Abstract:
An apparatus and a method for filtering glitches in a data communications controller receiving asynchronous input data signals varying between two signal levels representing two bit values and having a predetermined input bit period, and sending output data signals corresponding to the input data signals. Glitches are detected in the input data signals by detecting reversals of signal level having a duration less than the input bit period. A glitch time value corresponding to the glitch duration is determined, and then a sampling clock rate is set at a rate determined from the glitch time value. The input data signals are sampled at the sampling clock rate to generate a sequence of input data samples. A voting number of input data samples are monitored and an output signal is provided, representing the value of a majority of the sequential input data samples.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates to asynchronous data communication interface controllers, and more particularly relates to a method and apparatus for detecting and filtering glitches in asynchronous digital signals received by such controllers. 
   BACKGROUND OF THE INVENTION 
   In asynchronous transmission, data is coded into a series of pulses, including a start bit, and including a stop bit or a guard band. The start bit is sent by a sending unit to inform a receiving unit that a character is to be sent. The character is then sent, followed by the stop bit or guard band, designating that the transfer of that character is complete. Modes of asynchronous communication are frequently defined in standards that are established by standards setting bodies, such as the American National Standards Institute (ANSI), the International Telecommunications Union (ITU) and the International Organization for Standardization (ISO). 
   Asynchronous communication is frequently used to transfer data to and from plug-in units, such as modems, memory cards, and the like, that are plugged into host units, such as digital cameras, personal computers, and the like. An interface controller in the host unit manages the asynchronous data communication between the plug-in unit and the host unit. An exemplary asynchronous communication standard is the ISO7816 standard, adopted by the ISO. Plug-in units communicating with an interface controller in a manner that complies with the ISO7816 standard are frequently referred to as Smart Cards, or Integrated Circuit Cards. 
     FIG. 1  shows a typical Smart Card interface arrangement. A Smart Card unit  10  is electrically connected to a host unit  14  by way of an interface controller  12  in the host unit  14  that manages the transfer of data between the Smart Card unit  10  and the host unit  14 . The transfer of data between the host unit  14  and the interface controller  12  is shown by way of example in  FIG. 1  as being by way of a PCI bus  16  in the host unit  14 . Numerous other means for communicating data between the interface controller and the host unit are possible, including EISA bus, universal serial bus (USB), and so on. The Smart Card connection to the host unit  14  is by way of a two-way serial line  11 , which is split in the host unit  14  into a transmitting line  26  and a receiving line  30 , using well known techniques. The rate of data exchange between the Smart Card unit  10  and the interface controller  12  is controlled by an interface clock, which can be at one of five different clock frequencies, 4 MHz, 6 MHz, 8 MHz, 12 MHz and 20 MHz. 
   The interface unit  12  includes a Smart Card interface subunit  18  and a PCI interface subunit  20 . The Smart Card interface subunit  18  includes a Smart Card block  22  and a parity checker block  24 . The Smart Card block  22  receives the signals transmitted on line  26  by the Smart Card unit  10 , recovers the data in those signals, and then sends that data on line  27  to the PCI interface subunit where it is placed on the PCI bus  16  according to the well-known PCI standard protocol, for transmission to other parts (not shown) of the host unit  14 . The parity checker block  24  monitors the data on line  26  to detect whether a parity error exists in a character of data. If such a parity error is so detected, the parity checker block  24  asserts a signal on line  28 , which causes a gate  29  to block the Smart Card block  22  from receiving the error, and sends a signal on line  30  to the Smart Card unit  10  notifying it of the error, which prompts an attempted re-send of the affected character from the Smart Card unit  10 . 
   When parity errors exist, it is frequently because of errors made in the coding of the data in the Smart Card unit  10 . However, sometimes parity errors are detected in the parity checker block  24  because of the occurrence of glitches occurring on the signals sent from the Smart Card unit  10 . This problem is not limited to asynchronous data communicated according to the ISO7816 standard, but is a problem with respect to asynchronous data communication generally. Further, it is not limited to systems in which errors are determined by parity checking; rather, it applies to such systems in which errors in the data can occur because of glitches. It would be desirable to be able to be able to successfully detect data in asynchronous communication, even if glitches are present. It would also be desirable to avoid the time expenditure involved in error detecting and signaling, and re-send of data in systems wherein an error detecting and data re-send protocol is provided. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an apparatus and a method are provided for filtering glitches in a data communications controller receiving asynchronous input data signals varying between two signal levels representing two bit values and having a predetermined input bit period, and sending output data signals corresponding to the input data signals. The glitches comprise reversals of signal level, having a glitch duration less than the predetermined bit period, on the input data signals. Glitches are detected in the input data signals by detecting reversals of signal level having a predetermined duration less than the predetermined input bit period. A glitch time value corresponding to the glitch duration is determined, and then a sampling clock rate is determined from the glitch time value. The input data signals are sampled at the sampling clock rate to generate a sequence of input data samples. A predetermined voting number of input data samples are monitored and an output signal is provided, representing the value of a majority of the sequential input data samples. Finally, a voting number of subsequent input data samples are monitored and output signals are provided, representing the value of a majority of those subsequent input data samples. 
   The invention may be utilized in systems with error detection, such as parity check, but is not limited to use in such systems. However, when used in systems with error detection, the inventive glitch filtering may be turned on when a programmable number of errors is detected, but maintained in an off state otherwise. 
   These and other features of the invention will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art Smart Card-to-PCI bus interface arrangement. 
       FIG. 2  is a signal timing diagram for a signal complying with the ISO7816 standard. 
       FIG. 3  is a signal timing diagram like that of  FIG. 2 , and also showing the presence of glitches. 
       FIG. 4  is a block diagram of a Smart Card interface unit incorporating a preferred embedment of the present invention. 
       FIG. 5  is a block diagram of the glitch filter of  FIG. 4 . 
       FIG. 6  is a block diagram of the glitch detector and adjustment block of  FIG. 4 . 
       FIG. 7  is a block diagram of a glitch filter like that of  FIG. 5 , but also including an optional hysteresis function. 
       FIG. 8  is a block diagram of the glitch detector of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The numerous innovative teachings of the present invention will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. For example, while the embodiment of the invention described herein is with respect to an ISO7816 standard data interface, it is applicable to any asynchronous data interface. In general, statements made in the specification of the present application do not necessarily delimit the invention, as set forth in different aspects in the various claims appended hereto. Moreover, some statements may apply to some inventive aspects, but not to others. 
     FIG. 2  is a timing diagram for a signal that is compliant with the ISO7816 standard, with time represented on the horizontal axis. A complete character is shown in the figure, starting at time  0  and ending at time tn. The character is composed of bits, each bit being of a defined duration referred to in the ISO7816 standard as an Elementary Time Unit, or etu. At any given time the signal is at either an A state or a Z state. At the beginning of a character, between time  0  and time t 1 , a start bit having a value of A is sent, followed by eight data bits between time t 1  and time t 9 . The eight data bits are followed, between time t 9  and t 10 , by a parity bit. The parity bit is followed by a guard time having a variable number of etus duration, between time t 10  and the end of the character, time tn. Further details on this and other aspects of the ISO7816 standard can be found in the ISO7816 standard publication, which may be obtained from the International Organization for Standardization at 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20, Switzerland. A basic overview of the ISO7816 standard may be obtained from the Smart Card Industry Association at 191 Clarksville Road, Princeton Junction, N.J. 08550. 
     FIG. 3  is a signal timing diagram like that of  FIG. 2 , but also showing two glitches, at  32  and  34 , that appear in the first and sixth data bits, respectively. These glitches take the form of a signal superimposed on the transmitted signal, having a voltage opposite from that of the data bit. Thus, for example, if the first data bit in  FIG. 3  has a value of Z, the glitch would be a negative-going pulse that could cause a detector to see a value of A during the glitch. Likewise, if such first data bit has a value of A, the glitch would be a positive-going pulse that could cause a detector to see a value of Z during the glitch. 
   Glitches are typically less than 0.2 bits in duration. However, if the glitch exists at the time the Smart Card block  22  attempts to resolve the value of the bit, an erroneous detection of the bit value occurs. Generally, there is seldom more than one glitch in a given character, and so a parity error is usually generated by the glitch. Worse, however, if two glitches occur in a given character, no parity error is generated, since the two glitches cancel for the purposes of parity, but corrupted data is transmitted to the host unit. 
