Abstract:
An emulation unit/target processor interface apparatus senses the target processor I/O voltages using filters to reduce the noise level and provides the rest of the interface apparatus with a target reference voltage level. The reference voltage is used to create threshold voltages, termination voltages and drive levels appropriate to provide an interface with the target processor. Power loss in the target processor is also detected so that drive signals can be removed from the target processor to avoid damaging the target processor and to prevent the target processor from being energized by the emulation unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to digital processing units and more particularly, to the test and debug of a target processor by an emulation unit.  
         [0003]     2. Background of the Invention  
         [0004]     In the past, testing and debugging digital signal processors was performed using interface electronics with a fixed voltage capability. Older test and debug units such as emulation units were designed to work only with 5 volt digital signal target processors. When the digital signal processor under test operates with a different supply voltage, the user has to provide interface logic apparatus to translate between the older style emulation unit signal levels and the signal levels of the processor under test. Emulation units soon started using 3.3 volt logic apparatus with a tolerance of 5 volts which reduced the effort in providing interface apparatus for the digital signal processor.  
         [0005]     Advanced emulators are designed to operate over a wide range of supply voltages, typically between 0.5 volts and 5 volts. To determine the operating voltage of the emulation unit, a sense pin is provided to detect the target processor I/O voltage and to scale the emulation unit drive signals and set the logic threshold voltages.  
         [0006]     A need has therefore been felt for apparatus and an associated method having the feature of providing improved test and debug capabilities. It is a further feature of the apparatus and associated method to provide an emulation unit that is able to sense the voltage of the target processor and adjust the output voltage levels of an emulation unit. It is yet another feature of the apparatus and associated method to create a threshold voltage for received signals that is based on the target I/O voltage level. It is a still further feature of the apparatus and associated method to provide an emulation unit that can detect the loss of power by the target processor. It is still a further feature of the apparatus and associated method to provide a clamping voltage to protect the emulation unit against electrostatic discharge. It would be a more particular feature of the apparatus and associated method to limit voltage excursions by signals from the target processor.  
       SUMMARY OF THE INVENTION  
       [0007]     The aforementioned and other features are accomplished, according to the present invention, by providing an interface circuit associated with the emulation unit to sense the target I/O voltage, to limit the output voltage of the emulation unit to a maximum value, to provide a suitable threshold voltage and a clamping voltage, and to detect the loss of target power.  
         [0008]     Other features and advantages of present invention will be more clearly understood upon reading of the following description and the accompanying drawings and the claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a block diagram of the interface circuit invention illustrating the relationship of the various components of the present invention.  
         [0010]      FIG. 2  is a schematic diagram of the voltage sensing device for sensing the target voltage according to the present invention.  
         [0011]      FIG. 3  is a schematic diagram of the clamp voltage generator according to the present invention.  
         [0012]      FIG. 4  is a schematic diagram of the target voltage limiting circuit according to the present invention.  
         [0013]      FIG. 5  is a schematic diagram of the threshold generation circuit according to the present invention.  
         [0014]      FIG. 6  is a schematic diagram of the power loss detection circuit according to the present invention.  
         [0015]      FIG. 7  is a schematic diagram of the input comparator circuit according to the present invention.  
         [0016]      FIG. 8  is diagram of the output switches according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0000]     1. Detailed Description of the Figures  
         [0017]     Referring to  FIG. 1 , the block diagram of the interface circuit between the emulation unit  10  and the target processor  12 , according to the present invention, is shown. The output terminals of the emulation unit  12  are coupled to an input terminal of an FET output switch  80  and to the control terminals of the FET switch  80 . The output terminals of the FET switch are coupled to the target processor  12 , through ESD protection diode  114  connected to ground potential, and through ESD protection diode  113  connected to an output terminal of clamp generator  30 . The target processor applies the I/O voltage to an input terminal of the target sensing device  20 . The output voltage of the target sensing device is coupled to the clamp generator  30 , to the limiter  40 , to the power loss detection  60 . The power loss detection  60  applies an output signal to the emulation unit  10 . The limiter  40  applies an output voltage to the threshold generation unit  50  and output signal to the power terminal of FET output switch  80 . The target processor output terminals are coupled to the positive terminal of comparator  70 , are coupled through diode  116  to ground potential, and are coupled through diode  115  to the output terminal of clamp generator  30 . An output terminal of the threshold generation unit is coupled to a negative input terminal of comparator  70 .  
