Abstract:
A high-speed universal serial bus (USB) transceiver includes a voltage-mode architecture for generating a USB signal. The voltage mode architecture reduces power consumption by reducing the current requirements for high-speed USB communications. The USB transceiver can include a reference voltage generator, a resistive element, and a switching element for completing and breaking a circuit including the reference voltage generator, the resistive element, and a data pin of a USB port to generate half of the differential USB signal (e.g., the D+ signal). A similar circuit can be used to generate the other half of the differential USB signal (i.e., the D− signal). The resistive element can be a set of parallel resistors in the transceiver, with the set of parallel resistors being specifically selected from a larger population of resistors to provide the specified resistance (45Ω±10%) in the USB transceiver.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/192,871, entitled “USB 2.0 HS Voltage-Mode Transmitter With Tuned Termination Resistance” filed Jul. 29, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the field of computer electronics, and in particular, to a system and method for enabling low-power universal serial bus communications. 
         [0004]    2. Related Art 
         [0005]    The universal serial bus (USB) protocol is a popular communications protocol that allows a wide range of modern electronic devices and peripherals (e.g., scanners, digital cameras, personal digital assistants, and digital music players) to communicate with another peripheral. The present USB 2.0 specification (“Universal Serial Bus Specification”, Revision 2.0, Apr. 27, 2000) defines three signaling levels that can be supported by USB-compliant devices. The three levels include a low-speed mode operating at 1.5 Mbps at 3.3 V, a full-speed mode operating at 12 Mbps at 3.3 V, and a high-speed mode that signals at 480 Mbps at 400 mV. 
         [0006]    Modern high-speed USB 2.0-compliant devices include USB transceivers that are based on a current mode architecture for the high-speed transmitter, in which a current source drives the outgoing USB communications. For example,  FIG. 1  shows a high-speed USB transmitter  100  (i.e., a device generating a high-speed USB signal) and a high-speed USB receiver  150  (i.e., a device receiving a high-speed USB signal). A USB cable  140  connects a USB port  111  on USB transmitter  100  to a USB port  151  on USB receiver  150  to enable communication between the two devices. 
         [0007]    A USB cable (i.e., USB cable  140 ) is a four-line serial data bus. Two of the lines are power lines (i.e., VBUS and ground lines), and the other two lines form a pair of differential signal lines (i.e., D+ and D− lines). For clarity, communication between high-speed USB transmitter  100  and high-speed USB receiver  150  will be described with respect to only one half of the differential USB signal (i.e., with respect to signal D+). The inverted signal forming the other half of the differential USB signal (i.e., the D− signal) is generated in a manner substantially similar to that described with respect to the generation of the D+ signal. In accordance with the USB 2.0 specification, USB receiver  150  includes a termination resistor  160  connected between USB port  151  and ground (i.e., the ground supply voltage or lower supply voltage). A D+ signal received at USB port  151  is read from the junction between USB port  151  and resistor  160 . 
         [0008]    To generate the D+ signal, a high-speed USB transceiver  110  in USB transmitter  100  includes a current source  120 , a switch  5105 , and a voltage-setting resistor  130 . Current source  120 , switch  5105 , and resistor  130  are connected in series between a supply voltage VDD and ground. When switch S 105  is closed, a portion of current I 121  supplied by current source  120  flows through resistor  130  to ground (i.e., current I 131 ) to set the output signaling voltage at USB port  111 , while another portion of current I 121  flows through USB cable  140  and through resistor  160  in USB receiver  150  to ground (i.e., current I 161 ). By switching switch S 105  on and off, USB transceiver  110  generates the D+USB signal transmitted from USB transmitter  100  to USB receiver  150  by USB cable  140 . 
