Abstract:
Methods and apparatuses to multiplex logic data pseudo synchronously are described. A representation of a multiplexer logic is generated to transmit data items asynchronously relative to a design clock. The data items may be transmitted under control of a transmission clock from a first integrated circuit to a second integrated circuit. A representation of a counter logic may be generated to couple with the multiplexer logic for transmitting the data asynchronously. Additionally, a representation of reset logic may be generated for a configuration to repeatedly reset the counter logic. Synchronization signals may be generated for a design clock cycle of a design clock driving the data items. The synchronization signals may be transmitted via the transmission clock asynchronous with the design clock. The data items may be transmitted via a number of transmission slots determined based on the clock cycles of the transmission clock and the design clock The total time for the transmission slots for transmitting the logic data may be less than the clock cycle of the design clock. One or more transmission slots within the clock cycle of the design clock may be used to transmit the synchronization data to indicate a new cycle to transmit the data items according to the design clock.

Description:
FIELD OF THE INVENTION 
     The invention relates to digital circuits, and more particularly to the design of digital circuits with Time Division Multiplexing (TDM) of digital signals. 
     BACKGROUND OF THE INVENTION 
     One of the advantages of a time domain multiplexing scheme to transmit digital signals is that a plurality of digital signals or channel signals can be transmitted sharing a single transmission channel. For example, TDM is commonly applied in ASIC (application specific integrated circuit) and FPGA (field programmable gate array) designs to reduce pin counts used for communication to other chips. As chip densities have increased, pin counts have not kept pace making TDM based communication more attractive. In ASIC, prototypes may be built by mapping partitions of the ASIC design onto FPGAs. Because ASIC modules were designed to be connected on a single chip, they often have 5-10 times the connections between them as are available in IC packages. If a connection between chips is being shared many times per design clock cycle, then the performance of transmission will limit the speed of the system. Improvements in the TDM transmission rate are then highly valuable. 
     Typically, TDM is either synchronous or asynchronous Synchronous TDM can be high performance because signals to be transmitted can be assigned a single time slot per clock cycle, allowing efficient use of the available time. However, maintaining accurate clock synchronization between multiple ICs can be both expensive and difficult, requiring the creation and distribution of high frequency clocks associated with each clock involved in chip interconnection. Failure to maintain accurate synchronization between chips limits the rate of transmission. The use of source synchronous transmission and buffering in a FIFO (First In First Out) can be used to overcome the synchronization issue where latency is not important. However, in many uses such as ASIC prototyping, latency has tight constraints and a FIFO can&#39;t be used. 
     Asynchronous TDM uses a high frequency transmission clock that is not synchronized to the user clock. Instead, the data is sampled and transmitted multiple times per design clock cycle, such as describe in, for example, U.S. Pat. No. 7,007,254 entitled “Methods and Apparatus for the Design and Analysis of Digital Circuits with Time Division Multiplexing” to Drazen Borkovic and Kenneth S. McElvain. The virtues of the asynchronous scheme are that a single TDM clock can be distributed with loose synchronization constraints and there is no complex slot assignment to determine as the transmission slots are not synchronized to the design clocks anyway. In such a scheme the time between successive samples of a given design signal becomes part of the chip-to-chip delay. For high TDM ratios where many distinct signals are sharing the same channel, this oversampling delay can become quite large and is the major deficiency of the scheme. 
     Therefore, traditional TDM schemes fail to meet the performance and resource requirements in heavily interconnected multi-device systems such as ASIC prototypes and there is a need for improvement. 
     SUMMARY OF THE DESCRIPTION 
     Methods and apparatuses for designing digital circuits with pseudo synchronous time division multiplexing are described here. Some embodiments of the present inventions are summarized in this section. 
     In at least one embodiment of the present invention, the number of data item transmission slots can be determined from the periods of the transmission clock and the design clock. A data item is a multi-valued symbol, a special case being a Boolean value. A number of transmission slots for data items can be determined as the number of slots such that the total time for the data item slots is less than the period of the design clock. Most of the slots may be used to transmit data item values with some slots used for synchronization information. 
     In one aspect of the invention, a representation of a slot counter with a range larger than the number of transmission slots can be generated. The counter can be reset by the edge of the design clock that generates the data to be transmitted. The counter value can be used to control input selection logic which selects and transmits the data item associated with each numbered slot. 
