Abstract:
An electronic device, includes a USB interface, a processing unit, a master-slave select triggering circuit, a controlling unit, a selection circuit and a switching circuit. The master-slave selection triggering circuit including an input port, a first output port and a second output port. The master-slave select triggering circuit is used to detect the type of external electronic devices connected to the USB interface, if the external electronic device is a master device, the first and second output port output a slave triggering signal, the controlling unit switches off the switching circuit t and the selection circuit selects the processing unit into slave mode; if the external electronic device is the slave device, the first and second output port output a master triggering signal, the controlling unit switches on the switching circuit and the selection circuit select the processing unit into master mode.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to electronic devices and, particularly, to an electronic device capable of automatically switching between a master and a slave mode. 
         [0003]    2. Description of Related Art 
         [0004]    In general, an electronic device has at least two control modes, for example, a master mode and a slave mode. When the electronic device, such as digital photo frame (DPF), is connected with a computer, the electronic device enters the slave mode, in which the DPF is controlled by the computer; when the DFP is connected with a memory device, such as a flash memory, the DPF enters the master mode, in which the DPF controls the flash memory. There is a need to change the control mode when the DPF connects to different external device. 
         [0005]    However, a present switching circuit of the electronic device to switch between the master mode and the slave mode usually is relatively complex and has a high cost. 
         [0006]    Therefore, an electronic device capable of automatically switching between a master mode and a slave mode is desired to overcome the above-described deficiencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the electronic device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views 
           [0008]      FIG. 1  is a block diagram of an electronic device capable of automatically switching between a master mode and a slave mode. 
           [0009]      FIG. 2  is a circuit diagram of an electronic device capable of automatically switching between a master mode and a slave mode of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a block diagram of an electronic device capable of automatically selecting between a master mode and a slave mode. The electronic device  1  includes a mode switching circuit  10  and a USB interface  20 . The USB interface  20  is used for connecting an external electronic device  2 , and the mode switching circuit  10  is used for detecting the type of the external electronic device  2  and switching the electronic device  1  to the master mode or the slave mode according to the detected type of the external electronic device  2 . For example, the mode switching circuit  10  switches the electronic device  1  to the slave mode, when the external electronic device  2  is a host device, such as a computer. When the external device  2  is a slave device, such as a flash memory, the switching circuit  10  switches the electronic device  1  to the master mode. 
         [0011]    The mode switching circuit  10  includes a master-slave selection triggering circuit  101 , a controlling unit  102 , a selection circuit  103 , and a switching circuit  104 . The master-slave selection triggering circuit  101  includes an input port  1011 , a first output port  1012 , and a second output port  1013 . The USB interface  20  includes a power pin  201  and a data pin  202 . The input port  1011  of the master-slave selection triggering circuit  101 , is connected to the power pin  201  of the USB interface  20 . The first output port  1012  of the master-slave selection triggering circuit  101 , is connected to the controlling unit  102 , and the second output port  1013  of the master-slave selection triggering circuit  101 , which is connected to the selection circuit  103 . 
         [0012]    The electronic device  1  also includes a processing unit  30  and a power source  40 . The processing unit  30  includes a slave mode controlling pin  301  and a master mode controlling pin  302 . 
         [0013]    When the external electronic device  2  connects to the USB interface  20 , the master-slave selection triggering circuit  101  determines the type of external electronic devices  2  by detecting the voltage of the power pin  201  of the USB interface  20 . When the master-slave selection triggering circuit  101  detects the power pin  201  is at high voltage. The master-slave selection triggering circuit  101  determines the external electronic device  2  is a host device and sends a slave triggering signal to the controlling unit  102  and the selection circuit  103  via the first output port  1012  and the second output port  1013  respectively. When the master-slave selection triggering circuit  101  detects the power pin  201  is at low voltage, the master-slave selection triggering circuit  101  determines the external electronic device  2  is a slave device. Then the master-slave selection triggering circuit  101  sends a master triggering signal to the controlling unit  102  and the selection circuit  103  via the first output port  1012  and the second output port  1013  respectively. 
         [0014]    The controlling unit  102  is further connected to the switching circuit  104 , and the switching circuit  104  is connected between the power source  40  and the power pin  201 . When the controlling unit  102  receives the slave triggering signal from the first output port  1012 , the controlling unit  102  switches off the switching circuit  104 , then disconnects the connection between the power source  40  and the power pin  201 . When the selection circuit  103  receives the slave triggering signal from the second output port  1013 , the selection circuit  103  connects the slave mode controlling pin  301  of the processing unit  30  with the data pin  202 . Therefore, the electronic device  1  works in the slave mode. 
         [0015]    When the controlling unit  102  receives the master triggering signal from the first output port  1012 , the controlling unit  102  switches on the switching circuit  104 , then the power source  40  is electrically connects to the power pin  201  via the switching circuit  104  and provides power to the power pin  201 . When the selection circuit  103  receives the master triggering signal from the second output port  1013 , the selection circuit  103  connects the master mode controlling pin  302  of the processing unit  30  with the data pin  202 . Therefore, the electronic device  1  works in the master mode. 
         [0016]    Therefore, when the external electronic device  2  is a host device, such as a computer, the controlling unit  102  of the electronic devices  1  switches off the switching circuit  104  to disconnect the connection between the power source  40  and the power pin  201 . At the same time, the selection circuit  103  of the electronic device  1  connects the slave mode controlling pin  301  with the data pin  20 , then the electronic device  1  is switched to the slave mode. When the external electronic device  2  is a slave device, the controlling unit  102  of the electronic devices  1  switches on the switching circuit  104  to connect the power source  40  with the power pin  201 , and the power source  40  provides power to the power pin  201 . At the same time, the selection circuit  103  of the electronic device  1  connects the master mode controlling pin  302  with the data pin  20 , then the electronic device  1  is switched to the master mode. 