         [0009]    The USB 2.0 specification requires that termination resistor  160  (i.e., the resistor coupled between the USB port and ground in the downstream device) have a resistance value equal to 45Ω±10%. Voltage setting resistor  130  is sized similarly, so that the currents flowing through both resistors are the same (i.e., current I 131  is equal to current I 161 ) and that the voltage drops across resistors  130  and  160  are the same. Therefore, to provide the requisite 400 mV D+ signal (i.e., half of the total 800 mV signal specified for high-speed USB 2.0 communications), both currents I 131  and I 161  must be equal to roughly 8.9 mA (equal to 400 mV (signal amplitude) divided by 45Ω (resistance)). Accordingly, current source  120  must provide a total current I 121  equal to roughly 17.8 mA (i.e., the sum of currents I 131  and I 161 ). 
         [0010]    As the proliferation of USB-compatible devices continues to increase, the importance of power efficiency for those USB devices also increases. Unfortunately, the current-mode architecture used in conventional USB 2.0 transceivers (as shown in  FIG. 1 ) is less than ideal in the realm of power efficiency. Specifically, because resistor  130  in transceiver  110  is used to generate the required signal voltage (i.e., 400 mV), current source  120  must effectively drive two parallel resistance paths to ground (i.e., through resistor  130  in USB transmitter  100  and through resistor  160  in USB receiver  150 ). Hence, current source  120  must generate twice as much current as is required by USB receiver  150 . 
         [0011]    Accordingly, it is desirable to provide a system and method for providing high-speed USB communications that reduces power consumption over the conventional current-mode transceiver architecture shown in  FIG. 1 . 
       SUMMARY OF THE INVENTION 
       [0012]    To reduce power consumption in a high-speed USB transceiver, the conventional current-mode architecture can be replaced with a voltage-mode architecture. The voltage-mode architecture drives the USB signal using a voltage source, thereby eliminating the need for a voltage setting resistor coupled to ground in the USB transceiver. As a result, the additional current path to ground provided by the voltage-setting resistor is eliminated, and current requirements for the voltage-mode USB transmitter can be effectively halved over the current requirements for a current-mode USB transmitter. 
         [0013]    In one embodiment, a high-speed USB transceiver (e.g., in a computer, computer peripheral, digital cameral, PDA, or digital music player) can include a voltage source, a resistive element, and a data pin in a USB port to generate half of a high-speed differential USB signal (e.g., the D+ signal). By having the voltage source provide a signal that switches between 0 and 800 mV, the USB signal level at the data pin in the USB port will be the specified 400 mV (due to the voltage dividing effect of the resistive element in the transceiver and the similarly sized termination resistor in the USB receiver). In one embodiment, the transceiver can include a set of parallel resistors and selection logic that defines the resistive element actually used in the generation of the USB signal by selecting a subset of those parallel resistors that provides a total resistance equal to the 45Ω±10% specified in the USB 2.0 specification. In another embodiment, the transceiver can include tuning resistors that can be used in place of the actual output resistors during tuning (i.e., during the selection process used to determine which combination of resistors will provide the 45Ω±10%). In another embodiment, the transceiver can include low-speed/full-speed circuitry for making and breaking a circuit including a second reference voltage source (e.g., providing 3.3 V), the resistive element, and the data pin in the USB port to generate half of a low-speed or full-speed differential USB signal. 
         [0014]    A method for operating a USB device can include generating a reference voltage (e.g., 800 mV), and controlling switches in a circuit that includes the reference voltage generator, a resistive element, and a data pin in a USB port of the device to generate half of a differential USB signal. The method can further include selecting a portion of a set of resistors to define the resistive element, with the selection process involving selecting various combinations of resistors from the set until a total resistance of 45Ω±10% is achieved. In one embodiment, this selection process can be performed using replica tuning resistors in place of the actual output resistors, where the tuning resistors are matched to the output resistors. The use of tuning resistors allows the selection process to be performed without involving the actual output resistors. In another embodiment, the reference voltage can be switched from the 800 mV to 3.3 V to enable the generation of low-speed or full-speed USB signals. 
         [0015]    The invention will be more fully understood in view of the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a circuit diagram of a conventional USB transceiver based on a current-mode architecture. 
           [0017]      FIG. 2  shows circuit diagram of a USB transceiver based on a voltage-mode architecture. 
           [0018]      FIG. 3A  shows a circuit diagram of an embodiment of the USB transceiver of  FIG. 2 . 
           [0019]      FIG. 3B  shows a circuit diagram of another embodiment of the USB transceiver of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    To reduce power consumption in a high-speed USB transceiver, the conventional current-mode architecture can be replaced with a voltage-mode architecture. The voltage-mode architecture drives the USB signal using a voltage source, thereby eliminating the need for a voltage setting resistor coupled to ground in the USB transceiver. As a result, the additional current path to ground provided by the voltage-setting resistor is eliminated, and current requirements for the voltage-mode USB transmitter can be effectively halved over the current requirements for a current-mode USB transmitter. 
         [0021]      FIG. 2  shows an embodiment of a high-speed USB apparatus  200  (e.g., a computer, computer peripheral, digital cameral, PDA, or digital music player) based on a voltage-mode architecture. A USB port  211  on USB apparatus  200  is connected to a USB port  251  on a high-speed USB receiver  250  by a USB cable  240 . As described above with respect to  FIG. 1 , high-speed USB receiver  250  (in a downstream USB device not shown for clarity) is required by the USB 2.0 specification to include a termination resistor  260  (45Ω±10%) coupled between USB port  251  and ground. A USB signal D+ is generated by a high-speed USB transmitter  210  in a high-speed USB transceiver  205  in USB apparatus  200 , and is transmitted from USB apparatus  200  to USB receiver  250  by USB cable  240  to enable USB communications between the two devices. 
         [0022]    Note that, as described above with respect to  FIG. 1 , USB communications are performed using differential signaling, in which half of a differential signal (i.e., a D+ signal) is transmitted over one wire in USB cable  240 , and the other half of the differential signal (i.e., a complementary D− signal) is transmitted over a second wire in USB cable  240 . Therefore, USB cable  240  will include four wires (data D+ and D− wires and power and ground wires, not shown for clarity), and each USB port (e.g., USB port  211 ) will likewise include four pins (data D+ pin and data D− pin, power and ground pins, not shown for clarity). For exemplary purposes, the operation of high-speed USB transmitter  210  is described with respect to the generation of half of the differential USB signal (e.g., the D+ signal). However, it is understood that complementary USB signal generator circuitry  215  included in USB transmitter  210  can generate complementary signal D− forming the other half of the differential USB signal in a manner substantially similar to that described below with respect to the generation of signal D+. Note further that USB receiver  250  will also include a second resistor to ground (not shown) that is substantially similar to resistor  260  to receive the complementary D− signal. 
         [0023]    High-speed USB transmitter  210  includes a voltage source  220 , signaling circuitry  225 , and a resistance element  230 . Voltage source  220 , signaling circuitry  225  and resistance element  230  are connected in series between a ground terminal (indicated by the inverted triangle) and USB port  211 . Voltage source  220  provides a reference voltage VREF, which for reasons described in greater detail below, is 0.8 V. Signaling circuitry  225  provides a switching output between voltage VREF and ground (0 V) to resistance element  230 , which, in accordance with the USB 2.0 specification, is sized to provide a resistance equal to 45Ω±10% (i.e., equal to termination resistor  260  in USB receiver  250 ). Note that while resistance element  230  is depicted as a single resistor for exemplary purposes, in other embodiments (such as described below with respect to  FIG. 3B ), resistance element  230  can comprise multiple resistors connected in parallel (or series) that provide a total resistance equal to 45Ω±10%. 
         [0024]    The switching output of signaling circuitry  225  (between voltage VREF and ground) is provided by resistance element  230  at USB port  211  as USB signal D+ (i.e., half of the differential USB signal). When signaling circuitry  225  passes voltage VREF to resistance element  230 , the voltage divider formed by resistance element  230  and resistor  260  pulls the D+ USB pin (in USB port  251  in receiver  250 ) high with an output resistance of 45 ohms. When signaling circuitry  225  switches to a grounded output (0 V), the D+ signal output from transmitter  210  is pulled to a logic LOW level with an output resistance of 45Ω. Because resistance element  230  and resistor  260  (in receiver  250 ) have equal resistances (i.e., 45Ω±10%), the amplitude of signal D+ is simply half of voltage VREF. Because the required amplitude for high-speed USB communications is 800 mV, and because half of the differential signal is carried on each data line in USB cable  240 , signal D+ must have an amplitude of 400 mV. Therefore, voltage VREF provided by voltage source  220  is simply 800 mV (i.e., 2×400 mV). 