     In another aspect of the invention, in the receiving chip, a representation of a slot counter is generated. The values of the receiver counter can be used to distribute and/or de-multiplex the stream of data values 
     In yet another aspect of the invention, after all the data items have been transmitted, which can be determined by the transmit counter counting past the number of transmission slots, the input selection logic can select and/or transmit ‘0’ values. When the counter is reset to 0, the input selection logic can select a value of ‘1’ and/or transmit it in a specific numbered slot, such as slot  0  or slot  1 . In the receiving chip or circuit, a reset generation circuit can be generated for the receiver counter. After the receive counter has counted through all the determined slot numbers, the receive reset generation circuit can wait for a ‘0’ to ‘1’ transition in the input data stream, which identifies the transmit slot number of the ‘1’ value and the counter can be reset to match the identified slot number. This mechanism can maintain agreement on the slot numbers between the transmit and receive circuits despite variation of the clock frequencies as long as the TDM clock is sufficiently faster than the design clock. The result of this mechanism is to roughly synchronize the positions of the numbered data item transmission slots with respect to the design clock edges without synchronizing the design clock and the transmission clock. This slot synchronization without clock synchronization is why we call the method pseudo-synchronous TDM. 
     In yet another aspect of this invention, multiple parallel TDM channels may share some or all of the synchronization elements including reset generation circuits and counters. Only a single member of the group of parallel channels would then need to send synchronization data, freeing a few more transmission slots for sending data items. 
     In yet another aspect of this invention, a representation of multiplexer logic may be generated to transmit data items asynchronously relative to a design clock. The data items may be transmitted under control of a transmission clock from a first integrated circuit to a second integrated circuit. A representation of a counter logic may be generated to couple with the selection logic for transmitting the data items in a sequence of time slots within a clock cycle of the design clock. Additionally, a representation of reset logic may be generated for a configuration to repeatedly reset the counter logic. 
     In yet another aspect of the invention, a representation of de-multiplexing logic may be generated to receive data asynchronously relative to a design clock. The data may be received under control of a transmission clock from a first integrated circuit to a second integrated circuit. A representation of a counter logic which is coupled to the representation of the extraction logic may be generated to receive the data asynchronously. Additionally, a representation of a reset logic may be generated with a configuration to repeatedly reset the counter logic 
     In another aspect of the invention, synchronization signals may be generated for a design clock cycle of a design clock driving a logic device. The synchronization signals may be transmitted via a transmission clock asynchronous with the design clock. Subsequently, data items from the logic device may be transmitted within a clock cycle of the design clock via a multiplexing scheme clocked by the transmission clock. 
     In yet another aspect of the invention, synchronization signals may be detected as received from an input port of a logic device according to a transmission clock. One or more signals may be de-multiplexed from the input port clocked by the transmission clock subsequent to the detection of the synchronization signals. De-multiplexed signals may be buffered within a clock cycle of a design clock driving the logic device asynchronous to the transmission clock. 
     In yet another aspect of the invention, a digital circuit may include design logic, synchronization logic and a multiplexing logic for communicating multiple signals over a communication port of a chip. The design logic may generate the signals clocked by a design clock. The synchronization logic driven by the design clock and a transmission clock may generate synchronization signals to approximately align the transmission slots based on the asynchronous transmission clock with the design clock. The multiplexing logic coupled with the design logic and the synchronization logic may transmit the synchronization signals and the multiple signals over the communication port according to the transmission clock 
     In yet another aspect of the invention, the earliest possible sampling time relative to the design clock edge of each transmission slot is computed. This computation may take into account the earliest possible reset of the transmission counter. The latest possible delivery time relative to the receiving design clock edge may also be computed. This computation may take into account the latest possible time of reset of the transmission counter. Then, design signals may be assigned to slots so that they meet the timing analysis requirements implied by the computed sampling and delivery times 
     The present invention includes methods and apparatuses which perform these methods, including data processing systems which perform these methods, and computer readable media which when executed on data processing systems cause the systems to perform these methods. 
     Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIG. 1  illustrates a multi chip digital design including a pseudo synchronous TDM according to one embodiment of the present invention; 
         FIG. 2  illustrates an exemplary digital circuit including pseudo synchronous time division multiplexer according to one embodiment of the present invention; 
         FIG. 3  is a timing diagram illustrating one example of pseudo synchronization between asynchronous clocks for time division multiplexing according to one embodiment of the present invention; 
         FIG. 4  illustrates a multi chip digital design including a pseudo synchronous TDM multiplexer transmitting synchronization signals according to one embodiment of the present invention; 
         FIG. 5  is a flow diagram illustrating an embodiment of a control technique to multiplex data pseudo synchronously based on asynchronous clocks according to one embodiment of the present invention; 
         FIG. 6  is a flow diagram illustrating an embodiment of a control technique to de-multiplex data pseudo synchronously based on asynchronous clocks according to one embodiment of the present invention; 
         FIG. 7  is a flow diagram illustrating a process to create a design across multiple chips communicated pseudo synchronously according to one embodiment of he present invention; 
         FIG. 8  is a flow diagram illustrating a process to perform timing analysis for a design according to one embodiment of he present invention; 
         FIG. 9  shows an example of a data processing system which may be used with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of the present invention. However, in certain instances, well known or conventional details are not described in order to avoid obscuring the description of the present invention. 
     Many of the methods of the present invention may be performed with a digital processing system, such as a conventional, general purpose computer system. Special purpose computers which are designed or programmed to perform only one function may also be used. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     The processes depicted in the figures that follow, are performed by processing logic that comprises hardware (e.g., circuitry, dedicated logic, etc.), software (such as is run on a general-purpose computer system or a dedicated machine), or a combination of both. Although the processes are described below in terms of some sequential operations, it should be appreciated that some of the operations described may be performed in different order. Moreover, some operations may be performed in parallel rather than sequentially 
     At least one embodiment of the present invention seeks to transmit digital signals via a pseudo synchronous TDM system including positions of indexed time slots vary over the range of a clock cycle (or user clock cycle) of a source design clock. Using an edge of the source design clock, a counter or a state machine may be reset to position time slot  0  (first time slot within the clock cycle associated with the edge) periodically to be close to the clock edge. A source design clock may be identified by traversing from signals being transmitted back to flip-flops in a design. Positioning of time slot  0  or other time slots relative to the design clock edge may be based on timing analysis, such as in synchronous TDM based signal transmission. Each signal (or logic data) may be transmitted consuming only one time slot. As a result, bandwidth is increased and effective delay is dramatically decreased relative to asynchronous TDM implementations. 
     For example, at a 1 GHz transmission rate, a 64 to 1 multiplexing ratio, with 20 ns representing the delay of the multiplexer and demultiplexer plus the chip to chip interconnect delay, would result in an effective delay of 84 ns (or 64 ns+20 ns) for an asynchronous TDM scheme limiting the system frequency to about 10 MHz. In comparison, a corresponding pseudo synchronous TDM scheme would have an effective delay of about 28 ns (or 8 ns+20 ns) yielding a system speed over 25 MHz. 
       FIG. 1  illustrates a multi chip digital design including a pseudo synchronous TDM according to one embodiment of the present invention. A multiplexer may combine multiple signals into a single signal stream to share one transmission resource. Typically, a digital design may include design modules spread across multiple chips such as a sender chip  101  and a receiver chip  115 . In one embodiment, chips  101  and/or  115  may be programmable logic devices, such as FPGA devices to enable emulation of the digital design. Chip interconnects  117  may be physical wires between chips. In some embodiments, chip interconnects  117  may be based on optical or wireless connections. Usually, data signals passing among design modules across chips, e.g. chips  101  and  115 , may share the same physical connections  117  via different time slots. 
     Inside the sender chip  101 , design modules may be configured into one or more blocks  105 ,  107  coupled by interconnects, such as interconnect  111 . In one embodiment, a design clock  109  drives multiple logic blocks  105 ,  107  within the sender chip  101 . The design clock  109  may be based on a clock source driving multiple chips including the receiver chip  115  to perform computations. 
