Abstract:
Calibration circuitry and method for maintaining constant signal detection thresholds for multiple signal receivers that receive data signals in the form of current signals. A value of one of the incoming current signals having a predetermined signal pattern is detected and used to generate threshold control signals for each of the signal receivers to control the data signal detection thresholds.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to control circuitry for establishing and maintaining detection thresholds for data signal receivers, and in particular, to such circuits for establishing and maintaining signal thresholds for current signal receivers. 
   2. Related Art 
   As interface circuits operate with lower power supply voltages and lower magnitude signals, maintaining signal thresholds for properly detecting the states of the received signals becomes increasingly problematic. As a signal magnitudes continue to decrease, the smaller dynamic ranges make it all the more important to be able to consistently establish and maintain proper signal detection thresholds substantially irrespective of signal link conditions. 
   SUMMARY OF THE INVENTION 
   In accordance with the presently claimed invention, calibration circuitry and method are provided for maintaining constant signal detection thresholds for multiple signal receivers that receive data signals in the form of current signals. A value of one of the incoming current signals having a predetermined signal pattern is detected and used to generate threshold control signals for each of the signal receivers to control the data signal detection thresholds. 
   In accordance with one embodiment of the presently claimed invention, control circuitry for controlling a plurality of signal thresholds for a plurality of current signal receivers includes:
         a plurality of current signal receiver circuits each of which is responsive to reception of a respective one of a plurality of threshold control signals and a respective one of a plurality of current signals by providing a respective one of a plurality of data signals, wherein each one of the plurality of data signals corresponds to a respective one of the plurality of current signals; and   control circuitry coupled to the plurality of current signal receiver circuits and responsive to reception of at least a portion of one of the plurality of current signals by providing the plurality of threshold control signals, wherein each one of the plurality of threshold control signals has a respective value related to the one of the plurality of current signals.       

   In accordance with another embodiment of the presently claimed invention, control circuitry for controlling a plurality of signal thresholds for a plurality of current signal receivers includes:
         a plurality of current signal receiver means each of which is for receiving a respective one of a plurality of threshold control signals and a respective one of a plurality of current signals and providing a respective one of a plurality of data signals, wherein each one of the plurality of data signals corresponds to a respective one of the plurality of current signals; and   controller means for receiving at least a portion of one of the plurality of current signals and providing the plurality of threshold control signals, wherein each one of the plurality of threshold control signals has a respective value related to the one of the plurality of current signals.       

   In accordance with still another embodiment of the presently claimed invention, a method for controlling a plurality of signal thresholds for a plurality of current signal receivers includes:
         receiving a plurality of current signals including a selected current signal;   processing at least a portion of the selected current signal to provide a plurality of threshold control signals, wherein each one of the plurality of threshold control signals has a respective value related to the selected current signal; and   detecting the plurality of current signals in accordance with the plurality of threshold control signals to provide a plurality of data signals, wherein each one of the plurality of data signals corresponds to a respective one of the plurality of current signals.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a functional block diagram of a current signal link for which threshold control circuitry in accordance with the presently claimed invention is advantageous. 
       FIG. 2  is a functional block diagram of the signal transmitters of  FIG. 1 . 
       FIG. 3  is a functional block diagram of the signal receivers of  FIG. 1 . 
       FIG. 4  is a functional block diagram of a current signal receiver system using threshold control circuitry in accordance with the presently claimed invention. 
       FIG. 5  is a signal diagram depicting the signal averaging of threshold control circuitry in accordance with the presently claimed invention. 
       FIG. 6  is a circuit diagram of a preferred embodiment of an input stage for the averaging circuit of  FIG. 4 . 
       FIG. 7  is a circuit schematic of a preferred embodiment of the current mirror circuitry of  FIG. 6 . 
       FIG. 8  is a circuit schematic of a preferred embodiment of an output stage for the averaging circuit of  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
   Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
   Referring to  FIG. 1 , a low power interface circuit using small magnitude current signaling for which threshold control circuitry in accordance with the presently claimed invention is advantageous includes master circuitry  10  and slave circuitry  20  sharing a common reference, or ground, connection  11   g , and communicating via a half-duplex data signal  11   d  and clock signal  11   c . The clock signal  11   c  is transmitted by a line driver, or transmitter,  10   c  in the master  10 , and received by a receiver  20   c  in the slave  20 . A transmitter  10   t  and receiver  10   r  in the master  10  and a similar receiver  20   r  and transmitter  20   t  in the slave  20  provide for the half-duplex communication of the data signal  11   d . Low magnitude current signaling is used for low power operation and low electromagnetic interference (EMI), with the two binary logic states being determined by the magnitude of the current being sourced from the receiver to the transmitter. (The two currents associated with the two logic states can be visualized as an AC current riding on a DC bias current.) In accordance with a preferred embodiment, the two logic level currents are 150 microamperes and 750 microamperes, and are considered to be a signal of +/−300 microamperes riding on a DC bias current of 450 microamperes. The higher current (e.g., 750 microamperes) is considered a logic low level, while the lower current (e.g., 150 microamperes) is considered as the logic high level. 
   Referring to  FIG. 2 , the line driver  10   t / 20   t  uses a two-state current sink, with the magnitudes of the currents being determined by the input state. The line driver  10   t / 20   t  does not establish the voltage on the line; this is established by the receiver  10   r / 20   r . This allows the line driver to accommodate multiple receiver voltages, and allows for the line drivers and receivers to be operated with different power supplies. 
   When the internal enable signal is a logic low, switch S 1  connects current source I 1  to the signal line  11   c / 11   d , thereby causing current IDATA to be sunk by the driver  10   t / 20   t . When the internal data signal is low, switch S 2  is also connected to the signal line  11   c / 11   d , thereby causing a sum of currents of IDATA and 4IDATA to be sunk by the driver  10   t / 20   t . When the internal data signal is a logic high, switch S 2  opens, thereby causing only current IDATA to be sunk from the signal line  11   c / 11   d . When both switches S 1 , S 2  are open, the driver  10   t / 20   t  is in its off state, and zero current is sunk from the signal line  11   c / 11   d . Accordingly, a logic low is the higher current magnitude, i.e., the sum of currents I 1  and I 2 , and a logic high is the lower current magnitude, i.e., current I 1  only. 
   Referring to  FIG. 3 , the receiver circuitry  10   r / 20   r  detects the current state on the signal line  11   c / 11   d , converts that current to a voltage, and amplifies that voltage to standard logic level, regardless of whether signal current is present or not. The receiver circuitry  10   r / 20   r  also provides line termination. 
   Referring to  FIG. 4 , a slave circuit  120  with current signal receivers  120   c ,  120   r   0 ,  120   r   1  controlled in accordance with one embodiment of the presently claimed invention receive the signal, or link, currents  111   c ,  111   d   0 ,  111   d   1  being sunk by the transmitters, as discussed above, detect these signal currents and provide corresponding clock  121   c  and data  121   d   0 ,  121   d   1  signals. As discussed in more detail below, the clock signal current  111   c  (which is normally periodic and, therefore, more susceptible to predictable averaging) is sampled by current averaging circuitry  200 . An averaged current signal  201  is provided to threshold duplication circuitry  300  (e.g., a current mirror circuit) which provides replicated, or mirrored, signal currents which serve as threshold control signals  301   c ,  301   d   0 ,  301   d   1  for the receiver circuits  120   c ,  120   r   0 ,  120   r   1 . 
   The current thresholds for the receivers  120   c ,  120   r   0 ,  120   r   1  are set at the middle value of the currents  111   c ,  111   d   0 ,  111   d   1  being conveyed via the link. This threshold determines what signal current corresponds to a logic high or logic low on the link. For example, with a logic high current defined as a 1i and the logic low as defined as a 5i current, the threshold is ideally set at a current of 3i. 
   Referring to  FIG. 5 , a preferred embodiment of the current averaging circuitry  200  has a finite response with a settling time of approximately of one microsecond. This allows lower signal frequencies to be received with a stable threshold, while still maintaining the ability to track the calibration point. 
   Referring to  FIG. 6 , a preferred embodiment  200   a  of the input stage of the current averaging circuitry  200  includes sampling circuitry substantially as shown. Metal oxide semiconductor field effect transistors (MOSFETs) M 6   a , M 6   b , M 6   c , M 6   d , M 6   e , M 6   f  are biased by the positive power supply rail VDD and a reference current IREF to provide a bias current IBIAS for a current mirror circuit (discussed in more detail below) and another bias current I 6 . The clock signal current  111   c  is conveyed by transistor M 6   h  which provides a low impedance termination. This produces a sample current ICIN which is provided to the current mirror  200   b . Transistor M 6   h  is biased by amplifier and current mirror circuitry formed by transistors M 6   i , M 6   j  and M 6   k . Transistor M 6   l  provides for disabling this bias circuitry in accordance with a disable control signal DISABLE. (This disable control signal is also used in other areas of the circuitry described herein, and, when activated, disables operation of this averaging circuitry  200 .) 
   Transistor M 6   g  is used for gating the calibration current ICAL provided by the current mirror circuit in accordance with the disable control signal DISABLE. 
   Referring to  FIG. 7 , a preferred embodiment of the current mirror circuit  200   b  of  FIG. 6  is implemented substantially as shown. Disablement control transistors M 7   a , M 7   b  are biased by the positive power supply rail VDD and provide a current scaling ratio of 4:1 so that the magnitude of the output signal  17  is approximately one-fourth the magnitude of the input current ICIN. The bias current IBIAS is used by diode-connected transistor M 7   e  to bias transistors M 7   c , M 7   d , M 7   g  and M 7   h , which, in turn, provide biasing for input current scaling transistors M 7   a  and M 7   b , and a bias voltage V 7  for output current mirror and scaling transistors M 7   i  and M 7   j . These output transistors M 7   i , M 7   j  should be well matched and have relative device characteristics (e.g., channel widths) such that the ratio of the calibration current ICAL to the output current IOUT is 1:12. As a result, the calibration current ICAL has a magnitude scaled in a ratio of 1:48 relative to the magnitude of the input current ICIN. 
   Referring to  FIG. 8 , a preferred embodiment of the output stage  200   c  of the current averaging circuitry  200  ( FIG. 4 ) is implemented as a filtered current mirror circuit substantially as shown. The calibration current ICAL is received by a diode-connected input transistor M 8   a , thereby causing a corresponding voltage V 8   a  to be produced its gate electrode, due to the voltage drop across its source resistor R 8   a . This voltage V 8   a  is filtered by a low pass filter formed by series resistor R 8   b  and shunt capacitor C 8 . The resulting filtered voltage V 8   b  drives the gate electrode of output transistors M 8   c  to produce a corresponding output current IAVG, which is substantially equal to a scaled average of the original clock signal current  111   c  ( FIG. 4 ). The source resistors R 8   a , R 8   c  provide degeneration to help the response to remain substantially linear. 
   Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.