Abstract:
A timestamp generator generates a timestamp value having a predetermined number of most significant bits and a predetermined number of least significant bits. The least significant bits are transmitted to a client via a parallel data bus. The most significant bits are transmitted to the client sequentially via a series data bus. Each client receives the parallel least significant bits and the series most significant bits and assembles a complete time stamp value.

Description:
CLAIM OF PRIORITY 
     This application is a Divisional of prior application Ser. No. 12/971,930, filed Dec. 17, 2010, now U.S. Pat. No. 8,924,767, issued Dec. 30, 2014; 
     And claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/287,444 filed Dec. 17, 2009. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is time stamping for emulation and debug of electronic systems. 
     BACKGROUND OF THE INVENTION 
     Advanced wafer lithography and surface-mount packaging technology are integrating increasingly complex functions at both the silicon and printed circuit board level of electronic design. Diminished physical access to circuits for test and emulation is an unfortunate consequence of denser designs and shrinking interconnect pitch. Designed-in testability is needed so the finished product is both controllable and observable during test and debug. Any manufacturing defect is preferably detectable during final test before a product is shipped. This basic necessity is difficult to achieve for complex designs without taking testability into account in the logic design phase so automatic test equipment can test the product. 
     In addition to testing for functionality and for manufacturing defects, application software development requires a similar level of simulation, observability and controllability in the system or sub-system design phase. The emulation phase of design should ensure that a system of one or more ICs (integrated circuits) functions correctly in the end equipment or application when linked with the system software. With the increasing use of ICs in the automotive industry, telecommunications, defense systems, and life support systems, thorough testing and extensive real-time debug becomes a critical need. 
     Functional testing, where the designer generates test vectors to ensure conformance to specification, still remains a widely used test methodology. For very large systems this method proves inadequate in providing a high level of detectable fault coverage. Automatically generated test patterns are desirable for full testability, and controllability and observability. These are key goals that span the full hierarchy of test from the system level to the transistor level. 
     Another problem in large designs is the long time and substantial expense involved in design for test. It would be desirable to have testability circuitry, system and methods that are consistent with a concept of design-for-reusability. In this way, subsequent devices and systems can have a low marginal design cost for testability, simulation and emulation by reusing the testability, simulation and emulation circuitry, systems and methods that are implemented in an initial device. Without a proactive testability, simulation and emulation plan, a large amount of subsequent design time would be expended on test pattern creation and upgrading. 
     Even if a significant investment were made to design a module to be reusable and to fully create and grade its test patterns, subsequent use of a module may bury it in application specific logic. This would make its access difficult or impossible. Consequently, it is desirable to avoid this pitfall. 
     The advances of IC design are accompanied by decreased internal visibility and control, reduced fault coverage and reduced ability to toggle states, more test development and verification problems, increased complexity of design simulation and continually increasing cost of CAD (computer aided design) tools. In the board design the side effects include decreased register visibility and control, complicated debug and simulation in design verification, loss of conventional emulation due to loss of physical access by packaging many circuits in one package, increased routing complexity on the board, increased costs of design tools, mixed-mode packaging, and design for produceability. In application development, some side effects are decreased visibility of states, high speed emulation difficulties, scaled time simulation, increased debugging complexity, and increased costs of emulators. Production side effects involve decreased visibility and control, complications in test vectors and models, increased test complexity, mixed-mode packaging, continually increasing costs of automatic test equipment and tighter tolerances. 
     Emulation technology utilizing scan based emulation and multiprocessing debug was introduced more than 10 years ago. In 1988, the change from conventional in circuit emulation to scan based emulation was motivated by design cycle time pressures and newly available space for on-chip emulation. Design cycle time pressure was created by three factors. Higher integration levels, such as increased use of on-chip memory, demand more design time. Increasing clock rates mean that emulation support logic causes increased electrical intrusiveness. More sophisticated packaging causes emulator connectivity issues. Today these same factors, with new twists, are challenging the ability of a scan based emulator to deliver the system debug facilities needed by today&#39;s complex, higher clock rate, highly integrated designs. The resulting systems are smaller, faster, and cheaper. They have higher performance and footprints that are increasingly dense. Each of these positive system trends adversely affects the observation of system activity, the key enabler for rapid system development. The effect is called “vanishing visibility.” 