     In one embodiment, the sender chip  101  transmits logic data from design blocks within the chip, such as  105  and  107 , to an outside chip, such as receiver chip  115  according to a transmission clock driving an IO logic, such as TDM IO  111 . A transmission clock  113  may drive a corresponding IO port of the receiver chip  115  to receive logic data from the sender chip  101  via chip interconnects  117 . In one embodiment, transmission clocks for logic data between a source chip, e.g. the sender chip  101 , and a target chip, e.g. the receiver chip  115 , are synchronous or based on a single clock source. In another embodiment, the transmission clock on each chip is not controlled with respect to the transmission clocks on other chips and receive logic in a receiver chip is clocked by a source synchronous clock sent alongside the data. 
     TDM IO  111  may transmit logic data from the sender chip  101  to a corresponding TDM IO  119  of the receiver chip  115  in a pseudo synchronous TDM manner. TDM IO  119  may include a pseudo synchronous TDM de-multiplexer which extracts multiple signals from a single signal stream received. In one embodiment, TDM IO  111  are coupled with both the transmission clock  103  and the design clocked  109  to transmit logic data based on a TDM scheme which separately synchronizes each clock cycle of the design clock  109  with multiple clock cycles or time slots of the transmission clock  103 . In one embodiment, the design clock  109  driving logic operations and the transmission clock  103  driving data transmission across chips are asynchronous (e.g. without known phase or specific frequency relationships between each other). 
       FIG. 2  illustrates an exemplary digital circuit including pseudo synchronous time division multiplexer according to one embodiment of the present invention. Exemplary circuit  200  may be implemented in a chip such as sender chip  101  of  FIG. 1 . IO logic, such as TDM IO  111  of  FIG. 1  may include reset generator logic  211 , a slot counter logic  213  and/or a multiplexer  215 . Logic data  217  may be output data from user logic  209 , such as logic  105  and/or logic  107  of  FIG. 1 . In one embodiment, multiple logic data items  217  are transmitted to another chip via the multiplexer  215  in a pseudo synchronous TDM manner sharing interconnect wires  219 , such as interconnects  117  of  FIG. 1 , across separate chips. 
     Exemplary circuit  200  may be driven by multiple asynchronous clocks, such as clock design_clk  203  and clock tdm_clk  201 . User logic  209  may perform logic operations or computations to generate logic data  217 , e.g. including multiple binary values, clocked by clock design_clk  203 . Clock design_clk  203  may be generated according to, for example, the design clock  109  of  FIG. 1 . Sequence elements  205 ,  207  may be registers, flip-flops, or memory elements to pipeline clock signals to drive user logic  209 . In one embodiment, clock tdm_clk  201  may be generated according to, for example, the transmission clock  103  of  FIG. 1 , to drive transmission of digital data across chips. Typically, clock tdm_clock  201  will be of higher frequency than clock design_clk  203 . 
     In one embodiment, slot counter  213  generates a count for the multiplexer  215  to select logic data  217  to transmit over a time slot according to a transmission clock such as clock tdm_clk  201 . Slot counter  213  may control the selection order of logic data  217  for transmission. Typically, a time slot for the multiplexer  215  corresponds to a clock cycle of clock tdm_clk  201 . In one embodiment, the multiplexer  215  provides a separate time slot for each logic data  217  within a clock cycle of clock design_clk  203 . As a result, all values of logic data  217  generated from the user logic  209  for each clock of clock design_clk  203  are transmitted across interconnects  219  during the corresponding clock cycle of clock design_clk  203 . 
     Synchronization signals (or synchronization data) to align asynchronous clock design_clk  203  and clock tdm_clk  201  may be generated for each clock cycle of clock design_clk  203 . In one embodiment, a multiplexer  215  is configured to generate the synchronization signals according to certain counts from the slot counter  213 . For example, the slot counter  213  may include a saturation count and an initial count for the multiplexer  215  to select signal values to generate the synchronization signals. 
     A saturation count may be the largest count configured for a counter. A counter is saturated when the counter counts to its saturation count (or counter range) from an associated driving clock. A saturated counter may not perform additional counting in response to a driving clock. Typically, when a counter is reset, it is set to an initial count, e.g. 0, on the next clock cycle of its driving clock. A counter may be reset when receiving a reset signal. 