     FIG. 4  is a block diagram of a Smart Card interface subunit  40  incorporating a programmable glitch filter  41  in accordance with a preferred embedment of the present invention, a gate  29 , a Smart Card block  22 , a parity checker block  24 , and a control logic block  42 . The gate  29 , the Smart Card block  22  and the parity checker block  24  may be the same as the same numbered blocks of  FIG. 1 . The programmable glitch filter  41  and control logic block  42  may be implemented in hardware, firmware or software, as desired. In the preferred embodiment described herein the control logic block  42  is implemented as a microcontroller, such as an 8052 microcontroller, controlled by firmware, while the programmable glitch filter  41  is implemented in hardware. 
   The programmable glitch filter (PGF)  41  is comprised of a glitch detector and adjustment block (GDA)  43  and a glitch filter  44 , interconnected, and connected to logic block  42 , Smart Card block  22  and parity checker block  24 , as shown. The PGF  41  is generally initialized in an Off state, although it may be programmed to be initialized in an On state. In the Off state, the glitch filter  44  receives signals from a Smart Card unit, provided to the PGF  41  on line  26 , and samples those signals at a reference clock rate, for example a system clock rate at 48 MHz, to recover the Smart Card signals. The recovered Smart Card signals are sent to the Smart Card block  22  and to the parity checker block  24  on line  45 . As in the prior art, if the parity checker block  24  detects an error, it sends a blocking signal on line  28  to the gate  29 , and sends a signal on line  30  to the Smart Card unit. The parity checker block  24  also sends a signal on line  46  to control logic  42 , indicating that a parity error has been detected. This can be the same signal as that sent on line  30 , inverted if required. 
   Briefly, after the control logic  42  detects a programmable number of parity errors, it sends a signal on line  47  to the GDA  43 , which enables the finding of glitches. The GDA  43  is programmed to detect glitches having a width less than 0.2 etu. When the GDA  43  detects a glitch, it provides a signal to the control logic on line  48 . Once the control logic  42  receives this signal, it causes a newly calculated glitch width to be stored, and turns on the glitch filter  44  by asserting an enable bit on line  68 . The newly calculated glitch width is provided to the glitch filter  44  on line  59 , where it is used to set the sample clock at a frequency that optimizes glitch filtering, described in detail below. 
   In this way, glitches are detected, and the width of the latest detected glitch is used to set the sample clock, thus providing advantageous adaptive glitch detection and filtering, maximizing the detecting and filtering of glitches in an environment where glitch widths may be varying over time. The operation of PGF  41  is described in detail below. 
     FIG. 5  shows a detailed diagram of the glitch filter  44 . A three-bit shift register is provided, comprising, for example, three latches  50 ,  51 ,  52 , connected in series, as shown. Three two-input multiplexers  53 ,  54 ,  55 , are also provided in series with the multiplexers  53 ,  54 ,  55 , for a data hold function described in detail below. The data from a Smart Card on line  26  is provided to one input of multiplexer (MUX)  53 . The output of MUX  53  is provided to the data input of latch  50 . The output of latch  50  is provided to one input of MUX  54 , as well as to the other input of MUX  53 , and to a first input of a three-input voting unit  36 . Likewise, the output of MUX  54  is provided to the data input of latch  51 , and the output of latch  51  is provided to one input of MUX  55 , as well as to the other input of MUX  54 , and to a second input of three-input voting unit  36 . The output of MUX  55  is provided to the data input of latch  52 , and the output of latch  52  is provided to the other input of MUX  55 , and to a third input of three-input voting unit  36 . 
   The output of a sampling clock generator  56  is provided to the control input of MUXes  53 ,  54  and  55 . Each of latches  50 ,  51  and  52 , of the three-bit shift register is clocked by a reference clock, which in this embodiment is a system clock, CLK, e.g., at 48 MHz. A results output of voting unit  36  is provided to the control input of a MUX  58  on line  57 . The two inputs of MUX  58  are held to a 0 and to a 1, respectively. The output of MUX  58  is provided to line  45 . 
   Note that while the latches  50 ,  51  and  52 , are clocked by the system clock CLK, the MUXes  53 ,  54  and  55 , are strobed by the output of sampling clock generator  56 . Thus, the contents of the latches  50 ,  51  and  52  are controlled by the sampling clock generator  56 , since the contents of each latch is simply circulated, i.e., held, unless a sample clock is asserted. The sampling clock generator  56  receives an input on line  59  from the GDA  43  ( FIG. 4 ), while the voting unit  36  receives an enable signal on line  68  from the GDA  43 . 
   The voting unit  36  is an unclocked block of logic that provides an output that is the result of a two-out-of-three “vote,” i.e., according to the following table: 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Output 
             
