Abstract:
A tool for facilitating automatic test pin assignment for a programmable platform device including a process for collecting information related to the programmable platform device, a process for automatically initializing a test pin assignment for the programmable platform device, a process configured to receive user specifications for IOs and a process for performing dynamic test pin reassignment in response to the user specifications.

Description:
FIELD OF THE INVENTION 
   The present invention relates to Very Large Scale Integrated (VLSI) circuit design generally and, more particularly, to a method and/or tool for automatic test pin assignment. 
   BACKGROUND OF THE INVENTION 
   In a conventional design methodology for a programmable platform device (e.g., platform and structured application specific integrated circuits (ASICs)), a developer assigns up to 256 Low Pin Count (LPC) pins for each slice. Each of the pins of a slice are categorized as No Test, Dedicated, Shared, or Reserved. If a pin is marked as Dedicated, Shared, or Reserved, the pin is considered a LPC pin. 
   The conventional methodology can be too restrictive. With the conventional methodology, scan pins are fixed and there are certain classifications of buffers that cannot be used for test sharing. For example, if a layout includes a 32 bit stub series termination logic (SSTL) bus, the probability is quite high that a pin to be used will be marked as a test_share pin. However, SSTL buffers cannot be used with test_share pins. 
   The conventional solution involves the manufacturer manually changing the limited amount of test pins. The process is manual, error prone, and time consuming. In addition, changes can only be performed by the manufacturer. Therefore, a vendor must rely on the manufacturer to make the changes. Only non-scan pins are allowed to change with the current methodology. 
   It would be desirable to have a method and/or tool for automatic test pin assignment. 
   SUMMARY OF THE INVENTION 
   The present invention concerns a tool for facilitating automatic test pin assignment for a programmable platform device comprising: a process for collecting information related to the programmable platform device, a process for automatically initializing a test pin assignment for the programmable platform device, a process configured to receive user specifications for IOs and a process for performing dynamic test pin reassignment in response to the user specifications. 
   The objects, features and advantages of the present invention include providing a method and/or tool for automatic test pin assignment that may (i) automatically assign test pins during development, (ii) increase quality, (iii) decrease time to market (TAT), (iv) reduce non-recurring engineering (NRE) costs, (v) automatically re-assign test pins based on user interaction, (vi) perform re-assignment transparently to the user, (vii) automatically change a test function when an IO that is non-sharable is selected for test and/or (viii) allow for moving scan pins within valid locations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
       FIG. 1  is a block diagram illustrating an example programmable platform device; 
       FIG. 2  is a flow diagram illustrating a process for initializing test pins on a slice in accordance with a preferred embodiment of the present invention; 
       FIG. 3  is a flow diagram illustrating a process for automatic assignment of test pins on a particular instance of a slice in accordance with a preferred embodiment of the present invention; and 
       FIG. 4  is a more detailed flow diagram illustrating a process for a dynamic test pin reassignment process of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a block diagram of a programmable platform device (or die or slice)  50  is shown in accordance with a preferred embodiment of the present invention. In one example, the device  50  may comprise one or more regions of diffused memory  52 , one or more regions of R-cell memory  54 , one or more hard macros  56  and a diffused region  58 . In another example, multiple diffused regions  58  may be implemented. The regions  52 ,  54 ,  56 , and  58  may be distributed around the slice  50 . 
   The regions  56  may include diffused patterns of a circuit design that is customized and optimized for a particular function. The hard macros implemented in the regions  56  may also be referred to IP (intellectual property) blocks. The hard macros generally act similarly to an ASIC design. In general, the hard macros  56  may be implemented to provide a number of functions on the device  50 . For example, the hard macros  56  may comprise phase locked loops (PLLs), instances of processors, input/output PHY level macros, etc. The regions  56  may comprise similar and/or different hard macros. 
   The diffused regions  58  may be customized, in one example, as logic, memory and/or firm or soft IP. For example, the regions  58  may be implemented as a sea of gates array. In one example, the regions  58  may be implemented with a number of R-cells. As used herein, R-cells generally refer to an area of silicon designed (or diffused) to contain one or more transistors that have not yet been personalized (or configured) with metal layers. Wire layers may be added to the R-cells to make particular transistors, logic gates, storage elements (e.g., the R-cell memories  54 ) and/or soft or firm IP. 
