Driving circuit for stepping motor

Abstract

A direction setting pin DIR receives a direction signal ROT that indicates the rotational direction of the stepping motor. A clock pin CLK receives a clock signal CLK that indicates the rotational direction of the stepping motor. A logic circuit generates a first internal signal and a second internal signal that respectively indicate the states of the first H-bridge circuit and the second H-bridge circuit according to the direction signal ROT and the clock signal CLK. When the clock signal CLK remains in a predetermined state for a predetermined judgment time, the logic circuit transits to a predetermined mode.

Claims

1. A driving circuit for a stepping motor comprising a first coil and a second coil, the driving circuit comprising: a first H-bridge circuit to be coupled to the first coil; a second H-bridge circuit to be coupled to the second coil; a direction setting pin to be coupled to receive a direction signal that specifies a rotational direction of the stepping motor; a clock pin to be coupled to receive a clock signal that indicates a rotational operation of the stepping motor; a logic circuit structured to generate a first internal signal and a second internal signal indicative of respective states of the first H-bridge circuit and the second H-bridge circuit according to the direction signal and the clock signal; a first pre-driver structured to drive the first H-bridge circuit according to the first internal signal; and a second pre-driver structured to drive the second H-bridge circuit according to the second internal signal, wherein, when the clock signal remains in a predetermined state for a predetermined judgment time, the logic circuit transits to a predetermined mode; wherein the predetermined mode is a weak excitation mode, and wherein, in the weak excitation mode, the first internal signal and the second internal signal are generated such that an output of the first H-bridge circuit and an output of the second H-bridge circuit are each reduced as compared with a corresponding output current in a normal excitation mode.

2. The driving circuit according to claim 1, wherein the predetermined mode is a high-impedance mode, and wherein, in the high-impedance mode, the first internal signal and the second internal signal are generated such that an output of the first H-bridge circuit and an output of the second H-bridge circuit are each set to a high-impedance state.

3. The driving circuit according to claim 2, wherein, in the high-impedance mode, the logic circuit maintains operations of ail circuit blocks of the driving circuit.

4. The driving circuit according to claim 2, wherein, in the high-impedance mode, the logic circuit is structured to shut down a circuit block that is not required to maintain the outputs of the first H-bridge circuit and the second H-bridge circuit in the high-impedance state.

5. A system comprising: a processor; a stepping motor; and the driving circuit according to claim 1, structured to drive the stepping motor according to a direction signal and a clock signal received from the processor.

6. A control method for a stepping motor comprising a first coil and a second coil, the control method comprising: providing a first H-bridge circuit coupled to the first coil; providing a second H-bridge circuit coupled to the second coil; generating, by means of a processor, a clock signal so as to rotationally drive the stepping motor; generating, by means of the processor, a direction signal that indicates a rotational direction of the stepping motor; switching a state of the first H-bridge circuit and a state of the second H-bridge circuit according to the direction signal and the clock signal; and reducing an output current of the first H-bridge circuit and an output current of the second H-bridge circuit when the processor fixes a state of the direction signal and a state of the clock signal over a predetermined period of time.

7. A driving circuit for a stepping motor comprising a first coil and a second coil, the driving circuit comprising: a first H-bridge circuit coupled to the first coil; a second H-bridge circuit coupled to the second coil; a clock pin structured to receive a clock signal that indicates a rotational operation of the stepping motor; a control pin structured to receive a control signal that sets an output of the first H-bridge circuit and an output of the second H-bridge circuit to a predetermined state; a logic circuit structured to determine a rotational direction based on a state of the clock signal and a state of the control signal, and to generate a first internal signal and a second internal signal that respectively indicate a state of the first H-bridge circuit and a state of the second H-bridge circuit according to the clock signal; a first pre-driver structured to drive the first H-bridge circuit according to the first internal signal; and a second pre-driver structured to drive the second H-bridge circuit according to the second internal signal; wherein, when the clock signal and the control signal each remain in a predetermined state for a predetermined judgment time, the logic circuit switches a rotational direction to a direction that is the reverse of an immediately previous rotational direction.

8. The driving circuit according to claim 7, wherein the control signal is an enable signal, and wherein, when the enable signal is set to a predetermined level, the logic circuit shuts down at least a part of the driving circuit.

9. The driving circuit according to claim 7, wherein the control signal is an enable signal, and wherein, during a period of time in which the enable signal is set to a predetermined level, the logic circuit maintains operations of all circuit blocks of the driving circuit.

10. A system comprising: a processor; a stepping motor; and the driving circuit according to claim 7, structured to drive the stepping motor according to a control signal and a clock signal received from the processor.

11. A driving circuit for a stepping motor comprising a first coil and a second coil, the driving circuit comprising: a first H-bridge circuit coupled to the first coil; a second H-bridge circuit coupled to the second coil; a clock pin structured to receive a clock signal that indicates a rotational operation of the stepping motor; a control pin structured to receive a control signal that sets an output of the first H-bridge circuit and an output of the second H-bridge circuit to a predetermined state; a logic circuit structured to determine a rotational direction based on a state of the clock signal and a state of the control signal, and to generate a first internal signal and a second internal signal that respectively indicate a state of the first H-bridge circuit and a state of the second H-bridge circuit according to the clock signal; a first pre-driver structured to drive the first H-bridge circuit according to the first internal signal; and a second pre-driver structured to drive the second H-bridge circuit according to the second internal signal; wherein a control signal is a signal for switching an excitation mode between a weak excitation mode and a normal excitation mode, and wherein, when the control signal indicates the weak excitation mode, the logic circuit reduces an output current of the first H-bridge circuit and an output current of the second H-bridge circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:

(2) FIG. 1 is a block diagram showing a system including a driving circuit for a stepping motor;

(3) FIG. 2 is a block diagram showing a system including a driving circuit according to a first embodiment;

(4) FIG. 3A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and I.sub.OUT2 in the 1-2 phase excitation method, and FIG. 3B is an operation waveform diagram showing an operation of the driving circuit shown in FIG. 2;

(5) FIG. 4 is a block diagram showing a system including a driving circuit according to a third embodiment;

(6) FIG. 5A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and I.sub.OUT2 in the 2-2 phase excitation method, and FIG. 5B is an operation waveform diagram showing an operation of the driving circuit shown in FIG. 4;

(7) FIG. 6 is another operation waveform diagram showing the operation of the driving circuit shown in FIG. 4;

(8) FIG. 7 is a block diagram showing a system including a driving circuit according to a fourth embodiment;

(9) FIG. 8 is a circuit diagram showing an example configuration of a PWM generator;

(10) FIG. 9A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and I.sub.OUT2 in the weak excitation mode in the 1-2 phase excitation method, and FIG. 9B is an operation waveform diagram showing the operation of the driving circuit shown in FIG. 7;

(11) FIG. 10 is a block diagram showing a system including a driving circuit according to a fifth embodiment;

(12) FIG. 11 is an operation waveform diagram showing the operation of the driving circuit shown in FIG. 10; and

(13) FIG. 12 is another operation waveform diagram showing the operation of the driving circuit shown in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

(14) The invention will now be described based on preferred embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention.

(15) In the present specification, the state represented by the phrase the member A is coupled to the member B includes a state in which the member A is indirectly coupled to the member B via another member that does not substantially affect the electric connection between them, or that does not damage the functions or effects of the connection between them, in addition to a state in which they are physically and directly coupled.

(16) Similarly, the state represented by the phrase the member C is provided between the member A and the member B includes a state in which the member A is indirectly coupled to the member C, or the member B is indirectly coupled to the member C via another member that does not substantially affect the electric connection between them, or that does not damage the functions or effects of the connection between them, in addition to a state in which they are directly coupled.