         [0025]    Due to the serial configuration of resistance element  230  in USB transmitter  210  and resistor  260  in USB receiver  250 , a current I 231  through resistance element  230  is the same as a current I 261  through resistor  260 , which in turn is the same as a current I 221  supplied by voltage source  220 . Specifically, currents I 221 , I 231 , and I 261  are all roughly equal to 8.9 mA (i.e., voltage VREF (800 mV) divided by the sum of the resistances of resistance element  230  and resistor  260  (90Ω)). Note that this current is half of the current required from the current-mode architecture used in a conventional USB transceiver (i.e., USB transceiver  110  shown in  FIG. 1 ). In this manner, the voltage-mode architecture in USB transmitter  210  results in only a single path to ground for the D+ signaling circuitry (as opposed to the two parallel paths to ground that result from conventional current-mode USB transceiver architectures, as described with respect to  FIG. 1 ), thereby halving power consumption over conventional USB transceivers. 
         [0026]    Note that in one embodiment, voltage source  220  can be used to generate both USB signals D+ and D− (as indicated by the dotted line between the output of voltage source  220  and complementary signal generation circuitry  215 ). Because signals D+ and D− are complementary signals, only one of those signals will be driven by voltage source  220  at any given time. This voltage source “sharing” between the D+ and D− circuitry can beneficially create a constant (and therefore less taxing) loading condition on voltage source  220 . 
         [0027]      FIG. 3A  shows an exemplary circuit diagram of high-speed USB transmitter  210  that includes voltage source  220  for providing reference voltage VREF (i.e., 800 mV) and an output sliver  330  for generating USB signal D+. Note that the voltage source  220  can be implemented using any circuit capable of providing a stable voltage VREF. 
         [0028]    Output sliver  330  comprises an output resistor R 323  and an embodiment of signaling circuitry  225  that includes NMOS switching transistors N 321 , N 322 , and an optional NMOS enable transistor N 324 . USB transmitter  210  can include optional digital tune logic  335  for controlling the signals provided to the gates of transistors N 321 , N 322 , and N 324  (if present). Transistors N 321  and N 322  are connected in series between the output of voltage source  220  and ground, whereas transistor N 324  is connected in series between resistor R 323  and the junction between transistors N 321  and N 322 . Resistor R 323  is connected between USB port  311  and the transistor N 324 . In one embodiment, transistors N 321 , N 322 , and N 324  (if present) can be implemented as thick oxide devices to enable robust operation for a wide range of operating voltages (e.g., up to 3.3 V for low-speed/full-speed operation). 
         [0029]    During high-speed operation, high-speed switching control signals HS_DATA and HS_DATA_N are supplied to the gates of switching transistors N 321  and N 322 , respectively. Signals HS_DATA and HS_DATA_N operate in opposition, such that when signal HS_DATA is asserted, signal HS_DATA_N is deasserted, and vice versa. Thus, signal HS_DATA acts as a signaling control signal while signal HS_DATA_N acts as a pulldown control signal. Specifically, when signal HS_DATA is asserted to turn on transistor N 321 , signal HS_DATA_N is deasserted to turn off transistor N 322 , and a positive voltage (i.e., reference voltage VREF (i.e., 800 mV) minus the voltage drop across resistor R 323  and NMOS devices N 321  and N 324  (i.e., 400 mV)) is provided at USB port  311 . Likewise, when signal HS_DATA is deasserted to turn off transistor N 321 , signal HS_DATA_N is asserted to turn on transistor N 322 , and USB port  311  is pulled to ground. In this manner, switching signals HS_DATA and HS_DATA_N control the signaling pattern of USB signal D+. 