         [0018]     Referring to  FIG. 2 , the schematic diagram of the target voltage sense circuit  20  is shown. The I/O voltage from the target processor  12  is applied through ESD protection diode  21  to ground potential, through ESD protection diode  22  to the  5  volt supply voltage, through resistor  23  to ground potential, and to a first terminal of resistor  24 . A second terminal of resistor  24  is coupled to the positive terminal of operational amplifier  26  and through capacitor  25  to the ground potential. The power terminal of operational amplifier  26  is coupled to a 5 volt supply voltage. The output terminal of operational amplifier  26  is coupled to the negative input terminal of operation amplifier  26 , and to a first terminal of resistor  27 . The second terminal of resistor  27  is coupled through capacitor  28  to ground potential and provides the TREF signal.  
         [0019]     Referring to  FIG. 3 , a schematic diagram of the clamp voltage generation circuit  30  is shown. The TREF signal is applied to the positive terminal of an operational amplifier  31 . The negative input terminal of the operational amplifier  31  is coupled to through resistor  32  to the ground potential and through resistor  33  to the output terminal of operational amplifier  31 .  
         [0020]     The power terminal of operational amplifier  31  is coupled to a 5 volt supply voltage. The output terminal of operational amplifier  31  is coupled through resistor  34  to the terminal providing the TVS CLAMP signal and to a first terminal of capacitor  35 , the second terminal of capacitor  35  being coupled to ground potential.  
         [0021]     Referring to  FIG. 4 , a schematic diagram of the target voltage limiter circuit  40 , according to the present invention, is shown. The TREF signal is applied to an input terminal of output switch  41  while the ground potential is applied to the control terminal of output switch  41 . A 5 volt supply voltage is coupled through resistor  43  to the substrates of output switches  41 ,  42 ,  44  and  45 , and to the output terminals of output switches  41  and  42 . This same signal is output as TREF2DRIVER. A 5 volt supply is coupled to the input terminals of output switches  44  and  45 . The ground potential is applied to control terminals of output switches  42 ,  44  and  45  are coupled to the ground potential. The output terminal of output switch  44  provides the MAX THRESH signal while the output terminal of output switch  45  is coupled through resistor  46  to ground potential, through capacitor  47  to ground potential, and supplies the TVR_LIMIT signal.  
         [0022]     Referring to  FIG. 5 , a schematic diagram of the threshold generator circuit  50 , according to the present invention is shown. The MAX THRESH signal is applied through resistor  51  to grounded capacitor  52 , to grounded resistor  53 , to the positive input terminal of operational amplifier  54 , and to the terminal providing the TVR THRESH signal. The power terminal of operational amplifier  54  is coupled to a  5  volt supply. The output terminal of the operational amplifier  54  is coupled to the negative input terminal of operational amplifier  54 , and is coupled through resistor  55  to grounded capacitor  56  and to the TVR TERM signal terminal.  
         [0023]     Referring to  FIG. 6 , a schematic diagram of the target power loss detection unit  60 , according to the present invention, is shown. The TREF signal is applied to a positive input terminal of comparator  67 , to a positive input terminal of comparator  66 , and through resistor  61  to a terminal of grounded capacitor  62  and the positive input terminal of operational amplifier  63 . The power terminals of operational amplifier  63  and comparators  66 , and  67  are coupled to a 5 volt supply. The output terminal of operational amplifier  63  is coupled to a negative input terminal of operational amplifier comparator  63  and through resistor  64  to a negative input terminal of comparator  66  and to grounded resistor  65 . The negative input terminal of comparator  67  is coupled through capacitor  601  to ground potential, through resistor  69  to the ground potential, and through resistor  68  to a 5 volt power supply. The output terminals of comparators  66  and  67  are coupled together to provide a TV_GOOD signals and are coupled through resistor  602  to a 5 volt supply.  
         [0024]     Referring to  FIG. 7 , the configuration of the input comparator  70  is shown. The input comparator  70  has the TVR_THRESH signal is applied to the positive input terminal of comparator  70 , while the INPUT signal is applied to the negative input terminal of comparator  70 . The power terminal of comparator  70  is coupled to a 5 volt supply.  
         [0025]     Referring to  FIG. 8 , the configuration of the output switch  80  is shown. An input signal (e.g., from the emulation unit signals) is applied to an input terminal of FET switch  80  while the output terminal FET switch  80  provides the output signal (e.g., to the target processor. The control terminal of the FET switch has the control signal applied thereto. The TREFZDRIVER signal is applied to the power terminal of the FET switch. The FET switch limits the output voltage to TREF2DRIVER-1V.  