First Embodiment

(17) FIG. 2 is a block diagram showing a system 200A including a driving circuit 100A according to a first embodiment. A stepping motor 202 includes a first coil L1 and a second coil L2. A power supply voltage is supplied to a power supply (VCC) pin of the driving circuit 100A. A ground (GND) pin thereof is grounded.

(18) The driving circuit 100A includes a logic circuit 110A, a first pre-driver 120_1, a second pre-driver 120_2, a first PWM generator 122_1, a second PWM generator 122_2, a first H-bridge circuit 130_1, and a second H-bridge circuit 130_2. The first H-bridge circuit 130_1 is arranged such that its output is coupled to the first coil L1. The second H-bridge circuit 130_2 is arranged such that its output is coupled to the second coil L2. The driving circuit 100A is provided with an OUT1A pin, an OUT1B pin, an OUT2A pin, and an OUT2B pin. The first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 each include a sensing resistor R.sub.S for detecting a driving current. A voltage drop occurs across the sensing resistor R.sub.S in proportion to the driving current. The voltage drop is supplied to the corresponding PWM generator 122.

(19) The driving circuit 100A drives the stepping motor 202 according to a control signal received from the controller 204. The controller 204 is configured as a microcontroller or a CPU. The driving circuit 100A is provided with a direction setting (DIR) pin and a clock (CLK) pin. The DIR pin receives, as an input signal, a direction indication (ROT) signal that indicates the rotation direction (clockwise direction or counterclockwise direction) to be set for the stepping motor 202. Furthermore, the CLK pin receives, as an input signal, a clock (CLK) signal that indicates the rotational operation of the stepping motor 202.

(20) The logic circuit 110A generates a first internal signal S.sub.INT1 and a second internal signal S.sub.INT2 that respectively indicate the states of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 according to the ROT signal and the CLK signal. The logic circuit 110A may generate the internal signals using known techniques. Examples of the method for generating the internal signals include: a 1-2 phase excitation method in which the current phase is rotated by 45 degrees for every edge of the CLK signal; and a 2-2 phase excitation method in which the current phase is rotated by 90 degrees for every edge of the CLK signal. The logic circuit 110A may perform a micro-step driving operation in which the internal signal is modulated with a frequency that is N (N is an integer) times the frequency of the CLK signal.

(21) The first PWM generator 122_1 generates a first PWM signal S.sub.PWM1 based on the voltage drop across the sensing resistor R.sub.S of the corresponding first H-bridge circuit 130_1 such that the driving current is stabilized to a setting value. Similarly, the second PWM generator 122_2 generates a second PWM signal S.sub.PWM2 based on the voltage drop across the sensing resistor R.sub.S of the corresponding second H-bridge circuit 130_2 such that the driving current is stabilized to a setting value. In order to suppress audible-band noise, the frequency of each of the PWM signals S.sub.PWM1 and S.sub.PWM2 (PWM frequency) is designed to be equal to or higher than 20 kHz that is outside the audible noise band. Accordingly, the maximum period of the PWM signal is 50 s.

(22) The first pre-driver 120_1 drives the first H-bridge circuit 130_1 based on the first internal signal S.sub.INT1 and the first PWM signal S.sub.PWM1. Similarly, the second pre-driver 120_2 drives the second H-bridge circuit 130_2 based on the second internal signal S.sub.INT2 and the second PWM signal S.sub.PWM2.

(23) When the CLK signal remains in a predetermined state (e.g., low level) for a predetermined time , the logic circuit 110A is set to the high-impedance mode. In the high-impedance mode, the logic circuit 110A generates the first internal signal S.sub.INT1 and the second internal signal S.sub.INT2 such that the outputs of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are set to the high-impedance state, i.e., such that all the transistors that form each H-bridge circuit are turned off at the same time.

(24) The number of pulses of the PWM signal to be output for a given excitation position (required PWM pulse number) M is determined according to the rotational load and the rotational speed of the stepping motor. The maximum period of the PWM signal is 50 s. Accordingly, when the stepping motor is to be rotated at a constant rotational speed, the CLK signal is input with a period of 50 sM (or otherwise 50 sM or more). Accordingly, the judgment time is preferably designed to be longer than 50 sK (KM).

(25) The logic circuit 110A may include a timer circuit 112A that monitors the state of the CLK signal. The configuration of the timer circuit 112A is not restricted in particular. For example, the timer circuit 112A may be configured such that, during a period in which the CLK signal remains at the low level, the timer circuit 112A counts up (or otherwise counts down) in synchronization with a high-speed internal clock CLKINT of the driving circuit 100A, and such that the timer circuit 112A is reset every time the CLK signal becomes the high level. When the count value of the timer circuit 112A reaches a value that corresponds to the judgment time , the timer circuit 112A asserts (sets to the high level, for example) a high-impedance setting (Hi-Z) signal. The logic circuit 110A sets the internal signals S.sub.INT1 and S.sub.INT2 using an assertion of the Hi-Z signal as a trigger such that the outputs of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 each become the high-impedance state.

(26) With the embodiment, in the high-impedance mode, the logic circuit 110A maintains the operation states of all the circuit blocks of the driving circuit. The above is the configuration of the driving circuit 100A. Next, description will be made regarding the operation thereof.

(27) FIG. 3A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and I.sub.OUT2 in the 1-2 phase excitation method. For example, when the ROT signal is set to the low-level state, the driving circuit 100A changes the excitation position by +1, i.e., in the counterclockwise (CCW) direction, every time an edge occurs in the CLK signal. Conversely, when the ROT signal is set to the high-level state, the driving circuit 100A changes the excitation position by 1, i.e., in the clockwise (CW) direction, every time an edge occurs in the CLK signal. The driving circuit 100A may change the excitation position with a finer resolution by means of the micro-step driving operation.

(28) FIG. 3B is an operation waveform diagram showing the operation of the driving circuit 100A shown in FIG. 2. In this example, the ROT signal is set to the low level, which is an indication for the rotation in the counterclockwise (CCW) direction. Before the time point t.sub.0, the driving circuit 100A is set to the high-impedance mode Hi-Z. In this state, the stepping motor 202 suspends its operation at the excitation position 1.

(29) After the time point t.sub.0, the cyclic CLK signal is input. This sets the Hi-Z signal to the low level, which switches the driving circuit 100A from the high-impedance mode to the normal operation mode. In the normal operation mode, the driving circuit 100A changes the currents I.sub.OUT1 and I.sub.OUT2 in synchronization with an edge of the CLK signal. This instructs the stepping motor 202 to rotate by an angle according to the number of pulses.

(30) When the judgment time has elapsed after the last CLK signal is generated at the time point t.sub.1, the Hi-Z signal is asserted at the time point t.sub.2, which sets the driving circuit 100A to the high-impedance mode. In the high-impedance mode, the currents I.sub.OUT1 and I.sub.OUT2 are set to zero, which fixes the rotational position of the stepping motor 202.

(31) The above is the operation of the driving circuit 100A. With the driving circuit 100A, this arrangement requires only the two external interface pins to provide the high-impedance mode to be implemented as an additional function. In the high-impedance mode, current supply to the stepping motor 202 is suspended, thereby allowing the current consumption to be reduced.

Second Embodiment

(32) A driving circuit 100B according to a second embodiment has the same basic configuration as that of the driving circuit 100A shown in FIG. 2. The point of difference from the driving circuit 100A is that the driving circuit 100B is set to the standby state in the high-impedance mode. In the standby state, at least a part of the circuit block (e.g., protection circuit or bias circuit) of the driving circuit 100B is shut down. In this state, the consumed current (power supply current) I.sub.CC of the driving circuit 100B is reduced to substantially zero. The power supply current I.sub.CC of the driving circuit 100B is represented by the line of alternately long and short dashes shown in FIG. 3B.