     In one embodiment, synchronization signals are based on signal values selected by the multiplexer  215  according to the initial and reset counts of the slot counter  213 . A reset generator  211  may generate a reset signal for the slot counter  213 . When the slot counter  213  counts between the initial and saturation counts, the multiplexer  215  may select logic data  217  for transmission. In one embodiment, once the slot counter  213  saturates, the multiplexer  215  may output a configured initial synchronization signal value, e.g. logic low or 0. 
     On receipt of a reset signal from the reset generator  211 , the slot counter counts to its initial count (reset) on the next clock cycle of clock tdm_clk  201 . In one embodiment, the multiplexer  215  outputs subsequent synchronization signal values configured based on the slot counter  213  counting from the initial count according to clock tdm_clk  201 . For example, the synchronization signal transmitted via clock tdm_clk  201  may include a single logic high and/or a transition from a logic low to a logic high which can be unambiguously recognized. 
     According to one embodiment, the reset generator  211  derives reset signals from edges of design_clk  203 . For example, the reset generator  211  can generate a reset signal to reset the slot counter  213  on the edge of each clock cycle of clock design_clk  203 . The edge used may depend on the clock edge that generates the data to be transmitted. Typically, the number of time slots according to clock tdm_clk  201  within a clock cycle of clock design_clk  203  is more than the required number of time slots to transmit the logic data  217  via the multiplexer  215 . Extra time slots may be allocated for digital circuit  200  to accommodate the synchronization signals. 
     In certain embodiments, multiple interconnects may transmit data in parallel sharing the same logic blocks for synchronizing data transmissions, such as reset generator  211  and/or slot counter  213  of  FIG. 2 . Each interconnect may be associated with a channel for transmitting data. For example, multiple channels, including the channel for interconnect  219 , may share synchronization logic blocks such as slot counter  213  and reset generator  211  to transmit data in parallel for design clock  203 . Consequently, additional data slots can be provided via parallel channels without the cost of extra synchronization logic blocks. In some embodiments, parallel channels share synchronization signals transmitted via a particular one of the parallel channels. A channel which does not need to transmit synchronization signals may transmit data concurrently while the particular channel transmits shared synchronization signals, e.g. during the same transmission slots. 
       FIG. 3  is a timing diagram illustrating one example of pseudo synchronization between asynchronous clocks for time division multiplexing according to one embodiment of the present invention. For example, design clock  301  may include a clock cycle of a design clock driving user logics in a chip, such as design_clk  203  of  FIG. 2 . Slot sequences  305 ,  307  may illustrate two exemplary assignments of time slots by a pseudo synchronous TDM multiplexer, such as multiplexer  215  of  FIG. 2 , driven by a transmission clock, such as tdm_clk  201  of  FIG. 2 , aligned differently with the deign clock  301 . A time slot or transmission slot may correspond to an edge (e.g. rising edge and/or falling edge) of a clock cycle of the transmission clock. A time slot may not include a complete clock cycle of the transmission clock. 
     In one embodiment, slot sequence  307  includes eight time slots slot_ 0  . . . slot_ 7 , for transmitting logic data  315  and two slots, sync slots  317 ,  319 , for synchronization signals  313 . Time slot slot_ 0  may be designated with label “0”, time slot slot_ 1  may be designated with label “1”, . . . etc. Two design clock cycles (or periods) may include commonly labeled time slots, e.g. time slots labeled as “0”, time slots labeled as “1”, . . . etc. Slot sequence  307  may correspond to an 8 by 1 multiplexer. Synchronization signals may include a pattern of transition from logic low, e.g. sync slot  317 , to logic 1, e.g. sync slot  319 . In one embodiment, a receiving port of the synchronization signals identifies the next time slot, slot  321 , subsequent to receiving the synchronization signals (e.g. based on recognition of the unique signal pattern), as the first slot assigned to transmit logic data. Synchronization signals (or data) may indicate positions of time slots (or transmission slots) positioned within a clock cycle of a design clock. Thus synchronization logic generating the synchronization signals may select or determine which label to assign to each time slot. The timing of a time slot may be determined relative to a clock edge of a clock cycle (or period) of the transmission clock. In certain embodiments, multiple time slots may be positioned overlapping in time to transmit multiple data concurrently, e.g. using multiple voltage levels and/or different frequency light. 