   An R-cell generally comprises one or more diffusions for forming the parts of transistors and the contact points where wires may be attached in subsequent manufacturing steps (e.g., to power, ground, inputs and outputs). In general, the R-cells may be, in one example, building blocks for logic and/or storage elements (e.g., the R-cell memories  54 ). For example, one way of designing a chip that performs logic and storage functions may be to lay down numerous R-cells row after row, column after column. A large area of the chip may be devoted to nothing but R-cells. The R-cells may be personalized (or configured) in subsequent production steps (e.g., by depositing metal layers) to provide particular logic functions. The logic functions may be further wired together (e.g., a gate array design). 
   In one example, a number of slices  50  may be fabricated having different varieties and/or numbers of hard macros and diffused memories. The slices  50  may be fabricated with different varieties and numbers of IOs around the periphery of the slice. By fabricating a variety of slices with a variety of hard macros and diffused memories, a wide variety of applications may be supported. For example, a particular slice may be selected for customization because the particular hard macros implemented are suitable for a customized application. Once a slice has been customized, the slice may be referred to as an instance. 
   Referring to  FIG. 2 , a flow diagram is shown illustrating an example operation of a design flow (or tool)  100  implemented in accordance with a preferred embodiment of the present invention. In one example, a number of types of information may be input into the design flow  100 . For example, the design flow  100  may receive information regarding generic test specifications (e.g., the block  102 ), information regarding slice restrictions (e.g., the block  104 ), information regarding tester restrictions (e.g., the block  106 ), information regarding common hardware classes (e.g., the block  108 ) and information regarding assignment rules (e.g., the block  110 ). The information regarding common hardware classes may also be used by a common test fixture (e.g., the block  112 ). The design flow  100  may be configured to automatically initialize test pin assignments for a slice based upon the information received (e.g., the block  114 ). 
   Referring to  FIG. 3 , a flow diagram is shown illustrating an example instance creation process  120  implemented in accordance with a preferred embodiment of the present invention. The instance creation process  120  may begin by examining a design to determine whether IO selection for the design is complete (e.g., the block  122 ). If the selection of IO pins is complete, the instance creation process  120  generally moves to a step of test pin assignment (e.g., the block  124 ). The test pin assignment process  124  may be implemented, for example, to (i) guarantee functionality, (ii) provide for additional uses of data in multiple formats and/or (iii) guarantee compatibility with common test fixtures. 
   Test pin assignment may be accomplished through setting up the correct IO properties so that the assignment process may be automatically performed. Prior to completing the slice design, the slice designer may, in one example, set up a file with the constraints for each IO. It is common in the industry today to manually select IO test pin assignment by understanding the rules or constraints for each IO. 
   In order to maximize tester memory usage, each Tester Memory Segment (TMS) has a definition of how many scan_in and scan_out pins may be located on a particular TMS. The automatic test pin assignment  124  generally looks at the TMS definitions for each slice to determine how to properly distribute the scan pin assignments. The remaining (non-scan pin) test functions may be, in one example, randomly assigned to shared IO classifications. 
   When the process  120  determines that the IO selection is not complete, the process  120  may move to a state  126 . In the state  126 , the process  120  generally provides for developer input to select IOs (e.g, a prompt may be generated to request information from the developer). In one example, a graphic user interface (GUI) may be implemented with pull down menus for aiding the designer in selecting the IOs. A selected IO is generally examined to determine whether the selected IO is compatible with the test function (e.g., the block  128 ). If the IO is compatible with the test function, the process  120  generally returns to the block  122  to determine whether the IO selection is complete. 
   When the IO selected by the user is not compatible with the test function, a dynamic test pin re-assignment operation may be initiated (e.g., the block  130 ). A rule checker operation may be performed. The rule checking may be performed using convention techniques. Following performance of the rule checker operation, an examination may be performed to see whether additional IOs are available for re-assignment. When additional IOs are available, the dynamic test pin re-assignment process may indicate the automatic re-assignment of the test pin (e.g., a return code equal to 1). When additional IOs are not available, the dynamic test pin re-assignment process may indicate that the test pin was not re-assigned (e.g., a return code equal to 0). When the dynamic test pin re-assignment process completes re-assigning a test pin, the process  120  generally returns to the step  122  (the YES path from the block  132 ). When additional IOs are not available, a message may be provided to the developer instructing that another IO be selected (e.g., the block  134 ). 
