circuit  20  includes a four-channel ADC  70 . Note that in some embodiments a three channel ADC may be used to perform back EMF and field oriented control, as desired. Using four-channel ADC  70 , however, allows one channel to be used for motor supply voltage sensing for the field oriented control mode of operation, or for the DC link current for the back EMF mode of operation. 
         [0073]    Synchronizing the ADC sampling to PWM (using the “ADC trig” trigger signal shown in  FIG. 4 ) provides relatively accurate low-noise samples. Simultaneous sampling of three or more channels enables field oriented control with minimum or reduced sampling time error, and supports relatively high PWM duty cycle. In exemplary embodiments, the sampling time is about 1 microsecond, although other values may be used in other embodiments, as persons of ordinary skill in the art understand. 
         [0074]    ADC  70  may be implemented in a variety of ways, as persons of ordinary skill in the art understand. For example, ADC  70  might use four sample capacitors in some embodiments. As an alternatively, ADC  70  might alternate sampling between the four channels, as desired. 
         [0075]    The embodiment shown in  FIG. 4  also uses a multiplexer (MUX)  68  for use in conjunction with ADC  70 . MUX  68  provides a means for switching between inputs used for field oriented control (e.g., ±pga 0 , ±pga 1 , ±pga 2 , which constitute outputs of PGAs  65 A- 65 C, respectively; V M , etc.) and inputs used for back EMF control (e.g., V A , V B , V C , V Y , etc.). 
         [0076]    In the embodiment of  FIG. 4 , MUX  68  is implemented as a four channel 2-input multiplexer (4×2-to-1) for both positive and negative inputs, with the multiplexer sections labeled as  68 A- 68 B, respectively. Note, however, that MUX  68  may be implemented in a variety of ways and configurations, as persons of ordinary skill in the art understand. 
         [0077]    Regardless of implementation, MUX  68 A and MUX  68 B select one of their respective four inputs, and provide that input to ADC  70 .  FIG. 4  shows the input signals for each of MUX  68 A and  68 B. Signals amuxpsel and amuxnsel, provided by MCU  15  (e.g., by CPU  25 ) constitutes the select signals for MUX  68 A and  68 B, respectively. 
         [0078]    Similarly, the embodiment shown in  FIG. 4  uses a MUX  88  for use in conjunction with comparator  72 . MUX  88  provides a means for switching between inputs used for field oriented control and inputs used for back EMF control. 
         [0079]    In the embodiment of  FIG. 4 , MUX  88  is implemented as a four channel 2-input multiplexer (4×2-to-1) for both positive and negative inputs, with the multiplexer sections labeled as  88 A- 88 B, respectively. Note, however, that MUX  88  may be implemented in a variety of ways and configurations, as persons of ordinary skill in the art understand. 
         [0080]    Regardless of implementation, MUX  88 A and MUX  88 B select one of their respective four inputs, and provide that input to comparator  72 .  FIG. 4  shows the input signals for each of MUX  88 A and  88 B. Signals cmuxpsel and cmuxnsel, provided by MCU  15  (e.g., by CPU  25 ) constitutes the select signals for MUX  68 A and  68 B, respectively. 
         [0081]    In the embodiment shown, comparator  72  compares four inputs received from MUX  88 A with four respective inputs received from MUX  88 B. Comparator  72  also has a reference input driven by DAC  82 . Under control of MCU  15  (e.g., CPU  25  controlling DAC  82  and providing desired inputs to it), the reference value may be used to trim the offset value or set the current limit value. This scheme is equivalent to first converting the differential signal to a single ended signal and then comparing to a preset DAC value. 
         [0082]    The outputs of comparator  72  are used to kill or inhibit or disable the PWM signals when implementing a cycle-be-cycle current limit. Comparator  72  may also trigger an interrupt (e.g., to CPU  25 ) or trigger a timer capture for back EMF control, as desired. 
         [0083]    As noted, mixed signal motor control circuit  20  includes center-aligned PWM circuit  74 , and DSVM circuit  76 . Center-aligned PWM circuit  74  may be a conventional PWM block for most motor control applications. DSVM circuit  76  may be a dedicated DSVM block for field oriented control using a single sense resistor. 
         [0084]    Center-aligned PWM circuit  74  and DSVM circuit  76  may control or synchronize the operation of ADC  70  via an ADC trigger signal. Each of center-aligned PWM circuit  74  and DSVM circuit  76  provides an ADC trigger signal to MUX  80  as input signals. In response to select signal adctrigsel, provided by MCU  15  (e.g., by CPU  25 ), MUX  80  provides one of its inputs to ADC  70  as a trigger signal, labeled as “ADC trig” in  FIG. 4 . 
         [0085]    The outputs of center-aligned PWM circuit  74 , and DSVM circuit  76  are provided as the respective inputs of MUX  84 . In response to a select signal, pwmsel, provided by MCU  15  (e.g., by CPU  25 ), MUX  84  provides a set of six control signals (or four for a single phase implementation) to external inverter and motor  30  (see  FIG. 1 ). In the exemplary embodiment shown in  FIG. 4 , the signals constitute control or drive signals AH-CH and AL-CL, described above. 
         [0086]    By using the configuration shown in  FIG. 4 , MCU  15 , for example, CPU  25  operating in conjunction with control firmware circuit  30 , may provide various signals to control the operation of mixed signal motor control circuit  20 . The signals depend on the mode of operation, i.e., back EMF versus field oriented control. 
         [0087]      FIG. 5  illustrates in tabular form various signals and associated values used to implement or realize different types of motor control schemes according to an exemplary embodiment. More specifically,  FIG. 5  shows as table columns the various types of motor control, e.g., back EMF using ADC, back EMF using comparators, field oriented control using PWM and three or two sense resistors, and field oriented control using DSVM and a sense resistor. The table rows list the names of various signals or parameters used in the motor control schemes. The table cells show the status or value of the various signals or parameters for the different types of motor control scheme. 
         [0088]    Referring back to  FIG. 4 , PGA  65  receives input signals and feeds the inputs of comparator  72 , as described above.  FIG. 6A  shows a simplified block diagram showing that arrangement. Note that  FIG. 6A  omits MUX  68  and MUX  88  to facilitate presentation. 
         [0089]    In some embodiments, some of the blocks may be re-arranged. More specifically, in some embodiments, comparator  72  may be coupled before PGA  65  (i.e., the three PGAs  65 A- 65 C).  FIG. 6B  illustrates such an arrangement. (Note that  FIG. 6B  also omits MUX  68  and MUX  88  to facilitate presentation, although the MUXs may be used, as desired, for a given implementation.) In this arrangement, the input signals feed the input of comparator  72  and also PGA  65 . In other words, comparator  72  is arranged or coupled before PGA  65 . 
         [0090]    Referring to  FIG. 6B , voltages from the sense resistors feed inputs of comparator  72 . The circuit arrangement in  FIG. 6B  thus decouples the bandwidths of comparator  72  and ADC  70 . In some embodiments, comparator  72  may have a high enough bandwidth or speed to detect or respond to over-current situations that are expected to be encountered during operation of mixed signal motor control circuit  20 . The bandwidth of the ADC  70  may be reduced in order to provide increased signal to noise ratio and higher precision. 
         [0091]    In some embodiments, different MUX arrangements may be used. Specifically,  FIG. 4  illustrates a four channel, two input MUX for ADC  70  and comparator  72  positive and negative inputs. An extended or flexible multiplexer for each ADC channel or comparator input set provides the capability to measure any current or voltage on any channel.  FIG. 7  shows such a multiplexing circuit arrangement, using MUX  90  (consisting of MUX  90 A and MUX  90 B) for channel  0  of ADC  70 . The multiplexing circuit arrangement shown in  FIG. 7  is repeated for each ADC channel and comparator input set. 
         [0092]    The circuit arrangement in  FIG. 7  allows differential measurements or comparisons of any phase voltage to any other phase voltage (e.g., V A -V B , V B -V C , V C -V A ). Note that not all combinations of positive and negative inputs are necessarily useful. The first three positive MUX selections should be used with the corresponding negative MUX selections. (Selecting the same voltage for positive and negative inputs would normally not provide useful information.) 
         [0093]    According to another aspect, motor control systems according to exemplary embodiments may provide support for blanking. Specifically, when switching the switches or transistors (see  FIG. 2 ) on the active edge of the low PWM/DSVM signals, normally a relatively large current spike may occur due to diode recovery and switching capacitances. In some embodiments, the over-current protection is not active for a relatively short duration after the active edge of the lower switching or control signal to provide a blanking period. In some embodiments, the blanking period is programmable. 
         [0094]    In some embodiments, the blanking time (which may be programmable) is implemented in both center aligned PWM circuit  74  and DSVM circuit  76 .  FIG. 8  shows a circuit arrangement to provide that capability. Center aligned PWM circuit  74  and DSVM circuit  76  generate blanking signals that are labeled  95 A and  95 B, respectively. Center aligned PWM circuit  74  and DSVM circuit  76  have the ability in some embodiments to ignore the kill or inhibit signal during the blanking time. 
         [0095]    In addition, in some embodiments, the blanking signal is used to deactivate the front end circuitry of the PGA and comparators.  FIG. 9  shows a circuit arrangement according to an exemplary embodiment that provides this feature. More specifically, blanking signals (labeled as “blank  0 ,” “blank  1 ,” etc.) are provided to PGAs  65 A- 65 C and to comparators  72 A- 72 C. By configuring PGAs  65 A- 65 C and comparators  72 A- 72 C into auto-zero mode (part of the calibration of PGAs  65 A- 65 C and comparators  72 A- 72 C in exemplary embodiments) during the blanking time, the large scale current spike is not propagated through the gain stages. This scheme protects the gain stage from overload, and also provides a relatively fast recovery time after the current spike. 
         [0096]    Referring to the figures, persons of ordinary skill in the art will note that the various blocks shown might depict mainly the conceptual functions and signal flow. The actual circuit implementation might or might not contain separately identifiable hardware for the various functional blocks and might or might not use the particular circuitry shown. For example, one may combine the functionality of various blocks into one circuit block, as desired. 
         [0097]    Furthermore, one may realize the functionality of a single block in several circuit blocks, as desired. The choice of circuit implementation depends on various factors, such as particular design and performance specifications for a given implementation. Other modifications and alternative embodiments in addition to those described here will be apparent to persons of ordinary skill in the art. Accordingly, this description teaches those skilled in the art the manner of carrying out the disclosed concepts, and is to be construed as illustrative only. 
         [0098]    The forms and embodiments shown and described should be taken as illustrative embodiments. Persons skilled in the art may make various changes in the shape, size and arrangement of parts without departing from the scope of the disclosed concepts in this document. 
         [0099]    For example, persons skilled in the art may substitute equivalent elements for the elements illustrated and described here. Moreover, persons skilled in the art may use certain features of the disclosed concepts independently of the use of other features, without departing from the scope of the disclosed concepts.