     In certain embodiments, each labeled time slot is positioned with an offset (or time difference) from an edge of a clock cycle of a design clock associated with the labeled time slot. For example, slot_ 0   312  may be associated with an offset  325  relative to an edge  303  of design clock  301 . Offsets associated with commonly labeled time slots may vary from one design clock cycle to another design clock cycle. Variations of offsets for commonly labeled time slots among different design clock cycles may be maintained within a predetermined bound, e.g. one or two transmission clock cycles of a transmission clock corresponding to the time slots. 
     In one embodiment, a counter corresponding to slot sequence  307 , e.g. slot counter  213  of  FIG. 2 , saturates at value 9. As a result, a synchronization signal value of 0 (logic low) corresponding to the saturation count of the counter is forced at the next time slot, e.g. sync slot  317 , from an associated pseudo synchronous TDM multiplexer, e.g. multiplexer  215  of  FIG. 2 . At time instance  303 , in one embodiment, a rising edge of design clock  301  triggers a reset signal, e.g. via a reset generator  211  of  FIG. 2 , to reset the counter. Subsequently, at time instance  311  according to a transmission clock driving the counter, e.g. tdm_clk  201  of  FIG. 2 , the multiplexer generates a synchronization value of 1 (logic high) corresponding to the initial count of the counter for sync slot  319 . Thus, the clock cycle of the design clock  301  is calibrated with a transmission clock corresponding to slot sequence  307  via synchronization signals  313 . Logic data  315  is multiplexed in the following time slots as indexed from 0 to 7 starting at time slot  321 . The number of sync slots with value 0 may differ from one design clock cycle to another design clock cycle. 
       FIG. 4  illustrates a multi chip digital design including a pseudo synchronous TDM multiplexer transmitting synchronization signals according to one embodiment of the present invention. For example, exemplary digital design  400  may be included in the sender chip  101  and/or receiver chip  115  of  FIG. 1 . In one embodiment, logic data  419  from source logic block  413  driven by clock design_clk  403  are transmitted across chip interconnects  117  by a transmission clock tdm_clk  405  asynchronous to clock design_clk  403 . Interconnects  117  may be coupled to a transmitter logic driven by the transmission clock tdm_clk  405 . Logic data  419  may be transmitted within one clock cycle of clock design_clk  403  aligned (e.g. pseudo synchronously) with clock tdm_clk  405 , e.g. as illustrated in timing diagram  300  of  FIG. 3 . Clocks design_clk  403  and tdm_clk  405  may respectively correspond to design clock  109  and transmission clock  103  of  FIG. 1 . 
     In one embodiment, synchronization logic  411  is coupled with a multiplexer  415  to provide synchronization signal values for generating synchronization signals for the receiver chip  115 . The synchronization logic  411  may include storage elements configured with synchronization signal values selectable via the multiplexer, e.g. according to a count from a sender counter  409 . In one embodiment, the sender counter  409  clocked by clock tdm_clk  405  saturates at value N for the multiplexer  415  to select a first synchronization value from the sync logic  411  at the next time slot assigned according to clock tdm_clk  405 . A clock cycle of clock design_clk  403  may trigger a reset generator  407  to set the sender counter  409  at a reset state. In one embodiment, a rising edge of a clock cycle triggers the reset generator  407  to generate a reset signal to reset the sender counter  409 . Subsequently, at the next clock cycle of clock tdm_clk  405 , the sender counter resets to an initial count for the multiplexer  415  to select a second synchronization value from the sync logic  411  at the corresponding time slot. Synchronization signals including the first and second synchronization values may be transmitted to the receiver chip  115  to align the time slots for transmitting logic data  419  for the receiver chip  115 . 
     In one embodiment, the receiver chip  115  includes a de-multiplexer  431  to receive logic data and synchronization signals transmitted from the sender chip  101  in a pseudo synchronous manner. The receiver chip  115  may be a static or programmable logic device, such as an FPGA device. Logic data  419  collected over a series of time slots according to a transmission clock, such as clock r_tdm_clk  423 , may be de-multiplexed into logic data  433  and latched (or buffered) in order for target logic block  435 . In one embodiment, clock r_design_clk  439  drives target logic block  435  to perform computations. