Abstract:
A power supply detection circuit biased by at least two power supply voltages for controlling a signal driver circuit. Upstream and downstream amplifiers, powered by upstream and downstream power supply voltages, respectively, process an original control signal to produce a differential signal via output signal electrodes. Capacitances coupling respective ones of the output signal electrodes to the downstream power supply voltage and the circuit reference potential discharge and charge respective ones of the output signal electrodes in relation to initial receptions of the upstream and downstream power supply voltages and original control signal, following which voltage clamp circuitry maintains such discharged and charged states pending reception of the original control signal in a predetermined state.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to interface circuits for translating voltage levels of signals between circuits biased by unequal power supply voltages, and in particular, to such interface circuits with minimum power consumption during initial application of the power supply voltages. 
   2. Related Art 
   Referring to  FIG. 1 , a power supply detection circuit has become an essential part of the input/output (I/O) interface in digital and mixed signal integrated circuits, or “chips”, having multiple, e.g., two, power supplies. For example, a typical example of such a chip  10  includes, at a minimum, core circuitry  12  biased by a core power supply voltage VDD, and I/O interface circuitry  14  powered by another power supply voltage VDDIO. Typically, the core circuit  12  operates at a lower power supply voltage to minimize power consumption, while the I/O circuit  14  operates, at least in part, at a higher power supply voltage so as to provide appropriate signal levels to and receive higher level signals from circuitry outside the chip  10  which often operates at such higher power supply voltage. Additionally, the core circuit  12  is often primarily, if not exclusively, digital circuitry using insulated gate field effect transistors (IGFETs, often still referred to as metal oxide semiconductor field effect transistors, or MOSFETs) having thin gate oxides, while the I/O circuit  14  uses transistors having thicker gate oxides. Accordingly, the core power supply voltage VDD is less than the I/O power supply voltage VDDIO. 
   Depending upon the functions to be performed by the chip  10 , various signals  15  are conveyed between the core  12  and I/O  14  circuits. Some signals  15   a  may flow exclusively from the core  12  to the I/O  14  circuit while other signals  15   b  may flow exclusively in the opposite direction. Additionally, some signals  15   c  may be bi-directional between the circuits  12 ,  14 . Similarly, the I/O circuit  14  provides outgoing signals  11   a  and receives incoming signals  11   b , and may also exchange bi-directional signals  11   c.    
   During initial application of the power supply voltages VDD, VDDIO, a power supply detection circuit monitors the actual reception of the voltages. In many instances, the I/O power supply voltage VDDIO is applied, or asserted, prior to application of the core power supply voltage VDD. This is done to establish the appropriate interfaces between the I/O circuit  14  and the core circuit  12 . Moreover, there should be virtually no current consumption by the I/O driver output circuit. During this interval, however, the output interfaces, i.e., those electrodes responsible for providing communication between the chip  10  and other external circuits (not shown) must remain disabled. This is often problematic since the output enable signal is typically generated by the core circuit  12  which is powered by the core power supply voltage VDD, which has not yet been applied. 
   Accordingly, it is desirable to provide circuitry for detecting the core power supply voltage VDD while still disabling the output interfaces within the I/O circuit  14 . More importantly, it would be desirable to provide this capability while consuming minimal, if any, power, consistent with the goals of low power consumption achieved during normal circuit operation due to the lower core power supply voltage VDD. 
   SUMMARY OF THE INVENTION 
   In accordance with the presently claimed invention, a power supply detection circuit biased by at least two power supply voltages for controlling a signal driver circuit is provided. Upstream and downstream amplifiers, powered by upstream and downstream power supply voltages, respectively, process an original control signal to produce a differential signal via output signal electrodes. Capacitances coupling respective ones of the output signal electrodes to the downstream power supply voltage and the circuit reference potential discharge and charge respective ones of the output signal electrodes in relation to initial receptions of the upstream and downstream power supply voltages and original control signal, following which voltage clamp circuitry maintains such discharged and charged states pending reception of the original control signal in a predetermined state. 