             
                 
               Input Values 
               Value 
             
             
                 
                 
             
           
           
             
                 
               0, 0, 0 
               0 
             
             
                 
               1, 0, 0 
               0 
             
             
                 
               0, 1, 0 
               0 
             
             
                 
               0, 0, 1 
               0 
             
             
                 
               1, 1, 0 
               1 
             
             
                 
               0, 1, 1 
               1 
             
             
                 
               1, 0, 1 
               1 
             
             
                 
               1, 1, 1 
               1 
             
             
                 
                 
             
           
        
       
     
   
   In operation, when not enabled for glitch filtering, the sampling clock generator  56  is simply set to the CLK rate. This is controlled by the filter enable signal on line  68 . Thus, when the filter enable signal is not asserted (i.e., signaling disable) the sampling clock generator  56  is forced to output the sampling clock at the CLK rate. In this mode, as the samples are sensed by the voting unit  36 , it controls MUX  58  to provide a 0 or 1, thereby recovering the data from the signals on line  26 . Any but extremely short glitches (&lt;˜50 picoseconds) will result in multiple samples, which will be sensed by the voting unit  36  and thus appear at the output of MUX  58 , i.e, on line  45  in the recovered data. 
   However, when glitch filter  46  is enabled for glitch filtering, the latest glitch width setting is provided from the GDA  43  is provided on line  59 , and stored in the sampling clock generator  56 , and the enable signal is asserted on line  68 . The sampling clock generator  56  provides the sampling clock to the control input of MUXes  53 ,  54  and  55 , at a frequency corresponding to that stored value, as described below. The three latches  50 ,  51 ,  52 , continue to be clocked at the CLK rate. However, as mentioned above, the MUXes  53 ,  54  and  55 , are strobed at the sampling clock rate, and thus shift data samples in sequence through the latches  50 ,  51 ,  52 , at that rate. The frequency of the sampling clock is set to optimize glitch filtering, for example corresponding to approximately the frequency of a square wave signal having a period equal to the width of the detected glitch. As a result, for glitches approximately the width of the detected glitch, or smaller, the data value represented by that glitch will appear at most in only one of the three latches  50 ,  51 ,  52 , while the proper data value will appear in the other two latches. 
   The voting unit  36  outputs on line  57  a results value corresponding to the data value in at least two of the three latches  50 ,  51 ,  52 , as described above. Thus, even if a glitch is propagated through the latches  50 ,  51 ,  52 , the voting unit ignores it and controls MUX  58  to output the proper value. Note that a larger number of latches could be provided, and a majority vote taken on their outputs, but three latches is considered preferred. If a larger number of latches is used, the number is preferably odd, to ensure an unambiguous vote. Also note that the manner in which the sequential samples are stored for the voting process is a matter of design choice and is not limited to latches; it is only necessary to store them in some way so that they can be examined simultaneously for the voting process. 
     FIG. 6  is a block diagram of the GDA block  43  of  FIG. 4 . The GDA block  43  includes a glitch detector  60 , a glitch control register block  61 , a detected glitch width register  62 , and a filtering glitch width setting register  63 . The glitch control register block  61  includes a one-bit glitch finder enable register  64 , a one-bit glitch detected register  65 , a one-bit glitch filter enable register  66 , and a one-bit update glitch width register  67 . The glitch detector  60  receives the samples on line  45  from the glitch filter  44  ( FIG. 5 ). When glitch detector  60  detects a glitch it sets the bit in glitch detected register  65  to a 1, using line  99 , and stores a value corresponding to the width of the detected glitch in detected glitch width register  62 . The value stored in the filtering glitch width setting register  63  is provided on line  59 , for provision to the sampling clock generator  56  in the glitch filter  44 . The glitch finder enable register  64  receives a glitch finder enable signal from control logic  42  ( FIG. 4 ) on line  47 . The glitch detected register  65  provides the bit value stored therein to control logic  42  on line  48 . The glitch filter enable register  66  receives a glitch filter enable signal from control logic  42  on line  49 . The update glitch width register  67  receives an update glitch width signal from control logic  42  on line  69 . 