       FIG. 1  illustrates the trend in visibility and control over time and greater system integration in accordance with the prior art. Application developers prefer the optimum visibility level illustrated in  FIG. 1 . This optimum visibility level provides visibility and control of all relevant system activity. The steady progression of integration levels and increases in clock rates steadily decrease the actual visibility and control available over time. These forces create a visibility and control gap, the difference between the optimum visibility and control level and the actual level available. Over time, this gap will widen. Application development tool vendors are striving to minimize the gap growth rate. Development tools software and associated hardware components must do more with less resources and in different ways. Tackling this ease of use challenge is amplified by these forces. 
     With today&#39;s highly integrated System-On-a-Chip (SOC) technology, the visibility and control gap has widened dramatically over time. Traditional debug options such as logic analyzers and partitioned prototype systems are unable to keep pace with the integration levels and ever increasing clock rates of today&#39;s systems. As integration levels increase, system buses connecting numerous subsystem components move on chip, denying traditional logic analyzers access to these buses. With limited or no significant bus visibility, tools like logic analyzers cannot be used to view system activity or provide the trigger mechanisms needed to control the system under development. A loss of control accompanies this loss in visibility, as it is difficult to control things that are not accessible. 
     To combat this trend, system designers have worked to keep these buses exposed. Thus the system components were built in a way that enabled the construction of prototyping systems with exposed buses. This approach is also under siege from the ever-increasing march of system clock rates. As the central processing unit (CPU) clock rates increase, chip to chip interface speeds are not keeping pace. Developers find that a partitioned system&#39;s performance does not keep pace with its integrated counterpart, due to interface wait states added to compensate for lagging chip to chip communication rates. At some point, this performance degradation reaches intolerable levels and the partitioned prototype system is no longer a viable debug option. In the current era production devices must serve as the platform for application development. 
     Increasing CPU clock rates are also limiting availability of other simple visibility mechanisms. Since the CPU clock rates can exceed the maximum I/O state rates, visibility ports exporting information in native form can no longer keep up with the CPU. On-chip subsystems are also operated at clock rates that are slower than the CPU clock rate. This approach may be used to simplify system design and reduce power consumption. These developments mean simple visibility ports can no longer be counted on to deliver a clear view of CPU activity. As visibility and control diminish, the development tools used to develop the application become less productive. The tools also appear harder to use due to the increasing tool complexity required to maintain visibility and control. The visibility, control, and ease of use issues created by systems-on-a-chip tend to lengthen product development cycles. 
     Even as the integration trends present developers with a tough debug environment, they also present hope that new approaches to debug problems will emerge. The increased densities and clock rates that create development cycle time pressures also create opportunities to solve them. On-chip, debug facilities are more affordable than ever before. As high speed, high performance chips are increasingly dominated by very large memory structures, the system cost associated with the random logic accompanying the CPU and memory subsystems is dropping as a percentage of total system cost. The incremental cost of several thousand gates is at an all time low. Circuits of this size may in some cases be tucked into a corner of today&#39;s chip designs. The incremental cost per pin in today&#39;s high density packages has also dropped. This makes it easy to allocate more pins for debug. The combination of affordable gates and pins enables the deployment of new, on-chip emulation facilities needed to address the challenges created by systems-on-a-chip. 
     When production devices also serve as the application debug platform, they must provide sufficient debug capabilities to support time to market objectives. Since the debugging requirements vary with different applications, it is highly desirable to be able to adjust the on-chip debug facilities to balance time to market and cost needs. Since these on-chip capabilities affect the chip&#39;s recurring cost, the scalability of any solution is of primary importance. “Pay only for what you need” should be the guiding principle for on-chip tools deployment. In this new paradigm, the system architect may also specify the on-chip debug facilities along with the remainder of functionality, balancing chip cost constraints and the debug needs of the product development team. 
       FIG. 2  illustrates a prior art emulator system  100  including four emulator components. These four components are: a debugger application program  110 ; a host computer  120 ; an emulation controller  130 ; and on-chip debug facilities  140 .  FIG. 2  illustrates the connections of these components. Host computer  120  is connected to an emulation controller  130  external to host  120 . Emulation controller  130  is also connected to target system  140 . The user preferably controls the target application on target system  140  through debugger application program  110 . 
     Host computer  120  is generally a personal computer. Host computer  120  provides access the debug capabilities through emulator controller  130 . Debugger application program  110  presents the debug capabilities in a user-friendly form via host computer  120 . The debug resources are allocated by debug application program  110  on an as needed basis, relieving the user of this burden. Source level debug utilizes the debug resources, hiding their complexity from the user. Debugger application program  110  together with the on-chip trace and triggering facilities provide a means to select, record, and display chip activity of interest. Trace displays are automatically correlated to the source code that generated the trace log. The emulator provides both the debug control and trace recording function. 
     The debug facilities are preferably programmed using standard emulator debug accesses through a JTAG or similar serial debug interface. Since pins are at a premium, the preferred embodiment of the invention provides for the sharing of the debug pin pool by trace, trigger, and other debug functions with a small increment in silicon cost. Fixed pin formats may also be supported. When the pin sharing option is deployed, the debug pin utilization is determined at the beginning of each debug session before target system  140  is directed to run the application program. This maximizes the trace export bandwidth. Trace bandwidth is maximized by allocating the maximum number of pins to trace. 
     The debug capability and building blocks within a system may vary. Debugger application program  100  therefore establishes the configuration at runtime. This approach requires the hardware blocks to meet a set of constraints dealing with configuration and register organization. Other components provide a hardware search capability designed to locate the blocks and other peripherals in the system memory map. Debugger application program  110  uses a search facility to locate the resources. The address where the modules are located and a type ID uniquely identifies each block found. Once the IDs are found, a design database may be used to ascertain the exact configuration and all system inputs and outputs. 
     Host computer  120  generally includes at least 64 Mbytes of memory and is capable of running Windows 95, SR-2, Windows NT, or later versions of Windows. Host computer  120  must support one of the communications interfaces required by the emulator. These may include: Ethernet 10T and 100T, TCP/IP protocol; Universal Serial Bus (USB); Firewire IEEE 1394; and parallel port such as SPP, EPP and ECP. 
     Host computer  120  plays a major role in determining the real-time data exchange bandwidth. First, the host to emulator communication plays a major role in defining the maximum sustained real-time data exchange bandwidth because emulator controller  130  must empty its receive real-time data exchange buffers as fast as they are filled. Secondly, host computer  120  originating or receiving the real-time data exchange data must have sufficient processing capacity or disc bandwidth to sustain the preparation and transmission or processing and storing of the received real-time data exchange data. A state of the art personal computer with a Firewire communication channel (IEEE 1394) is preferred to obtain the highest real-time data exchange bandwidth. This bandwidth can be as much as ten times greater performance than other communication options. 
     Emulation controller  130  provides a bridge between host computer  120  and target system  140 . Emulation controller  130  handles all debug information passed between debugger application program  110  running on host computer  120  and a target application executing on target system  140 . A presently preferred minimum emulator configuration supports all of the following capabilities: real-time emulation; real-time data exchange; trace; and advanced analysis. 
     Emulation controller  130  preferably accesses real-time emulation capabilities such as execution control, memory, and register access via a 3, 4, or 5 bit scan based interface. Real-time data exchange capabilities can be accessed by scan or by using three higher bandwidth real-time data exchange formats that use direct target to emulator connections other than scan. The input and output triggers allow other system components to signal the chip with debug events and vice-versa. Bit I/O allows the emulator to stimulate or monitor system inputs and outputs. Bit I/O can be used to support factory test and other low bandwidth, non-time-critical emulator/target operations. Extended operating modes are used to specify device test and emulation operating modes. Emulator controller  130  is partitioned into communication and emulation sections. The communication section supports host communication links while the emulation section interfaces to the target, managing target debug functions and the device debug port. Emulation controller  130  communicates with host computer  120  using one of industry standard communication links outlined earlier herein. The host to emulator connection is established with off the shelf cabling technology. Host to emulator separation is governed by the standards applied to the interface used. 
     Emulation controller  130  communicates with the target system  140  through a target cable or cables. Debug, trace, triggers, and real-time data exchange capabilities share the target cable, and in some cases, the same device pins. More than one target cable may be required when the target system  140  deploys a trace width that cannot be accommodated in a single cable. All trace, real-time data exchange, and debug communication occurs over this link. Emulator controller  130  preferably allows for a target to emulator separation of at least two feet. This emulation technology is capable of test clock rates up to 50 MHZ and trace clock rates from 200 to 300 MHZ, or higher. Even though the emulator design uses techniques that should relax target system  140  constraints, signaling between emulator controller  130  and target system  140  at these rates requires design diligence. This emulation technology may impose restrictions on the placement of chip debug pins, board layout, and requires precise pin timings. On-chip pin macros are provided to assist in meeting timing constraints. 