(33) With the driving circuit 100B, this arrangement allows the operation current of the driving circuit 100B itself to be reduced in the high-impedance mode. Accordingly, this allows the operation current of the overall system to be reduced.

(34) It should be noted that, in a case in which several circuit blocks are shut down in the high-impedance mode, a predetermined period of time is required to restore the circuit blocks thus shut down. Accordingly, this arrangement involves a delay the next time the rotational operation is restarted.

(35) With the driving circuit 100A shown in FIG. 2, the circuit blocks are not shut down, i.e., the operations of all the circuit blocks are maintained. Accordingly, the driving circuit 100A has an advantage of being capable of switching from the stationary state to the rotational operation state without involving a delay.

(36) That is to say, whether or not such several circuit blocks are to be shut down in the high-impedance mode may be determined giving consideration to whether responsivity or power consumption is to be prioritized.

Third Embodiment

(37) FIG. 4 is a block diagram showing a system 200C including a driving circuit 100C according to a third embodiment.

(38) The driving circuit 100C is provided with an enable (EN) pin instead of the DIR pin shown in FIG. 2. An enable (ENABLE) signal is input from the controller 204 to the EN pin. When the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are to be set to the normal operation mode, the ENABLE signal is asserted (set to the high level, for example). When the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are to be set to the high-impedance mode, the ENABLE signal is negated (set to the low level, for example).

(39) With an embodiment, the logic circuit 110C may maintain the operation states of all the circuit blocks of the driving circuit in the high-impedance mode. Alternatively, at least a part of the circuit blocks may be shut down in the high-impedance mode.

(40) The logic circuit 110C determines the rotational direction based on the states of the CLK signal and the ENABLE signal, and generates a direction indication (ROT) signal that indicates the rotational direction. For example, when the rotational direction is to be set to the counterclockwise (CCW) direction, the ROT signal may be set to the low level. Conversely, when the rotational direction is to be set to the clockwise (CW) direction, the ROT signal may be set to the high level.

(41) The logic circuit 110C generates the first internal signal S.sub.INT1 and the second internal signal S.sub.INT2 that respectively indicate the states of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 according to the CLK signal and the ROT signal.

(42) When the CLK signal and the ENABLE signal remain in predetermined states for a predetermined judgment time , the logic circuit 110C logically inverts (toggles) the ROT signal so as to switch the rotational direction to the reverse direction. More specifically, when the ENABLE signal indicates the normal operation mode (high) and the CLK signal remains in the suspended state (i.e., non-input state) over the judgement time or more, the ROT signal is logically inverted. The judgement time may be designed in the same way as in the first embodiment.

(43) The logic circuit 110C may include a timer circuit 112C that monitors the states of the CLK signal and the ENABLE signal. The configuration of the timer circuit 112C is not restricted in particular. For example, the timer circuit 112C may be configured to count up (or otherwise count down) in synchronization with the high-speed internal clock CLKINT of the driving circuit 100C during a period in which the CLK signal remains in the low-level state and the ENABLE signal remains in the high-level state, and to be reset every time there is a state transition in the CLK signal or the ENABLE signal. When the count value of the timer circuit 112C reaches a value that corresponds to the judgment time , the ROT signal may be logically inverted.

(44) With a modification, the state in which the ENABLE signal indicates the high-impedance mode and the supply of the CLK signal is suspended exceeds the judgment time , the ROT signal may be logically inverted. With another modification, the state in which the ENABLE signal is fixed to the high level or otherwise the low level and the supply of the CLK signal exceeds the judgment time , the ROT signal may be logically inverted.