         [0000]     Operation of the Preferred Embodiment  
         [0026]     Referring once again to interface apparatus of  FIG. 1 , the inputs to the apparatus are protected by electrostatic discharge (ESD) clamp diodes. The target I/O voltage is sensed, filtered and fed to the remaining analog circuitry. The clamp generator generates the ESD clamping voltage for input and output signals. The limiter sets the maximum voltage into the threshold circuit. The threshold generator creates threshold and termination voltages. The power loss detection unit senses when the target voltage is off. As will be clear, the signal path between the target processor and the emulation unit will actually be comprised of a multiplicity of paths, i.e., a multiplicity of output switches will be used.  
         [0027]     Referring once again to the target voltage sensing circuit shown in  FIG. 2 , the input signal to this circuit has the ESD protection diodes clamped to ground and to the op amp power supply. The input circuit has a high value resistor  23  coupled to ground so that when disconnected, the output TREF signal will go to zero volts. The input signal is applied to the low pass-filter of resistor  24  and capacitor  25 , buffered by op amp  26 , and then applied to low pass of resistor  27  and capacitor  28 . The output TREF signal is a buffered and filtered equivalent of the target voltage. The time constants are chosen to reduce noise, but allow a reasonable response time when the voltage is turned on.  
         [0028]     Referring once again to the clamp voltage generation circuit shown in  FIG. 3 , this circuit multiplies the target reference voltage, TREF, by 1.33 to increase the clamping voltage for the ESD diodes on all of the other input and output signals. The clamping voltage is applied to a low pass filter and is decoupled.  
         [0029]     Referring once again to the target voltage limiter shown in  FIG. 4 , this circuit routes TREF through FET switch  41 . It also routes the 3.30 volt power supply through FET switch  42 . These two switches are in the same package and share the same substrate. The output voltage is limited to the lesser of the two input signals plus approximately 1 volt. This voltage, TREF2DRIVER, is used to power FET switches  44  and  45 , as well as the FET output switch  80 . The outputs of those switches are limited to TREF2DRIVER-1 volt. The output from FET switch  45  is low pass filtered by resistor  46  and capacitor  47  to become TUR_LIMIT. The TUR_Limit voltage can be used for pulling up target signals if required.  
         [0030]     Referring once again to the threshold generator as shown in  FIG. 5 , this circuit takes the MAX_THRESH voltage and divides the voltage using two resistors  51  and  53  and a low pass filter including capacitor  52  to provide the TVR THRESHOLD signal. For all target voltages 3.3 volts and less, the threshold is set to 50% of the target voltage, the standard CMOS threshold. For target voltages greater than 3.3 volts, typically 5 volts, the threshold is set to 1.65 volts, which is close to the nominal 1.4 volt TTL threshold voltage level. The TVR_THRESHOLD is buffered, passed through a low pass filter and is decoupled using resistor  55  and capacitor  56  to generate the TERMINATION voltage, TVR_TERM. The terminal voltage can be used to terminate signals from target processor to the Target I/O voltage/2 to minimize the DC current loading.  
         [0031]     Referring to the power loss detection circuit shown in  FIG. 6 , this circuit has two methods for detecting power loss. The first method is by comparing the target reference voltage to a fixed threshold of 0.35 volts. The threshold is provided by resistors  68  and  69 , capacitor  601  providing low pass filtering. The second method of power loss detection is to detect a drop in power from the existing level. This detection is accomplished by filtering the TREF voltage with a low pass filter of resistor  61  and capacitor  62  with a very large time constant. This signal is buffered and divided by resistors  64  and  65  to 75% of the target voltage. The comparator detects drops in the target voltage exceeding 25% of nominal. When the target processor loses power, the emulation unit is notified by an interrupt signal in order for the software to make appropriate adjustments.  
         [0032]     Referring to the input comparator circuit shown in  FIG. 7 , all of the active signals from the target are routed to input comparators to sense whether they are high or low. The threshold levels are derived from the target I/O voltage.  
         [0033]     Referring to the output switch logic as shown in  FIG. 8 , the input signals are applied to the target processor through FET switches in order to provide voltage level adjustment. The FET switches are controlled by control signals applied to the FET control terminal. The FET transistors can be used to stop the exchange of data signals between the emulation unit and the target processor. The FET switches are used in this implementation for several reasons, these switches have virtually no propagation delay, consume virtually no power, and the output voltage is constrained to no greater than the supply voltage minus the gate to drain voltage. Because the TREF2DRIVER voltage is applied to the power supply input of the FET switches, the output voltage can not exceed the target I/O voltage.  
         [0034]     As will be clear, the interface can be implemented using analog-to-digital converter to sense the target I/O voltage. A digital-to analog converter or programmable power supply can be programmed to supply the output voltage levels and threshold levels.  
         [0035]     While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, the scope of the invention being defined by the following claims.