   The present invention generally provides a methodology for assigning IOs for manufacturer testing. The assigned IOs may include IOs dedicated for testing and IOs that are shared for testing. Programmable platform devices are generally provided to reduce non-recurring engineering (NRE) costs and provide shorter product time-to-market (TAT). Design Center TAT may be shortened by creating IO definitions (e.g., in an IO definition file) that may be used by the design tools. Sharing information may be added to the IO definition file based on test information. In addition, test sharing functions may be pre-determined to save time for the Design Center. 
   To reduce NRE costs, the present invention may use common hardware for sort and common hardware for final test. The present invention may also enable sort hardware to be tested on a Low Pin Count (LPC) tester by assigning all test pins to 256 LPC pins. Within the 256 pins, pins may be categorized as No Test, Dedicated, Shared, or Reserved. When a pin is marked as Dedicated, Shared, or Reserved, the pin is considered a LPC pin. The No Test classification generally indicates that no test sharing is allowed. The Dedicated classification is generally used for designating manufacturer dedicated test signals in addition to dedicated test pins. 
   The Shared classification may be used for IOs that are shared with user function and manufacturing testing. In one example, a platform test info field may be implemented in the design tool with symbolic test names that are defined at slice development. The platform test info field may also comprise information (or rules) that may be used to determine what can and what cannot be done. The symbolic test names may be used to determine what test pins may be shared with the user defined IO. Examples may include, but are not limited to, scan_in, scan_out, scan_clock, and test_ip_in. The Reserved classification may be used to identify a LPC pin that is not used for test sharing. The Reserved classification may also be used for diffused IO that are used to make contact with a test head at sort testing. 
   In order to assign the pins for a slice, the developer first identifies the maximum number of test pins on the slice. The maximum number of test pins is generally identified based on a combination of diffused elements on the slice. The maximum number may also take into consideration what the user may add on a slice that will add additional test pins (e.g., High Impedance Controllers and soft or firm IP). There may be specific (diffused) elements on the slice that use test pins. For example, a slice may comprise one or more high-speed serial communication cores. Each core may have IO pins used for testing. The pins may be marked as RESERVED. In one example, a test information entry in the IO definition file may be left blank to indicate reserved pins. In addition, a particular core may have IOs (e.g., TEST_IP_IN[0:5] and TEST_IP_OUT[0:6]) implemented as shared IO test pins. Another core on the slice may also have dedicated IO pins. The second core may share the same shared IO test pins (e.g., TEST_IP_IN[0:5] and TEST_IP_OUT[0:6]) used for the first core. 
   In addition to known resources on the slice, test pins may be assigned for additional resources that may be implemented on the slice that are not diffused. Additional types of resources may include non-diffused memory (e.g., memory implemented in a programmable transistor fabric of the slice), a High Impedance Controller, and/or a second test access port (TAP) Controller (e.g., IP_TAP, ip_tap_tms, ip_tap_trstn, ip_tap_tdi, ip_tap _tck). Pins may also be added in order to accommodate IP legacy wrappers that may have older versions of test inserted. For example, the following pins may be added: SCAN_SETN, SCAN_RESETN, SCAN_SETRESETN. 
   Further slice considerations may include: scan in/out, scan clock, and High Impedance Controllers. Slices may have a number of scan chains. In one example, 16 scan chains may be implemented. In one example, a slice may be implemented with 16 scan clocks identified for testing. If more scan clocks are desired, the additional clocks may be assigned to other LPC pins by the design tool. In one example, a slice design may be limited to one High Impedance Controller. In another example, a slice may be implemented with several High Impedance Controllers. 
   The number of test inputs and test outputs is generally determined for all of the diffused IP on a slice. The number may be determined, in one example, in an IP netlist. Most of the signals may be shared among similar and dissimilar IP. Merging information may be provided in a documentation of the particular IP. 