     The on-chip debug facilities offer the developer a rich set of development capability in a two tiered, scalable approach. The first tier delivers functionality utilizing the real-time emulation capability built into a CPU&#39;s mega-modules. This real-time emulation capability has fixed functionality and is permanently part of the CPU while the high performance real-time data exchange, advanced analysis, and trace functions are added outside of the core in most cases. The capabilities are individually selected for addition to a chip. The addition of emulation peripherals to the system design creates the second tier functionality. A cost-effective library of emulation peripherals contains the building blocks to create systems and permits the construction of advanced analysis, high performance real-time data exchange, and trace capabilities. In the preferred embodiment five standard debug configurations are offered, although custom configurations are also supported. The specific configurations are covered later herein. 
     Transmission of a very large timestamp value to a number of timestamp clients uses substantial chip routing area. 
     SUMMARY OF THE INVENTION 
     This invention minimizes chip routing resources in transmission of timestamp values to timestamp clients. This invention employs a mix of both parallel and serial methods. 
     A timestamp generator generates a timestamp value having a predetermined number of most significant bits and a predetermined number of least significant bits. The least significant bits are transmitted to a client via a parallel data bus. The most significant bits are transmitted to the client sequentially via a series data bus. Each client receives the parallel least significant bits and the series most significant bits and assembles a complete timestamp value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this invention are illustrated in the drawings, in which: 
         FIG. 1  illustrates the visibility and control of typical integrated circuits as a function of time due to increasing system integration; 
         FIG. 2  illustrates a prior art emulation system to which this invention is applicable; 
         FIG. 3  illustrates in block diagram form a typical integrated circuit employing configurable emulation capability of the prior art; 
         FIG. 4  illustrates a first prior art timestamp distribution technique; 
         FIG. 5  illustrates a second prior art timestamp distribution technique; 
         FIG. 6  illustrates a third prior art timestamp distribution technique; 
         FIG. 7  illustrates schematically a first embodiment of timestamp value transmission of this invention; 
         FIG. 8  illustrates an example of the embodiment of  FIG. 7 ; 
         FIG. 9  illustrates the manner of transmitting the serial most significant bits of the time stamp value of the embodiment of  FIG. 7 ; 
         FIG. 10  illustrates schematically a second embodiment of timestamp value transmission of this invention; 
         FIG. 11  illustrates an example of the embodiment of  FIG. 10 ; 
         FIG. 12  illustrates the manner of transmitting the serial most significant bits of the time stamp value of the embodiment of  FIG. 10 ; 
         FIG. 13  illustrates schematically a third embodiment of timestamp value transmission of this invention; 
         FIG. 14  illustrates an example of the embodiment of  FIG. 13 ; 
         FIG. 15  illustrates the manner of transmitting the serial most significant bits of the time stamp value of the embodiment of  FIG. 13 ; 
         FIG. 16  illustrates schematically a fourth embodiment of timestamp value transmission of this invention; 
         FIG. 17  illustrates an example of the embodiment of  FIG. 16 ; 
         FIG. 18  illustrates the manner of transmitting the serial most significant bits of the time stamp value of the embodiment of  FIG. 16 ; 
         FIG. 19  illustrates various signals in a first embodiment of reception of the serial most significant bits of the time stamp of this invention; 
         FIG. 20  illustrates hardware for practicing the embodiment illustrated in  FIG. 19 ; 
         FIG. 21  illustrates various signals in a second embodiment of reception of the serial most significant bits of the time stamp of this invention; 
         FIG. 22  illustrates hardware for practicing the embodiment illustrated in  FIG. 21 ; and 
         FIG. 23  illustrates hardware for distribution of the timestamp value according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 3  illustrates an example of one on-chip debug architecture embodying target system  140 . The architecture uses several module classes to create the debug function. One of these classes is event detectors including bus event detectors  210 , auxiliary event detectors  211  and counters/state machines  213 . A second class of modules is trigger generators including trigger builders  220 . A third class of modules is data acquisition including trace collection  230  and formatting. A fourth class of modules is data export including trace export  240 , and real-time data exchange export  241 . Trace export  240  is controlled by clock signals from local oscillator  245 . Local oscillator  245  will be described in detail below. A final class of modules is scan adaptor  250 , which interfaces scan input/output to CPU core  201 . Final data formatting and pin selection occurs in pin manager and pin micros  260 . 
     The size of the debug function and its associated capabilities for any particular embodiment of a system-on-chip may be adjusted by either deleting complete functions or limiting the number of event detectors and trigger builders deployed. Additionally, the trace function can be incrementally increased from program counter trace only to program counter and data trace along with ASIC and CPU generated data. The real-time data exchange function may also be optionally deployed. The ability to customize on-chip tools changes the application development paradigm. Historically, all chip designs with a given CPU core were limited to a fixed set of debug capability. Now, an optimized debug capability is available for each chip design. This paradigm change gives system architects the tools needed to manage product development risk at an affordable cost. Note that the same CPU core may be used with differing peripherals with differing pin outs to embody differing system-on-chip products. These differing embodiments may require differing debug and emulation resources. The modularity of this invention permits each such embodiment to include only the necessary debug and emulation resources for the particular system-on-chip application. 
     The real-time emulation debug infrastructure component is used to tackle basic debug and instrumentation operations related to application development. It contains all execution control and register visibility capabilities and a minimal set of real-time data exchange and analysis such as breakpoint and watchpoint capabilities. These debug operations use on-chip hardware facilities to control the execution of the application and gain access to registers and memory. Some of the debug operations which may be supported by real-time emulation are: setting a software breakpoint and observing the machine state at that point; single step code advance to observe exact instruction by instruction decision making; detecting a spurious write to a known memory location; and viewing and changing memory and peripheral registers. 
     Real-time emulation facilities are incorporated into a CPU mega-module and are woven into the fabric of CPU core  201 . This assures designs using CPU core  201  have sufficient debug facilities to support debugger application program  110  baseline debug, instrumentation, and data transfer capabilities. Each CPU core  201  incorporates a baseline set of emulation capabilities. These capabilities include but are not limited to: execution control such as run, single instruction step, halt and free run; displaying and modifying registers and memory; breakpoints including software and minimal hardware program breakpoints; and watchpoints including minimal hardware data breakpoints. 
     Timestamps are generally regarding as part of instrumentation data. In a system where emulation or debug is desired, it is desirable to collect information about system operation with software and hardware monitors. This information becomes even more valuable when the time of information collection is also recorded and made available. The time at which information is collected is typically marked with a timestamp. 
     One method of timestamping presumes a common time base is available to all functions (timestamp clients) generating a timestamp. As the operating frequency of systems increase, the number of bits used for a timestamp can generally increase proportionally. When the timestamp must be delivered to a number of functions (timestamp Clients) within a chip, the number of routing channels required to deliver the timestamp to all destinations becomes significant. This invention covers methods that may be used to convey a timestamp value to many points in a system while minimizing the number of routing channels needed to provide the timestamp value to the functions. 
     A timestamp value is generally created with the highest frequency clock within the system when a timestamp with maximum resolution is desired. This may provide a timestamp of 64 or more bits in width. The timestamp value is represented as timestamp[n:00]: where bit n is the most significant bit (MSB); and bit 00 is the least significant bit (LSB). 
     The characteristics of this timestamp value are listed below. A timestamp value may be coded as: a binary value; a Gray coded value; or some other encoding format. When a timestamp is binary coded, beginning with bit [01], each bit n toggles at one half the rate of bit [n−1]. When a timestamp value is Gray coded, beginning with bit [01], each bit n toggles at one half the rate of bit [n−1]. For both binary coded and Gray coded timestamp values, the toggle rates of more significant bits are slow or very slow when compared to the toggle rates of less significant bits. The point at which bit [n] of the timestamp toggles is determined by the value of timestamp bits [n−1:0] 
       FIGS. 4 to 6  illustrate several techniques for presentation of timestamps in the prior art.  FIG. 4  illustrates a first prior art technique. Timestamp source  401  supplies a timestamp value as n parallel bits to client  402 . The n-bit timestamp is supplied each client  402  synchronous to a clock of client  402  or a sub-multiple of this clock. 
       FIG. 5  illustrates a second prior art technique. Timestamp source  501  supplies a timestamp value of parallel bits [n−1,0] as an asynchronous Gray coded value. Synchronizer  502  synchronizes each bit of the Gray coded value to a client clock or sub-multiple of the client clock. Synchronizer  502  supplies the bit synchronized MSB bits [n−1:m] of the timestamp value to client  503 . Note that the omitted LSBs are not used. Generally one synchronizer  502  is provided for each client  503 . 
       FIG. 6  illustrates a third prior art technique. Timestamp source  601  supplies a timestamp value of parallel bits [n−1,0] as an asynchronous Gray coded value. Synchronizer  602  synchronizes the timestamp value change in one of the LSBs to the client bit [x]. Synchronizer  602  transfers MSB bits [n−1:x+y] to client  603 . The omitted LSBs are not used. Generally one synchronizer  602  is provided for each client  603 . 
     In each of the prior art techniques illustrated in  FIGS. 4 to 6  an n-bit wide bus broadcasts a timestamp value to all timestamp clients. When some or all of these clients are located in areas of a chip where routing is congested, the number of routing channels required to provide the timestamp to these clients sometimes seems excessive. This invention is an alternate means of providing the timestamp value to clients on chip. 
     This invention supplies the MSBs of the timestamp value to the client serially. These MSBs of the timestamp value change infrequently when compared to the toggle rate of the client clock. This invention includes several variations of encoding the serial and parallel presentation of a timestamp value shown in Table 1 and described below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Serial Transmission (MSBs) 
                 Parallel Transmission (LSBs) 
               