         [0017]      FIG. 2  is a circuit diagram of the electronic device of  FIG. 1 . The master-slave selection triggering circuit  101   [Zi]  includes a first resistor R 1 , a second resistor R 2 , and a third resistor R 3 . Referring to  FIG. 2 , a connection node A between the first resistor R 1  and the second resistor R 2  serves as the input port  1011  of the master-slave selection triggering circuit  101 . A node B between the resistor R 1  and the controlling unit  102  serves as the output port  1012  of the master-slave selection triggering circuit  101 , and a connection node C between the first resistor R 2  and the second resistor R 3  serves as the output port  1013  of the master-slave selection triggering circuit  101 . 
         [0018]    The controlling unit  102  is a high voltage activated switch, and the controlling unit  102  includes a control terminal, a first conductive terminal, and a second conductive terminal. The exemplary embodiment, takes a negative-positive-negative (NPN) bipolar junction transistor (BJT) Q 1  as an example to illustrate the controlling unit  102 . A base, an emitter, and a collector of the NPN BJT Q 1  serve as the control terminal, the first conductive terminal, and the second conductive terminal of the controlling unit  102  respectively. 
         [0019]    The base of the NPN BJT Q 1  is connected to the first output port  1012  of the master-slave selection triggering circuit  101 , the collector of the NPN BJT Q 1  is connected to the switching circuit  104 , and the emitter of the NPN BJT Q 1  is connected to ground. 
         [0020]    The switching circuit  104  is a high voltage activated circuit, and the switching circuit  104  includes a control terminal, a first conductive terminal, and a second conductive terminal. The control terminal of the switching circuit  104  is connected to the collector of the NPN BJT Q 1 , the first conductive terminal is connected to the power source  40 , and the second conductive terminal is connected to the power pin  201 . In the exemplary embodiment, the switching circuit  104  is a high voltage activated switch, such as an n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) or a NPN BJT. 
         [0021]    In the exemplary embodiment, the slave mode controlling pin  301  of a processing unit  30  includes a “Device D+” pin and a “Device D−” pin, the master mode controlling pin  302  of a processing unit  30  includes a “Host D+” pin and a “Host D−” pin. The data pin  202  of the USB interface  20  includes a first data pin (D+ pin) and a second data pin (D− pin). 
         [0022]    The selection circuit  103  is a double-pole double-throw switch, which includes a control terminal, two public terminals, and four joint terminals. The control terminal of the selection circuit  103  is connected to the second output port  1013  of the master-slave selection triggering circuit  101 . The two public terminals of the selection circuit  103  are respectively connected to the first data pin and the second data pin of the data pin  202 . The four joint terminals of the selection circuit  103  are respectively connected to the “Device D+” pin, “Device D−” pin, “Host D+” pin, “Host D−” pin. When the control terminal of the selection circuit  103  is at high voltage, the two public terminals are connected to the joint terminals, which are connected to the “Device D+” pin, “Device D−” pin. When the control terminal of the selection circuit  103  is at low voltage, the two public terminals are connected to the joint terminals, which are connected to the “Host D+” pin, “Host D−” pin. 
         [0023]    When the external electronic device  2  connected to the electronic device  1  is the host device, the power pin  201  of the USB interface  20  receives a high voltage from the external device  2 , and the input port  1011  receives a high voltage from the USB interface  20  accordingly. The output port  1012  receives a high voltage from the input port  1011  via resistor R 1 , and the control terminal of the NPN BJT Q 1  receives a high voltage from the first output port  1012 , thereby causing the NPN BJT Q 1  to be switched on. The control terminal of the switching circuit  104  receives a low voltage from the NPN BJT Q 1 , the switching circuit  104  is switched off accordingly, and the connection between the power source  40  and the power pin  201  of the USB interface  20  is disconnected. 
         [0024]    When the input port  1011  receives a high voltage from the USB interface  20 , the second output  1013  receives a high voltage divided by resistor R 2  and resistor R 3 , then the second output  1013  outputs a high voltage slave triggering signal to the selection circuit  103 . The selection circuit  103  connects the “Device D+” pin and “Device D−” pin to the first data pin and second data pin of the USB interface  20  by connecting the two public terminals to the joint ends connected to the “Device D+” pin, “Device D−” pin. 
         [0025]    Therefore, the power supply of the USB interface  20  is from the external electronic device  2 , and the electronic device  1  is switched to the slave mode. 
         [0026]    When the external electronic device  2  connected to the electronic device  1  is the slave device, the power pin  201  of the USB interface  20  receives a low voltage from the external device  2 , and the input port  1011  receives a low voltage from the power pin  201  accordingly. The output port  1012  receives a low voltage from the input port  1011  via resistor R 1 , the control terminal of the NPN BJT Q 1  receives a low voltage from the output port  1012 , and the NPN BJT Q 1  is switched off. The control terminal of the switching circuit  104  receives a high voltage from the NPN transistor Q 1 , the switching circuit  104  is switched on, and the power source  40  is connected to the power pin  201  of the USB interface  20 . 
         [0027]    When the input port  1011  receives a low voltage from the USB interface  20 , the second output  1013  receives a low voltage divided by resistor R 2  and resistor R 3  then the second output  1013  outputs a low voltage master triggering signal to the selection circuit  103 . The selection circuit  103  connects the “Device D+” pin and “Device D−” pin separately to the first data pin and second pin of the USB interface  20  by connecting the two public terminals to the joint ends connected to the “Host D+” pin, “Host D−” pin. 
         [0028]    Therefore, the power supply of the USB interface  20  is from the electronic device  1 , and the electronic device  1  is switched to the master mode. 
         [0029]    It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the present disclosure.