   In accordance with one embodiment of the presently claimed invention, a power supply detection circuit biased by at least two power supply voltages for controlling a signal driver circuit includes: a circuit reference electrode; a first power supply electrode to convey a first power supply voltage; a second power supply electrode to convey a second power supply voltage; first amplifier circuitry coupled between said circuit reference and first power supply electrodes, and responsive to said first power supply voltage and a first signal having at least asserted and de-asserted signal states by providing a second signal having corresponding signal states; second amplifier circuitry including first and second output signal electrodes, coupled between said circuit reference and second power supply electrodes and to said first amplifier circuitry, and responsive to said second power supply voltage and said second signal by providing, via said first and second output signal electrodes, a differential signal having asserted and de-asserted signal states corresponding to said first asserted and de-asserted signal states; a first capacitance coupled between said circuit reference electrode and said first output signal electrode; a second capacitance coupled between said second power supply electrode and said second output signal electrode; and voltage clamp circuitry coupled to said circuit reference electrode and said first and second output signal electrodes, and responsive to at least a portion of said differential signal by selectively clamping said first output signal electrode at one of a plurality of voltages in relation to said differential signal states. 
   In accordance with another embodiment of the presently claimed invention, a power supply detection circuit biased by at least two power supply voltages for controlling a signal driver circuit includes: circuit reference means for providing a circuit reference voltage; first power supply means for providing a first power supply voltage; second power supply means for providing a second power supply voltage; first amplifier means for receiving and responding to said circuit reference and first power supply voltages and a first signal having at least asserted and de-asserted signal states by providing a second signal having corresponding signal states; second amplifier means for receiving and responding to said circuit reference and second power supply voltages and said second signal by providing, via first and second output signal electrodes, a differential signal having asserted and de-asserted signal states corresponding to said first asserted and de-asserted signal states; first capacitive means for capacitively coupling said circuit reference voltage to said first output signal electrode; second capacitive means for capacitively coupling said second power supply voltage to said second output signal electrode; and voltage clamping means for receiving and responding to said circuit reference voltage and at least a portion of said differential signal by selectively clamping said first output signal electrode at one of a plurality of voltages in relation to said differential signal states. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of a conventional integrated circuit powered by two power supply voltages. 
       FIG. 2  is a block diagram of control circuitry used for disabling a signal interface in response to a control signal generated by a power supply detection circuit in accordance with one embodiment of the presently claimed invention. 
       FIG. 3  is a schematic diagram of a power supply detection circuit in accordance with one embodiment of the presently claimed invention. 
   

   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. 2 , an I/O cell circuit  100  for enabling and disabling an I/O signal pad  111  that conveys a data signal includes level shifting circuits  102   d ,  102   e , a NAND gate  104 , a NOR gate  106 , and output transistors M 41  (P-type) and M 37  (N-type) for pulling up and pulling down, respectively, the output signal pad  111  according to the assertion and de-assertion states of the data signal  101   d . The outgoing data signal  101   d  is level-shifted by its level shifting circuitry  102   d  to produce data signal  103   d . Similarly, the output enable control signal  101   e  is level-shifted by its level shifting circuitry  102   e  to produce a differential enable control signal  103  having positive  103   ea  and negative  103   eb  signal phases. Clamping diodes D 1 , D 2  serve as “antenna” diodes for discharging the input nodes during the fabrication process. 
   During normal operation, both power supplies VDD, VDDIO are asserted. When the output is to be disabled, the enable control signal  101   e  is de-asserted, or low. This causes the differential signal  103  to be in its de-asserted state with its positive phase  103   ea  low and its negative phase  103   eb  high. These signal states disable the NAND  104  and NOR  106  gates, thereby causing the NAND output signal  105  to be high and the NOR output signal  107  to be low. This, in turn, causes both of the output driver transistors M 41 , M 37  to be turned off, thereby disabling the output signal pad  111 . Accordingly, with both output transistors M 41 , M 37  turned off, no current flows from the power supply VDDIO to the circuit reference VSS. 
   When the output signal pad  111  is to be enabled for driving a signal in conformance with the data signal  101   d , the enable control signal  101   e  will be asserted, or high. This causes the differential signal  103  to become asserted, with its positive phase  103   ea  high and its negative phase  103   eb  low. These signal states enable the NAND  104  and NOR  106  gates such that their respective output signals  105 ,  107  form differential signal phases corresponding to the level-shifted data signal  103   d . This, in turn, causes the signal at the output signal pad  111  to switch according to the level-shifted data signal  103   d.    
   As noted, this is the normal mode of operation, i.e., following the initial power up interval when DC power is first applied. However, since the two power supplies VDD, VDDIO are typically supplied through different voltage regulators, there is generally a time delay for the core power supply voltage VDD to be asserted after the I/O power supply voltage VDDIO has been asserted. This causes the states of various signals of the level shifter, both internal and external, to be unknown, since without its power supply voltage VDD many of its internal nodes (discussed in more detail below for  FIG. 3 ) will be at indeterminate or improper voltage levels. Hence, although the intent is to have the output driver disabled during the power up interval, it cannot be guaranteed that the internal nodes of the level shifter  102   e  will be in the proper states to provide the output signal phases  103   ea ,  103   eb  in the states necessary to keep the NAND output signal  105  high and the NOR output signal  107  low. 