               
                   
                   
               
             
             
               
                   
                 Gray coded 
                 Gray coded 
               
               
                   
                 Gray coded 
                 Binary coded 
               
               
                   
                 Binary coded 
                 Gray coded 
               
               
                   
                 Binary coded 
                 Binary coded 
               
               
                   
                   
               
             
          
         
       
     
     Other timestamp encoding are possible and are within the scope of this invention. With each of these techniques, the serial transmission of the MSBs of the timestamp is the next value of the MSBs so that this value may be presented to the timestamp in parallel when the count represented by the LSBs presented in parallel rolls over. 
       FIGS. 7 to 9  illustrate the first technique (Gray coded MSBs and Gray coded LSBs).  FIG. 7  illustrates the timestamp value or data transmission schematically. First timestamp source  711  supplies MSBs of one timestamp value or datum of serial bits [n,m+1] as an asynchronous Gray coded value. The MSB asynchronous Gray coded value is transmitted during periods of no interest. Because these MSBs change slowly this is generally possible. Second timestamp source  712  supplies LSBs of another timestamp value or datum of parallel bits [m,0] as an asynchronous Gray coded value. The LSB asynchronous Gray coded value is asynchronous to the client clock. First synchronizer  721  synchronizes each bit of the LSBs bits [m,0] to the client clock. Second synchronizer  722  keeps a timestamp value corresponding to the MSBs in a shadow register as updated by first timestamp source  711 . Second synchronizer  722  supplies the MSBs of the timestamp value synchronous with the client clock as triggered by a load signal from first synchronizer  721  to client  731 . This load is triggered by a rollover within the LSBs. Client  731  receives the MSB bits [n:m+1] from second synchronizer  722  and the LSB bits [m:0] from first synchronizer  721 . All bits are received by client  731  are synchronous with the client clock. 
       FIG. 8  illustrates an example of this operation. Column  810  is the serial MSBs, column  820  is the parallel LSBs and column  830  is resultant complete timestamp value. A rollover toggle in LSBs  820  at  811  triggers the load of MSBs  810 . As previously noted, this serial portion of the MSBs is coded for the next value. Therefore the load triggered by the rollover in the LSBs loads the correct value. 
       FIG. 9  illustrates the manner of transmitting the serial MSBs. Time slots  901 ,  903 ,  905  and  907  signal when a rollover toggle occurs in the LSBs. During time slot  902  the serial MSBs for the value 1 are transmitted to second synchronizer  722 . This value 1 is loaded into client  731  upon the value 1 signal in time slot  903 . During time slot  904  the serial MSBs for the value 2 are transmitted to second synchronizer  722 . This value 2 is loaded into client  731  upon the value 2 signal in time slot  905 . During time slot  906  the serial MSBs for the value 3 are transmitted to second synchronizer  722 . This value 3 is loaded into client  731  upon the value 3 signal in time slot  907 . 
       FIGS. 10 to 12  illustrate the second technique (Gray coded MSBs and binary coded LSBs).  FIG. 10  illustrates the timestamp value transmission schematically. First timestamp source  1011  supplies MSBs of a timestamp value of serial bits [n,m+1] as an asynchronous Gray coded value. The MSB asynchronous Gray coded value is transmitted during periods of no interest. Because these MSBs change slowly this is generally possible. Second timestamp source  1012  supplies LSBs of a timestamp value of parallel bits [m,p] as an asynchronous binary coded value. The LSB asynchronous Gray coded value is asynchronous to the client clock. First synchronizer  1021  synchronizes each bit of the LSBs bits [m,p] to the client clock. Second synchronizer  1022  keeps a timestamp value corresponding to the MSBs in a shadow register as updated by first timestamp source  1011 . Second synchronizer  1022  supplies the MSBs of the timestamp value synchronous with the client clock as triggered by a load signal from first synchronizer  1021  to client  1031 . This load is triggered by a rollover within the LSBs. Client  1031  receives the MSB bits [n:m+1] from second synchronizer  1022  and the LSB bits [m:P] from first synchronizer  1021 . All bits are received by client  1031  are synchronous with the client clock. Note that bits [p−1:0] are not transmitted to client  1031 . This results in a loss of precision. 
       FIG. 11  illustrates an example of this operation. Column  1110  is the serial MSBs, column  1120  is the parallel LSBs and column  1130  is resultant complete timestamp value which includes bits to be discarded  1135 . A toggle in LSBs  1120  at  1111  triggers the load of MSBs  1110 . A rollover toggle in LSBs  1120  at  1111  triggers the load of MSBs  1110 . A state change is detected at  1111  which marks this time to load the MSBs  1110 . As previously noted, this serial portion of the MSBs is coded for the next value. Therefore the load triggered by the rollover in the LSBs loads the correct value. As previously described discarded bits  1135  are not transmitted to client  1131 . 
