Abstract:
An integrated circuit module is designed with bypass switches in critical places to route signals around specific circuit blocks, e.g. an automatic gain control (AGC) system and an anti-aliasing filter. If there had been significant problems with either block, it can be bypassed and tests of the remaining circuits are possible. This allows all circuits in the module to be tested in the initial pass, reducing the risk of needing a third pass after the known problems were fixed in the second pass. Additionally, the bypass circuits are useful at module production test and for diagnostics in the final product.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to improved means for testing integrated circuits and in particular to test circuits for integrated circuit chips. Still more particularly, the present invention relates to an improved means for testing integrated circuits by bypassing failed elements. 
     2. Description of the Related Art 
     In developing analog modules involving complex blocks of circuitry there is significant risk that a design defect will cause a block to be inoperative. If so, the initial fabrication pass of the module may be totally useless, adversely affecting the development schedule of the products depending on the module. 
     Even if the module is tested, often these tests will not show which of the component circuits of the module is defective, or if more than one component is defective. Because the module components are often connected in series, if one component of the module is defective, it is impossible to test any components downstream of the known defective component. 
     This makes the process of testing the module extremely difficult. While it may be possible to repair a defective circuit component of the module, until it is repaired, it is impossible to even test the components downstream of the defective module. Since this is the case, it is difficult to determine if the module is even worth repairing until after too much time and expense has already been invested. It would therefore be desirable to have a means to bypass a defective component in order to test other components of the module. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide improved means for testing integrated circuits. 
     It is another object of the present invention to provide an improved means to test circuits for integrated circuit chips. 
     It is yet another object of the present invention to provide an improved means for testing integrated circuits by bypassing failed elements. 
     The foregoing objects are achieved as is now described. An integrated circuit module is designed with bypass switches in critical places to route signals around specific circuit blocks, e.g. an automatic gain control (AGC) system and an anti-aliasing filter. If there had been significant problems with either block, it can be bypassed and tests of the remaining circuits are possible. This allows all circuits in the module to be tested in the initial pass, reducing the risk of needing a third pass after the known problems were fixed in the second pass. Additionally, the bypass circuits are useful at module production test and for diagnostics in the final product. Further, by allowing specific stages of a module to be bypassed, the module can be easily customized for different applications. 
     The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  depicts an automatic gain control stage with a bypass circuit in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is an anti-aliasing stage with a bypass circuit in accordance with a preferred embodiment of the present invention; and 
         FIG. 3  depicts a flowchart of a process in accordance with a preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures, and in particular with reference to  FIG. 1 , a circuit is disclosed for selectively bypassing a circuit block in an integrated circuit module. While the innovative circuits are described below with reference to an analog front end (AFE) module and automatic gain control (AGC) and anti-aliasing filter (AAF) circuits, those of skill in the art will recognize that the bypass circuit described below will work very well in any number of circuits. 
     As shown in  FIG. 1 , the 2×1 ANALOG MUX circuit  140  in the AGC system  100 , which comprises blocks  110 ,  120 , and  130 , allows either the input to the AGC, at pins  115 , or the output of the AGC, at pins  145 , to be sent to the anti-aliasing filter (shown in FIG.  2 ). By sending the input of the AGC system through the bypass of MUX  140  to the anti-aliasing filter, the AGC system is disabled and has no effect on the signal output of the AFE. The 2×1 ANALOG MUX is controlled at the SELECT input by a register bit set by the serial interface in the AFE module. 
     The AGC module includes, for the sake of illustration, VGA_BIAS block  130 , ATT. block  110 , and VGA_MAIN block  120 . ATT. block  110  provides an attenuation of the differential signal to the input of the VGA_MAIN block  120 . Of course, the MUX bypass circuit can be used, as shown in this diagram, to bypass any circuit by connecting both the input and output of the circuit to be bypassed to the MUX. 
     With reference now to  FIG. 2 , a similar 2×1 ANALOG MUX  240  is included in the anti-aliasing filter stage  200  and allows the anti-aliasing filter block  220  to be bypassed. This MUX is also controlled, at its SELECT input, by a serial interface register bit. Here, the MUX  240  has inputs connected to both the VGA input of anti-aliasing filter block  220  and the output of anti-aliasing filter block  220 . According to the signal received at the SELECT input to MUX  240 , one of these signal sets will be passed to SGS block  250 . Also shown here is reference current generator AA_IREF  210 , connected to anti-aliasing circuit block  220 . 
     The 2×1 ANALOG MUX circuit  140 / 240  is, in the preferred embodiment, a very simple broadband two input, one output amplifier, with a gain of 2. Since the MUX is such a relatively simple device, the possibility of problems in the MUX circuit are much less than the far more complicated AGC system or anti-aliasing filter. 
     For early module testing to verify the performance of the individual blocks it is very useful to reduce the amount of extraneous circuitry. According to the preferred embodiment, bypassing the AGC allows the frequency response of the anti-aliasing filter to be studied more easily and bypassing the anti-aliasing filter allows the AGC response to be tested without being affected by the anti-aliasing filter. Should a problem develop with either the filter or AGC, it is still possible to make measurements on the other block. The ability to study each block in isolation increases the likelihood that problems can be quickly and correctly identified. 
     With reference now to  FIG. 3 , a flowchart of a process in a accordance with the preferred embodiment is shown. Here, the process starts when a signal is passed to a chip (step  300 ). The mode of operation is determined according to the MUX select input, as described above (step  310 ). If the module is operating in its normal mode, the signal is passed to a first stage, which can be any given circuit stage, at a common connection with a first input of the MUX (step  320 ). Next, the first stage processes the signal according to the function of the stage (step  330 ). After processing, the signal is output from the first stage and sent to the next stage, which again can be any given circuit stage, or can be the output of the module (step  340 ). The process then ends (step  380 ). 
     If the module is operating in its “bypass” mode, the signal is passed to the first stage, as in the normal mode, at a common connection with a first input of the MUX (step  350 ). Next, the MUX accepts the signal and bypasses the first stage (step  360 ). The signal is output from the MUX and sent directly to the next stage, which again can be any given circuit stage, or can be the output of the module (step  370 ). The process then ends (step  380 ). 
     The bypass circuits of the preferred embodiment are also useful for production module testing. They allow the outputs from the AGC system to be directly observed and inputs to the anti-aliasing filter to be made directly, without needing extra module pins. 
     A bypass circuit according to the preferred embodiment also provide an easy way to customize chip operation. For example, in the analog front-end module described above, there are applications where only the anti-aliasing filter is required. The AGC bypass allows the configuration of the AFE into a filter module to replace a potential end-of-life filter in an existing product. 
     The penalty for the advantages listed above is the inclusion of extra circuitry in the module. However, being a very simple circuit the MUX does not require a large amount of chip area and was deemed to be a reasonable trade-off for the advantages gained. 
     In conventional systems, AGC stages such as that described above are disabled by forcing a fixed voltage onto the AGC gain control line. This approach does not help in early first pass module tests as it assumes the AGC system is working correctly, which may not be the case. Further, this approach does not allow a defective AGC to be bypassed in order to use the preamplifier and anti-aliasing portions of the module for continued product development. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. In particular, while the specific examples above are drawn to an AFE module with automatic gain-control and anti-aliasing filter stages, the innovative bypass circuit can be used in any number of analog or digital circuits where specific circuit blocks might need to be bypassed.