   Referring to  FIG. 3 , a power supply detection circuit in accordance with one embodiment of the presently claimed invention includes a level shifter circuit  102   e  with relative sizing of the transistor channels driving the output signal phases  103   ea ,  103   eb , such that the negative signal phase  103   eb  is favored, plus additional components in the forms of accumulation capacitors C 1 , C 2  and a voltage clamp in the form of a pull down transistor MN 5 . 
   The enable control signal  101   e  is inverted by an input inverter circuit implemented with complementary transistors MP 1 , MN 1 , with the resulting signal  201  further inverted by another inverter circuit, also implemented with complementary transistors MP 2 , MN 2 . Both inverter circuits are powered by the core power supply voltage VDD. The input  201  and output  203  signals of this second inverter circuit MP 2 , MN 2  form a differential signal driving a differential output amplifier circuit implemented with two cross-coupled circuit branches containing complementary transistors MP 3 , MN 3 , MP 4 , MN 4 . This differential amplifier circuit is powered by the I/O power supply voltage VDDIO, and produces the differential output signal  103 . 
   An accumulation capacitor C 1  capacitively couples the electrode for the power supply voltage VDDIO to the electrode for the negative output signal phase  103   eb . Similarly, another accumulation capacitor C 2  capacitively couples the electrode for the circuit reference potential VSS to the electrode for the positive output signal phase  103   ea . Transistor MN 5  provides voltage clamping at the electrode for the positive output signal phase  103   ea  in response to the voltage at the electrode for the negative output signal phase  103   eb , which drives its gate electrode. 
   Operation of this level shifting circuit  102   e  occurs in two phases. The first phase is during the initial application of power, during which the I/O power supply VDDIO is active but the core power supply voltage VDD is not yet active. This makes the output amplifier MP 3 , MN 3 , MP 4 , MN 4  active, while the input amplifiers MP 1 , MN 1 , MP 2 , MN 2  are not yet active, thereby causing the inverted signals  201 ,  203  to have indeterminate signal states. The initial application of the I/O power supply voltage VDDIO causes transient current to flow through coupling capacitor C 1 , thereby charging the gate electrode of the voltage clamp transistor MN 5 . Similarly, coupling capacitor C 2  discharges the drain electrode of transistor MN 5 . Collectively, these actions cause transistor MN 5  to turn on, thereby actively pulling down the electrode for the positive output signal phase  103   ea , i.e., clamping its voltage down at approximately the circuit reference potential VSS. As a result, the differential output signal  103  is in its de-asserted state, with its positive  103   ea  and negative  103   eb  output signal phases in their low and high states, respectively. Hence, as discussed above, the output signal pad  111  is disabled with both output driver transistors M 41 , M 37  turned off ( FIG. 2 ). 
   Subsequently, during power up, immediately following the initial ramping of the I/O power supply voltage VDDIO and until the time that the core power supply voltage VDD is asserted, the charges that are accumulated from the parasitic body diodes of transistor MP 3 , (0.5 um channel pulling up the positive output signal phase  103   ea ) and transistors MN 3  and MN 5  (2 um and 1 um channels together pulling down the positive output signal phase  103   ea ), as compared to the charges that are accumulated from the parasitic body diodes of MP 4  (0.6 um pulling up the negative output signal phase  103   eb ) and transistor MN 4  (1.8 um pulling down the negative output signal phase  103   eb ), results in more leakage to the circuit reference VSS for the node providing the positive output signal phase  103   ea  than for the node providing the negative output signal phase  103   eb . Hence, the initial states of the positive  103   ea  and negative  103   eb  output signal phases as established by transistor MN 5  remain intact. Accordingly, the driver transistors M 41 , M 37  for the output signal pad  111  remain disabled. 
   Based upon the foregoing, it can be seen that a power supply detection circuit in accordance with the presently claimed invention advantageously disables the output signal driver during and following initial application of the power supply voltages in such a manner as to minimize power consumption, and even virtually eliminate power consumption for the output driver circuit itself. Particularly when many I/O signal pads are used, such as on a typical integrated circuit having a high pin count, the resulting power savings can be considerable. 
   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.