       FIG. 12  illustrates the manner of transmitting the serial MSBs. Time slots  1201 ,  1203 ,  1205  and  1207  signal when a rollover toggle occurs in the LSBs. During time slot  1202  the serial MSBs for the value 1 are transmitted to second synchronizer  1022 . This value 1 is loaded into client  1031  upon the value 1 signal in time slot  1203 . During time slot  1204  the serial MSBs for the value 2 are transmitted to second synchronizer  1022 . This value 2 is loaded into client  7101  upon the value 2 signal in time slot  1205 . During time slot  1206  the serial MSBs for the value 3 are transmitted to second synchronizer  1022 . This value 3 is loaded into client  731  upon the value 3 signal in time slot  1207 . Note that bits [3:0] of LSBs  1130  are discarded and not transmitted to client  1031 . This represents a loss of precision in the timestamp value. 
       FIGS. 13 to 15  illustrate the third technique (binary coded MSBs and Gray coded LSBs).  FIG. 13  illustrates the timestamp value transmission schematically. First timestamp source  1311  supplies MSBs of a timestamp value of serial bits [n,m+1] as an asynchronous binary coded value. The MSB asynchronous binary coded value is transmitted during periods of no interest. Because these MSBs change slowly this is generally possible. Second timestamp source  1312  supplies LSBs of a timestamp value of parallel bits [m,0] as an asynchronous Gray coded value. The LSB asynchronous Gray coded value is asynchronous to the client clock. First synchronizer  1321  synchronizes each bit of the LSBs bits [m,0] to the client clock. Second synchronizer  1322  keeps a timestamp value corresponding to the MSBs in a shadow register as updated by first timestamp source  1311 . Second synchronizer  1322  supplies the MSBs of the timestamp value synchronous with the client clock as triggered by a load signal from first synchronizer  1321  to client  1331 . This load is triggered by a rollover within the LSBs. Client  1331  receives the MSB bits [n:m+1] from second synchronizer  1322  and the LSB bits [m:0] from first synchronizer  1321 . All bits are received by client  1331  are synchronous with the client clock. 
       FIG. 14  illustrates an example of this operation. Column  1410  is the serial MSBs, column  1420  is the parallel LSBs and column  1430  is resultant complete timestamp value. A rollover toggle in LSBs  1420  at  1411  triggers the load of MSBs  1410 . As previously noted, this serial portion of the MSBs is coded for the next value. Therefore the load triggered by the rollover in the LSBs loads the correct value. 
       FIG. 15  illustrates the manner of transmitting the serial MSBs. Time slots  1501 ,  1503 ,  1505  and  1507  signal when a rollover toggle occurs in the LSBs. During time slot  1502  the serial MSBs for the value 1 are transmitted to second synchronizer  1322 . This value 1 is loaded into client  1331  upon the value 1 signal in time slot  1503 . During time slot  1504  the serial MSBs for the value 2 are transmitted to second synchronizer  1222 . This value 2 is loaded into client  1331  upon the value 2 signal in time slot  1505 . During time slot  1506  the serial MSBs for the value 3 are transmitted to second synchronizer  1322 . This value 3 is loaded into client  1331  upon the value 3 signal in time slot  1507 . 
       FIGS. 16 to 18  illustrate the fourth technique (binary coded MSBs and binary coded LSBs).  FIG. 16  illustrates the timestamp value transmission schematically. First timestamp source  1611  supplies MSBs of a timestamp value of serial bits [n,m+1] as an asynchronous binary coded value. The MSB asynchronous Gray coded value is transmitted during periods of no interest. Because these MSBs change slowly this is generally possible. Second timestamp source  1612  supplies LSBs of a timestamp value of parallel bits [m,p] as an asynchronous binary coded value. The LSB asynchronous binary coded value is asynchronous to the client clock. First synchronizer  1621  synchronizes each bit of the LSBs bits [m,p] to the client clock. Second synchronizer  1622  keeps a timestamp value corresponding to the MSBs in a shadow register as updated by first timestamp source  1611 . Second synchronizer  1622  supplies the MSBs of the timestamp value synchronous with the client clock as triggered by a load signal from first synchronizer  1621  to client  1631 . This load is triggered by a rollover within the LSBs. Client  1631  receives the MSB bits [n:m+1] from second synchronizer  1622  and the LSB bits [m:p] from first synchronizer  1611 . All bits are received by client  1631  are synchronous with the client clock. Note that bits [p−1:0] are not transmitted to client  1031 . This results in a loss of precision. 
       FIG. 17  illustrates an example of this operation. Column  1710  is the serial MSBs, column  1720  is the parallel LSBs and column  1730  is resultant complete timestamp value which includes bits to be discarded  1735 . A toggle in LSBs  1720  at  1711  triggers the load of MSBs  1710 . A rollover toggle in LSBs  1720  at  1711  triggers the load of MSBs  1710 . A state change is detected at  1711  which marks this time to load the MSBs  1710 . As previously noted, this serial portion of the MSBs is coded for the next value. Therefore the load triggered by the rollover in the LSBs loads the correct value. As previously described discarded bits  1735  are not transmitted to client  1731 . 