(45) Next, description will be made regarding the operation of the driving circuit 100C. Here, description will be made regarding an example employing a 2-2 phase excitation method. FIG. 5A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and I.sub.OUT2 in the 2-2 phase excitation method. For example, in a state in which the ROT signal is set to the low level, the driving circuit 100C changes the excitation position by +2, i.e., by 2 in the counterclockwise (CCW) direction, every time an edge occurs in the CLK signal. Conversely, in a state in which the ROT signal is set to the high level, the driving circuit 100C changes the excitation position by 2, i.e., by 2 in the clockwise (CW) direction, every time an edge occurs in the CLK signal. The driving circuit 100C may change the excitation position with a finer resolution by means of a micro-step driving operation.

(46) FIG. 5B is an operation waveform diagram showing the operation of the driving circuit 100C shown in FIG. 4. Before the time point to, the ENABLE signal is set to the low level. Accordingly, the driving circuit 100C is set to the high-impedance mode Hi-Z. In this state, the operation of the stepping motor 202 is suspended at the excitation position 8 (reset position). The ROT signal is set to the low level, which indicates a state in which the rotational direction is set to the counterclockwise (CCW) direction.

(47) When the ENABLE signal transits to the high level at the time point to, the driving circuit 100C transits to the normal operation mode. In this state, when a cyclic CLK signal is input, the excitation position is changed in increments of 2.

(48) At the time point t.sub.1, the last CLK signal is generated. In this stage, the ENABLE signal is set to the high level. Accordingly, the timer circuit 112C advances the counting operation. At the time point t.sub.2 after the judgment time has elapsed, the logical value of the ROT signal is inverted, which switches the rotational direction to the clockwise direction. Subsequently, when the next CLK signal is input at the time point t.sub.3, the excitation position changes in the direction that is the reverse of the rotational direction set before the time point t.sub.1.

(49) FIG. 6 is another operation waveform diagram showing the operation of the driving circuit 100C shown in FIG. 4. The operation before the time point t.sub.1 is the same as that shown in FIG. 5B. At the time point t.sub.1, the last CLK signal is generated, following which the rotational operation is suspended. Immediately after the generation of the last CLK signal, the ENABLE signal is set to the low level. In this case, the timer circuit 112C does not perform the counting operation, or otherwise is reset. Accordingly, in this case, the ROT signal is not logically inverted, which maintains the rotational direction in the counterclockwise direction. Subsequently, after the input of the CLK signal is restarted at the time point t.sub.3, the excitation position is changed in the same direction as that set before the time point t.sub.1.

(50) The above is the operation of the driving circuit 100C.

(51) With the driving circuit 100C, this arrangement requires only the two external interface pins to provide the high-impedance mode to be implemented as an additional function. In the high-impedance mode, current supply to the stepping motor is suspended, thereby allowing the current consumption to be reduced.

Fourth Embodiment

(52) FIG. 7 is a block diagram showing a system 200D including a driving circuit 100D according to a fourth embodiment. The driving circuit 100D supports the normal excitation mode and the weak excitation mode.

(53) The driving circuit 100D has the same input/output pins and the same basic configuration as those of the driving circuit 100A shown in FIG. 2.

(54) When the CLK signal remains in a predetermined state (e.g., low-level state) for a predetermined judgment time , the logic circuit 110D is set to the weak excitation mode. In the weak excitation mode, the output currents of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are set to the respective output currents set in the normal excitation mode.

(55) The logic circuit 110D may include a timer circuit 112D that monitors the state of the CLK signal. When the input of the CLK signal is continuously suspended over the judgment time , the timer circuit 112D asserts (sets to the high state, for example) a weak excitation (WE) signal.

(56) The WE signal is input to the first PWM generator 122_1 and the second PWM generator 122_2. In a state in which the WE signal is asserted, the first PWM generator 122_1 and the second PWM generator 122_2 lower a threshold value that defines the peak value of the driving current to a value that is lower than that set in a state in which the WE signal is negated (normal excitation mode).