   In one example, extra Low Pin Count pins may be identified (provided) to provide for new or additional test pins not identified during initial design stages. For example, five test pins may be selected, marked as shared in the design tool, and have a test info field left empty. The design tool may be configured to create five extra test buffers in the IO definition file. The extra test buffers may be marked flag. The extra test buffers may be used for testing. 
   In one example, a table (e.g., TABLE 1 below) may be used to identify all of the test pins for a slice. 
                                 TABLE 1                   EXAMPLE TEST CONNECTIONS            Object               Type   Non-Shared IO Pins   Shared IO Pins               Dedicated   IDDT*               TN*           PROCMON*           SCAN_ENABLE*           TCK*           TMS*           TDI*           TDO*           TRSTN*       PLL       Pins for each PLL               PLL_REF_CLK               PLL_LOCKOUT               Pins shared for all PLL               PLL_RESETN#               PLL_T[0:1]               Test control signals       PRO-   Pins may be shared   Pins shared for all ARM       CESSOR   for all cores with   IP_TAP_TDI           a second TAP   IP_TAP_TDK           controller. It is   IP_TAP_TMS           the responsibility   IP_TAP_TRSTN           of the designer to   Note: IP_TAP_TDO and           include this pin.   IP_TAP_TDO_ENABLEN           IP_TAP   will be connected to               primary output TDO       High   High-ZRSET   HIGH-Z_SCAN_OUT       Impedance   This IO may be   HIGH-Z_CLK#       Controller   managed through   HIGH-Z_RESETN#           the design tool           implemented in           accordance with           the present           invention.       Scan       SCAN_IN[0:14] (plus TDI)               SCAN_OUT[0:15]               SCAN_CLK[0:15]       Rcell       LBRAM_SCAN_IN[0:14]       memory       LBRAM_SCAN_CLOCK               LBRAM_SCAN_OUT[0:14]       MBIST       Pins shared for all memories               MBIST_CLK               MBIST_CMP_STAT       misc:       SCAN_SETN               SCAN_RESETN               SCAN_SETRESETN               IP_TAP_TDI               IP_TAP_TDK               IP_TAP_TMS               IP_TAP_TRSTN                    
Cells marked with an * are generally NOT counted as available IO for the designer. The term LBRAM generally refers to latch-based memory.
 
   After all of the specified test pins have been identified, valid sharing of the test pins may be assigned. A table (e.g., the TABLE 2 below) may be used to identify test pin sharing. 
                                                                                     TABLE 2                   EXAMPLE TEST PIN SHARING                Test Signal 1   Test Signal 2   Test Signal 3                        inputs                scan_in_0   lbram_scan_in_0   test_ip_in_0           scan_in_1   lbram_scan_in_1   test_ip_in_1           scan_in_2   lbram_scan_in_2   test_ip_in_2           scan_in_3   lbram_scan_in_3   test_ip_in_3           scan_in_4   lbram_scan_in_4   test_ip_in_4           scan_in_5   lbram_scan_in_5   test_ip_in_5           scan_in_6   lbram_scan_in_6   test_ip_in_6           scan_in_7   lbram_scan_in_7           scan_in_8   lbram_scan_in_8           scan_in_9   lbram_scan_in_9           scan_in_10   lbram_scan_in_10           scan_in_11   lbram_scan_in_11           scan_in_12   lbram_scan_in_12           scan_in_13   lbram_scan_in_13   pll_t0           scan_in_14   lbram_scan_in_14   pll_t1           scan_resetn   bz_resetn           scan_setn           scan_setresetn           ip_tap_tms           ip_tap_trstn           ip_tap_tdi           ip_tap_tck           pll_resetn           scan_clock_0   bz_clock           scan_clock_1           scan_clock_2           scan_clock_3           scan_clock_4           scan_clock_5           scan_clock_6           scan_clock_7           scan_clock_8           scan_clock_9           scan_clock_10           scan_clock_11           scan_clock_12           scan_clock_13           scan_clock_14           scan_clock_15           lbram_scan_clock           pll_ref_clk_0           pll_ref_clk_1           pll_ref_clk_2           pll_ref_clk_3           mbist_clock           rram_clock           rram_test_clock            outputs                scan_out_0   lbram_scan_out_0   test_ip_out_0           scan_out_1   lbram_scan_out_1   test_ip_out_1           scan_out_2   lbram_scan_out_2   