       FIG. 18  illustrates the manner of transmitting the serial MSBs. Time slots  1801 ,  1803 ,  1805  and  1807  signal when a rollover toggle occurs in the LSBs. During time slot  1802  the serial MSBs for the value 1 are transmitted to second synchronizer  1622 . This value 1 is loaded into client  1631  upon the value 1 signal in time slot  1803 . During time slot  1804  the serial MSBs for the value 2 are transmitted to second synchronizer  1622 . This value 2 is loaded into client  1631  upon the value 2 signal in time slot  1805 . During time slot  1806  the serial MSBs for the value 3 are transmitted to second synchronizer  1622 . This value 3 is loaded into client  1631  upon the value 3 signal in time slot  1807 . Note that bits [3:0] of LSBs  1730  are discarded and not transmitted to client  1631 . This represents a loss of precision in the timestamp value. 
     The second and fourth techniques have a loss in precision of the timestamp. Synchronization of one of the binary count values and its edge detection introduces a latency. This latency causes a truncation of timestamp precision. 
     There are several techniques for the serial transmission of the MSBs of the timestamp.  FIG. 19  illustrates a serial clock independent of the client clock. This serial clock is used to transfer the MSBs of the timestamp value to synchronizer for the MSBs. The serial clock triggers a read of serial timestamp_in in synchronism. This forms time_stamp_sr_value in the shadow register. The load_timestamp_value signal is triggered by a rollover toggle of the timestamp LSBs. This triggers a load of the accumulated serial MSBs (timestamp_MSBs) to the client in synchronism with client clock. 
       FIG. 20  illustrates hardware to implement the technique illustrated in  FIG. 19 . Shift register  2001  is clocked by the serial clock. Upon each instance of the serial clock shift register  2001  accumulates the timestamp data serial timestamp_in. The accumulated value within shift register  2001  is designated time_stamp_sr_value. When the load_timestamp_value signal is active commanding a load operation, register  2002  loads time_stamp_sr_value from shift register  2001  upon the next client clock. Once loaded into register  2002  the value is available to the client in parallel form as Timestamp MSBs. 
       FIG. 21  illustrates a second embodiment including an enable signal. This enable signal controls transfer of the MSBs of the timestamp value to synchronizer for the MSBs. A shift signal triggers a read of serial timestamp_in on the next client clock following the enable signal serial_timestamp_enable. This forms time_stamp_sr_value in the shadow register. The load_timestamp_value signal is triggered by a rollover toggle of the timestamp LSBs. This triggers a load of the accumulated serial MSBs (timestamp_MSBs) to the client in synchronism with client clock. 
       FIG. 22  illustrates hardware to implement the technique illustrated in  FIG. 21 . Edge detector  2201  is enabled by serial_timestamp_enable. Edge detector  2201  triggers upon the next clock edge from client clock. Edge detector  2201  enabled shift register  2201 . Upon each enable signal from edge detector  2201  serial clock shift register  2202  accumulates the timestamp data serial timestamp_in. The accumulated value within shift register  2202  is designated time_stamp_sr_value. When the load_timestamp_value signal is active commanding a load operation, register  2203  loads time_stamp_sr_value from shift register  2202  upon the next client clock. Once loaded into register  2203  the value is available to the client in parallel form as Timestamp MSBs. 
       FIG. 23  illustrates distribution of the timestamp value according to this invention. Timestamp generator  2301  produces the timestamp value including both the MSBs and the LSBs. Timestamp generator supplies the LSBs of the timestamp value in parallel on parallel timestamp bus  2303 . This includes data lines equal in number to the number of data bits in the timestamp LSBs. Timestamp generator supplies the MSBs of the timestamp value sequentially in series on serial timestamp bus  2305 . This includes a single data line to transmit all the timestamp MSBs. Both parallel timestamp bus  2303  and serial timestamp bus  2305  are connected to clients  2310 ,  2320  and  2330 . This will describe representative client  2310 . Parallel register  2311  receives the parallel LSBs of the timestamp value from parallel timestamp bus  2303 . Series register  2312  receives the serial MSBs from serial timestamp bus  2305  in the manner described above in conjunction with  FIGS. 19 to 22 . As detailed above the timestamp MSBs are serially accumulated and captured upon each load_timestamp_value signal. Merged timestamp register  2313  is connected to both parallel register  2311  and series register  2312  to assemble the timestamp value. The timestamp value stored in merged timestamp register  2313  is used as known in the prior art to capture the time of trace data used for debug. 
     Within a system, there may be a number of clock domains operating at a different frequency. It may be advantageous to serially transmit s different number of bits of the timestamp serially for each of the domains depending on the client clock frequency. The ratio between parallel LSBs and serial MSBs depends upon the timestamp precision required relative to the serial data transmission rate. The serial timestamp bus  2305  must be able to transmit all timestamp MSBs between each rollover toggle of the timestamp LSBs. Different client clock rates may require differing ratios of MSBs to LSBs.