(57) FIG. 8 is a circuit diagram showing an example configuration of the PWM generator 122. The PWM generator 122 may include a comparator 123, a variable voltage source 124, a flip-flop 125, and an oscillator 126. The variable voltage source 124 generates the threshold voltage V.sub.TH having a first voltage level V.sub.TH1 when the WE signal is set to the low level, and having a second voltage level V.sub.TH2 that is lower than the first voltage level V.sub.TH1 when the WE signal is set to the high level, i.e., is set to the weak excitation mode. The second voltage level V.sub.TH2 may be designed to be 25% of the first voltage level V.sub.TH1. The variable voltage source 124 may include a D/A converter. By bit shifting the input code of the D/A converter, this arrangement is capable of changing the voltage level of the threshold voltage V.sub.TH to be , , . . . . Alternatively, the variable voltage source 124 may include a constant voltage source and a dividing circuit having a variable voltage dividing ratio.

(58) The comparator 123 compares the voltage drop V.sub.S that occurs across the sensing resistor R.sub.S with the threshold voltage V.sub.TH. When V.sub.S>V.sub.TH, the comparator 123 asserts an OFF signal (reset signal). The oscillator 126 generates an ON signal (set signal) that is asserted for every predetermined PWM period. The flip-flop 125 generates the PWM signal S.sub.PWM having a level that is switched between the high level and the low level according to the ON signal and the OFF signal.

(59) FIG. 9A is a diagram showing the relation between the excitation position and the currents I.sub.OUT1 and T.sub.OUT2 in the weak excitation mode in the 1-2 phase excitation method. FIG. 9B is an operation waveform diagram showing the operation of the driving circuit 100D shown in FIG. 7. FIG. 9A shows the currents I.sub.OUT1 and I.sub.OUT2 in the normal excitation mode, which are represented by broken lines.

(60) FIG. 9B is an operation waveform diagram showing the operation of the driving circuit 100D shown in FIG. 7. In this example, the ROT signal is set to the low level, which indicates the rotational operation in the counterclockwise (CCW) direction. Before the time point t.sub.0, the driving circuit 100A is set to the weak excitation mode. In this mode, the stepping motor 202 is set to the stationary state at the excitation position 1.

(61) When the cyclic CLK signal is input after the time point to, the WE signal is set to the low level, which switches the driving circuit 100D from the weak excitation mode to the normal excitation mode. In the normal excitation mode, the driving circuit 100A changes the currents I.sub.OUT1 and I.sub.OUT2 in synchronization with an edge of the CLK signal. This rotates the stepping motor 202 by an angle corresponding to the number of pulses.

(62) When the judgment time has elapsed after the last CLK signal is generated at the time point t.sub.1, the WE signal is asserted at the time point t.sub.2, which switches the excitation mode to the weak excitation mode. In the weak excitation mode, I.sub.OUT1 and I.sub.OUT2 are each set to 25% of the corresponding value in the normal excitation mode. In the weak excitation mode, the rotational position of the stepping motor 202 is fixed.

(63) The above is the operation of the driving circuit 100D. With the driving circuit 100D, this arrangement requires only the two external interface pins to provide the weak excitation mode to be implemented as an additional function. In the weak excitation mode, this arrangement is capable of reducing current consumption while fixing the rotational position of the rotor using weak magnetic force.

Fifth Embodiment

(64) FIG. 10 is a block diagram showing a system 200E including a driving circuit 100E according to a fifth embodiment. The driving circuit 100E is provided with a weak excitation setting (WE) pin instead of the ENABLE pin shown in FIG. 4. The WE pin receives a control signal (WE signal) from the controller 204 for selecting the excitation mode from among the weak excitation mode and the normal excitation mode. When the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are to be set to the normal operation mode, the WE signal is negated (set to the low level, for example). When the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 are to be set to the weak excitation mode, the WE signal is asserted (set to the high level, for example).