         [0030]    As noted above, the complementary USB signal D− (i.e., the other half of the differential USB signal) can be generated in substantially the same manner by corresponding circuitry in complementary USB signal generation circuit  215  (e.g., circuit  215  can include a voltage source that generates a reference voltage at 800 mV and an output sliver that switches that reference voltage to the D− pin of USB port  311  based on signals HS_DATA (which sets signal D− to 400 mV) and HS_DATA_N (which sets signal D− to ground)) with opposite logic to the D+line. 
         [0031]    As further noted above, the USB 2.0 specification requires that the output resistance of a USB transceiver be equal to 45Ω±10%. However, on-chip sheet resistors used in ICs typically exhibit resistance variations in the +/−20-25% range. Therefore, the accurate resistance requirements of the USB 2.0 specification can be difficult to achieve using a single resistor. Therefore, in one embodiment, high-speed USB transmitter  210  can include multiple copies of output sliver  330  that can be selectively enabled by asserting an enable signal EN at the gate of transistor N 324 . The enabled slivers  330  can be selected such that USB signal D+ at USB port  311  is presented with a total resistance that is within the range required by the USB specification. 
         [0032]    For example,  FIG. 3B  shows a schematic diagram of another embodiment of high-speed USB transmitter  210  that includes multiple output slivers  330  (i.e., output slivers  330 - 1 , through  330 -N) and a sliver control circuit  490  for selectively coupling each of slivers  330 - 1  through  330 -N to USB port  211 . Each of slivers  330 - 1  through  330 -N can be substantially similar to output sliver  330  described with respect to  FIG. 3A , and therefore the outputs of output slivers  330  in response to switching control signals HS_DATA and HS_DATA_N are all synchronous. 
         [0033]    Sliver control circuit  490  selectively sends enable signals ENABLE-1 through ENABLE-N to control how many slivers are put in parallel to form the output driver. The selection logic  495  determines how the appropriate number of slivers to enable such that the total output resistance of the output slivers  330  will meet the USB 2.0 output resistance specification (i.e., 45Ω±10%). For example, selection logic  495  could enable various combinations of slivers  330  until a voltage at USB port  211  matches a precision reference voltage at 400 mV provided to selection logic  495 . Various other selection algorithms and techniques will be readily apparent. 
         [0034]    Note that because output resistors R 323  in output slivers  330  are all connected in parallel between voltage source  220  and USB port  211 , each individual resistor R 323  can be much larger than a single resistor providing the same total resistance (since the total resistance provided by parallel resistors is equal to the reciprocal of the sum of the reciprocals of the individual resistors). Therefore, a USB transceiver based on multiple output slivers  330  can exhibit improved manufacturability and improved electrostatic discharge (ESD) resistance (due to the larger resistors R 323 ). 
         [0035]    In certain circumstances, it may be desirable to avoid the use of output resistors R 323  for any purpose other than USB signal generation. Thus, in one embodiment, each sliver  330  can include a replica circuit  331  that includes duplicates of transistors N 321 , N 322 , N 324  and/or resistor R 326  for use in the resistance tuning process described above. As is well known in the art, matched resistors in an IC (e.g., output resistor R 326  and its matching resistor in replica circuit  331 ) can be created with very similar resistance values, even if the absolute values of those resistors cannot be set to a precise resistance value. Therefore, the results obtained from a resistance tuning process using a replica of the output driver will be valid for the actual output resistors R 323 . 
         [0036]    The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the particular embodiments described. For example, transistors N 321 - 1  through N 321 -N in each of output slices  330 - 1  through  330 -N, respectively, could be replaced with a single transistor that connects and disconnects output slices  330 - 1  through  330 -N to voltage regulator  310 . Similarly, transistors N 322 - 1  through N 322 -N in each of output slices  330 - 1  through  330 -N, respectively, could be replaced with a single transistor that connects and disconnects output slices  330 - 1  through  330 -N to ground. Thus, the invention is limited only by the following claims and their equivalents.