test_ip_out_2           scan_out_3   lbram_scan_out_3   test_ip_out_3           scan_out_4   lbram_scan_out_4   test_ip_out_4           scan_out_5   lbram_scan_out_5   test_ip_out_5           scan_out_6   lbram_scan_out_6   test_ip_out_6           scan_out_7   lbram_scan_out_7   test_ip_out_7           scan_out_8   lbram_scan_out_8   test_ip_out_8           scan_out_9   lbram_scan_out_9           scan_out_10   lbram_scan_out_10           scan_out_11   lbram_scan_out_11   rram_test_out           scan_out_12   lbram_scan_out_12   pll_lock_0           scan_out_13   lbram_scan_out_13   pll_lock_1           scan_out_14   lbram_scan_out_14   pll_lock_2           scan_out_15   pll_lock_3           mbist_cmp_stat           bz_scan_out_0           bz_scan_out_1           bz_scan_out_2           bz_scan_out_3           bz_scan_out_4           bz_scan_out_5           bz_scan_out_6                        
The signal names in TABLE 2 may be consistent with the symbolic signal names used by other applications to input into the design tool. In one example, multiple shared signal names may be input into the test info field in the tool separated by a space. The term lbram generally refers to latch-based memory.
 
   In an automatic test pin assignment process in accordance with the present invention, the slice developer assigns limited test pins during slice development. The assigned test pins may include dedicated test pins and non-configurable IO that are used for sort testing. The assigned pins may be included in the LPC pin count. The remaining LPC test pins are generally identified by the slice developers. 
   Referring to  FIG. 4 , a more detailed flow diagram is shown illustrating an example implementation of the dynamic test pin reassignment process  130  of  FIG. 3  in accordance with a preferred embodiment of the present invention. When the designer selects an IO that is on a LPC pin that is identified as Shared, a determination is made whether the IO selected is compatible with the Test Function (e.g., the block  128  of  FIG. 3 ). In the Dynamic test pin re-assignment  130 , a further determination is made whether the IO is assigned a scan test function (e.g., the block  140 ). When there is no scan test function assigned to the IO, an available reserved IO is sought to determine whether the test function may be swapped. Preference is generally given to an IO that the designer is already using (e.g., the block  142 ). If no IOs that are already in use are available, an IO is sought that the designer is not using to swap test functions (e.g., the block  144 ). When no IO is available, the process may set a flag indicating such (e.g., the block  146 ). When either an IO that is in use or an IO that is not in use is available, the test function and the test classifications between the IOs may be swapped (e.g., the blocks  148  and  150 ). The process may set a flag indicating the swap has been made (e.g., the block  152 ). 
   When the designer selects a scan pin, additional limitations may be placed on swapping test functions and test classifications. In order to select scan test data, both the IOs are checked for the same TMS property assigned to the IO (e.g., the blocks  154  and  156 ). To maximize scan assignment, an additional search may be performed for scan pins. In one example, a shared IO without scan data may be compared to the selected IO to determine whether the IOs have the same TMS property (e.g., the block  158 ). When no IO is available, the process may set a flag indicating such (e.g., the block  160 ). When either a RESERVED IO that is available on the same TMS and is in use, or a RESERVED IO that is available on the same TMS and is not in use, or a shared non-scan IO on the same TMS is available, the test function and the test classifications between the IOs may be swapped (e.g., the blocks  148  and  150 ). In general, the dynamic test pin re-assignment may be performed transparently to the designer. Transparent dynamic pin re-assignment makes test pin assignments as non-intrusive to the designer as possible. 
   When the designer selects an IO that is on a LPC pin that is identified as Shared and the IO selected cannot be used for test sharing, the design tool of the present invention is generally configured to check to see whether there is Reserved configurable IO available to swap test functions. In one example, preference may be given to Reserved IOs that the designer is currently using. 