   In operation, after the control logic  42  detects a programmable number of parity errors, it sends a signal on line  47  to the GDA  43 , setting the bit in the glitch finder enable register  64  to a 1, which enables the finding of glitches by glitch detector  60  in the signals provided on line  45 . The GDA  43  is preferably programmed to detect glitches having a width less than 0.2 etu, although selection of the width is a matter of design choice. When the glitch detector  60  detects a glitch, it sets the bit in the glitch detected register  65  to a 1. At the same time, it calculates the glitch width by counting the number of interface clock periods between the beginning edge and ending edge of the glitch, and stores that calculated value in the detected glitch width register  62 , overwriting any default value previously stored therein. 
   The glitch detector  60  of  FIG. 6  can be any logic that detects pulses and determines the time between a rising and a falling edge of the pulse.  FIG. 8  shows a preferred embodiment of glitch detector  60  in which the rising and falling edges of pulses on line  45  are detected, and a count is made of interface clock periods between them. It includes flip-flops  80 – 87 , counter  88 , inverters  91 – 93 , AND gates  94 – 97 , and OR gate  98 , interconnected as shown. All of the flip-flops  80 – 87  are clocked by CLK, i.e., at 48 MHz, but the counter  88 , under control of the interface clock provided on line  100 , counts at the interface clock rate. It will be recalled that the rate of data exchange between the Smart Card unit  10  and the interface controller  12  is controlled by an interface clock, which can be at one of five different clock frequencies, 4 MHz, 6 MHz, 8 MHz, 12 MHz and 20 MHz. 
   In operation, the output of flip-flop  82  goes to 1 when the glitch detector  60  detects a falling edge, e.g., the start of a negative glitch. On the other hand, the output of flip-flop  83  goes to a 1 when the glitch detector  60  detects a rising edge, e.g., the start of a positive glitch. The output of flip-flop  84  goes to a 1 when the first falling or rising edge is detected. This sets the output of flip-flop  85  to a 1, which starts the counter  88  counting interface clock periods. The output of flip-flop  86  is set to a 1 only when the end of a glitch, i.e., the second edge, is detected. This sets the output of flip-flop  87 , i.e., line  99 , to a 1, which causes the counter  88  to stop counting, and sets the bit in glitch detected register  65  ( FIG. 6 ) to a 1. Counter  88  is 15 bits wide, which is sufficient to catch the glitches it is desired to detect. The value in the counter is the value in the detected glitch width register  62 . Counter  88  may serve as the detected glitch width register  62 , in which case the final count value, when line  99  goes to a 1, is read directly from it, or a separate register may be provided as the detected glitch width register, in which case the final count is loaded into the separate register. If counter  88  counts to its highest count, e.g., 2 15 −1, a signal is asserted on line  101 , which is connected to the reset input of flip-flop  85 , thus resetting flip-flop  85 . 
   The value of the bit in glitch detected register  65  is provided to the control logic  42  on line  48 . Once the control logic  42  reads the value of 1 on line  48  it clears the glitch detected register  65 . At the same time, the control logic  42 , using line  69 . sets the bit in the update glitch width register  67  to a 1, which causes the newly calculated glitch width to be read from glitch detected register  62 , converted into system clock periods and stored in the filtering glitch width setting register  63 . At the same time, the control logic  42  turns on the glitch filter  44  ( FIG. 5 ) by setting the bit in the glitch filter enable register  66  to 1. The value in the glitch filter enable register  66  is provided to the glitch filter  44  on line  68 , as described above. 
   It will be recalled that the value in the filtering glitch width setting register  63  is provided on line  59  to the glitch filter  44 , where it is used to set the sample clock. The sample clock is derived from the system clock, and so the value stored in the filtering glitch width setting register is also in system clock periods in order to simplify the sample clock setting. To optimize glitch filtering, the frequency of the sampling clock is preferably set to the frequency of an oscillating signal having a period equal to the width of the detected glitch, i.e., the value in the filtering glitch width setting register  63 . Basically:
 