(65) The logic circuit 110E determines the rotational direction based on the states of the CLK signal and the WE signal, and generates a direction indication (ROT) signal that indicates the rotational direction. For example, when the rotational direction is to be set to the counterclockwise (CCW) direction, the ROT signal may be set to the low level. Conversely, when the rotational direction is to be set to the clockwise (CW) direction, the ROT signal may be set to the high level.

(66) The logic circuit 110E generates a first internal signal S.sub.INT1 and a second internal signal S.sub.INT2 that respectively indicate the states of the first H-bridge circuit 130_1 and the second H-bridge circuit 130_2 according to the CLK signal and the ROT signal.

(67) When the CLK signal and the WE signal each remain in a predetermined state for the predetermined judgment time , the logic circuit 110E logically inverts (toggles) the ROT signal so as to switch the rotational direction to a direction that is the reverse of the immediately previous rotational direction.

(68) More specifically, when the state in which the WE signal indicates the normal excitation mode (i.e., low level) and the supply of the CLK signal is suspended continues over the judgment time , the ROT signal is logically inverted.

(69) The logic circuit 110E may include a timer circuit 112E that monitors the states of the CLK signal and the WE signal. The configuration of the timer circuit 112E is not restricted in particular. Also, the timer circuit 112E may be configured in the same manner as that of the timer shown in FIG. 4.

(70) A modification may be made in which, when the state in which the WE signal indicates the weak excitation mode (i.e., high level) and the supply of the CLK signal is suspended continues over the judgment time , the ROT signal may be logically inverted. Also, another modification may be made in which, when the state in which the WE signal is fixed to the high level or otherwise the low level and the supply of the CLK signal is suspended continues over the judgment time , the ROT signal may be logically inverted.

(71) Next, description will be made regarding the operation of the driving circuit 100E. Here, description will be made regarding an example employing the 2-2 phase excitation method. FIG. 11 is an operation waveform diagram showing the operation of the driving circuit 100E shown in FIG. 10. Before the time point t.sub.0, the WE signal is set to the high level, and accordingly, the driving circuit 100E is set to the weak excitation mode. The ROT signal is set to the low level, which sets the rotational direction to the counterclockwise (CCW) state.

(72) When the WE signal is set to the low level at the time point t.sub.0, the driving circuit 100E transits to the normal excitation mode. In this state, when the cyclic CLK signal is input, the excitation position is changed in increments of +2.

(73) At the time point t.sub.1, the last CLK signal is generated. In this stage, the WE signal is set to the low level. Accordingly, the timer circuit 112E advances the counting operation. When the judgment time has elapsed at the time point t.sub.2, the logical value of the ROT signal is inverted, which switches the rotational direction to the clockwise direction. Subsequently, when the next CLK signal is input at the time point t.sub.3, the excitation position is changed in a direction that is the reverse of that set at the time point t.sub.1.

(74) FIG. 12 is another operation waveform diagram showing the operation of the driving circuit 100E shown in FIG. 10. The operation before the time point t.sub.1 is the same as that shown in FIG. 11. When the last CLK signal has been generated at the time point t.sub.1, i.e., when the supply of the CLK signal is suspended, the rotational operation is suspended. Immediately after the supply of the CLK signal is suspended, the WE signal is set to the high level. In this case, the timer circuit 112E does not perform the counting operation (or does not advance the counting operation). Accordingly, in this case, the ROT signal is not logically inverted, which maintains the rotational direction in the counterclockwise direction. Subsequently, after the input of the CLK signal is restarted at the time point t.sub.3, the excitation position is changed in the same direction as that set before the time point t.sub.1. The above is the operation of the driving circuit 100E.

(75) Description has been made in the embodiment regarding the driving circuit 100 that supports the PWM driving operation. However, the present invention is not restricted to such an arrangement. In a case in which the driving circuit does not support the stepping control operation for the phase currents, this arrangement does not require the PWM driving operation. In this case, the first PWM generator 122_1 and the second PWM generator 122_2 can be omitted.

(76) While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.