   When a Reserved IO is available, the design tool may be configured to swap the information that is in the platform test info field between the two IOs. The tool may be configured to also change the platform test usage values between the two IOs (e.g., Shared-ΔReserved and Reserved-ΔShared). In one example, the tool may be configured to generate a file containing test sharing information. 
   Scan pins generally include scan_in and scan_out (scan_clock pins are not included in this class). In order to maximize tester memory utilization, scan pins are generally distributed across the tester memory segment (TMS). When the designer selects a Shared IO that has scan_in or scan_out in the platform test info field, the tool is generally configured to swap test usage with a Reserved IO only if both the shared IO and the Reserved IO have the same TMS value. 
   If there are no swapable Shared IOs on a particular TMS, the tool may be configured to determine whether a non-scan Shared IO on the TMS may be moved to allow for the scan pin to be swapped. For example, when there are no available Reserved IOs on TMS 1  and the designer selects a non-shareable IO on TMS 1  that has a scan pin assignment, the tool generally checks to see whether there are non-scan pins being used on TMS 1  that may be moved to another TMS. Moving pins to another TMS may free up TMS 1  to be available for a scan pin swap. 
   The Tester Memory Segment (TMS) information may be used by the tool to facilitate swapping of all test pins, including scan pins. However, when the TMS information is not available, all test pins except scan pins may be moved. The slice developers are generally responsible for defining the TMS for each slice. In one example, a file may be created that maps the package ball information with the TMS. A package ball generally has a TMS listed only when the ball can be used for scan_in or scan_out. 
   In general, TMS information may be of the form TMSxx where xx denotes the specific segment (e.g., TMS 1 , TMS 2 , etc.). The TMS information generally includes an indication of how many scan chains may be in each segment (TMS 1 =2, TMS 2 =2, etc.). In one example, the TMS information may be input through an import function of the tool. 
   The tool may have backward compatibility. When backward compatibility is implemented in the tool, in order to take full advantage of the automation including scan pin swapping, TMS information may be put into the Slice database file(s). If the database has TMS information in the platform test info field, the tool may be configured to allow swapping of all test share functions including scan_in and scan_out. If the database does not have TMS information, the tool may be configured to only swap non-scan (scan_in and scan_out) test share functions. 
   The present invention may provide checking and verification. In order to use common hardware across multiple instances of a slice, checking is performed to verify common hardware will work. The tool may be configured to verify information between the slice data and test inserted netlist. For example, the tool may verify that the LPC pins in the Slice database are the same LPC pins in the finish database. LPC pins are defined in the platform test usage field as either Shared, Dedicated, or Reserved. The information may be obtained in the slice database and an instance database files. The data may be made available through a text out command of the tool. 
   The tool may verify that the number of scan_in and scan_out pins that are mapped to each TMS is the same in the Slice database and the finish netlist. All TMS data may be obtained from the slice database file. The platform test info field of the tool may contain the TMS data. In one example, the data may be in the format TMS 0 , TMS 1 , etc. The field may also contain test information as described in TABLE 2 above. The slice scan in and scan_out data may be obtained in the slice database file. In one example, the information may be in the platform test info field of the tool. In one example, the information may have the format scan_in_ 0 , scan_in_ 1 , scan_out_ 0 , etc. 
   When no TMS data is available, the tool may be configured to verify that the exact location of scan_in and scan_out pins are the same in the Slice database and the finish netlist. In general, the data is obtained as described above. The tool may be further configured to verify that all test share pins are assigned to LPC pins. The shared test pin assignment may be obtained from the finish netlist. For example, test pin assignments may be identified in the IO definition file. In one example, the pin assignments may be identified in share_port statements. LPC pins are generally identified in the slice database as described above. 
   The functions performed by  FIGS. 2-4  may be implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
   The present invention may also be implemented by the preparation of ASICs, FPGAs, or by interconnecting an appropriate network of conventional component circuits, as is described herein, modifications of which will be readily apparent to those skilled in the art(s). 
   The present invention thus may also include a computer product which may be a storage medium including instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROM, magneto-optical disks, ROMs, RAMs, EPROMS, EEPROMs, Flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
   While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.