 F   s ≦1/( P   sys   ·W   d ),  Equation (1)
 
where F s  is the sampling frequency, P sys  is the period of one cycle of the system clock, and W d  is the value in the filtering glitch width setting register  63 , converted to system clock periods. The “less than or equal to” symbol is used in order to indicate that an optional guard time may be provided in setting the sampling frequency, in order to ensure that the sampling is optimized. Since the embodiment described herein updates the glitch width on an ongoing basis, providing a guard time is not considered necessary. However, a designer may wish to provide such a guard time if, for example, glitch widths could be varying considerably between adjacent glitches.
 
   The guard time may be provided as follows. Assuming the counter  88  counts at the interface clock frequency, this is accomplished by determining a W ds :
 
 W   ds =(COUNT· F   sys ·GUARD)/ F   IC   Equation (2)
 
where W ds  is the glitch width in system clock periods, COUNT is the final count value in counter  88  (i.e., in detected glitch width register  62 ), F sys  is the system clock frequency, GUARD is a factor selected to provide a desired guard time and F IC  is the interface clock frequency. Thus, for example, say the counter  88  has a final count value of 4, the system clock frequency is 48 MHz, the interface clock frequency is 6 MHz, and GUARD has a value of 1, i.e, no extra guard time is selected. Then: 
               W   d     =       ⁢       (     4   ·   48   ·   1     )     /   6                 =       ⁢   32.             
 
Therefore, by Equation (1): 
               F   s     =       ⁢     48   ⁢           ⁢     MHz   /   32                   =       ⁢     1.5   ⁢           ⁢     MHz   .                 
 
   Now, if a guard time were desired, the sampling frequency would be decreased by the factor GUARD. In the above example, say the factor GUARD were selected to be equal to 1.1, then the sampling frequency would change from 1.5 MHz to 1.37 MHz. The resulting extra time in the sampling frequency period would provide the desired guard time. 
   Preferably, an initial default value is provided in the detected glitch width register  62 , of 2 μs. This is because it is believed that most glitches that are present on signals from Smart Cards are less than 2 μs wide. With such an initial default value, the control logic  42  could be programmed in a first mode, for example, to enable the glitch filter  44  by simply setting the bit in the glitch filter enable register  66  to a 1. There would then not be a need to set the bit in the glitch finder enable register  64  and then do the above-described calculations to find the glitch width. However, the control logic  42  could be programmed to switch to a second mode, in which the full operation, including glitch width calculation, all as described above, is followed, if the parity checker  24  continued to report more parity errors. 
   After detection of the first glitch, and the steps described above are taken, the glitch detector  60  continues to monitor the signals on line  45 . Operation is the same as described above, only instead of overwriting an initial default value in the detected glitch width register  62 , the previously calculated value is overwritten. Also, there is no need to re-set the bit in the glitch finder enable register  64 . 
   Advantageously, a hysteresis function may be provided in implementations of the present invention, if desired. By hysteresis function it is meant that the system is adapted to prevent toggling due to glitches at the end of bits. Referring now to  FIG. 7 , an implementation to provide this function is shown. This figure is similar to  FIG. 5 , but has an OR gate  70  added, in a feedback path from the output of the MUX  58 , with the other input being the output of latch  50 , and the output of OR gate  70  being an input to voting unit  36 . 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.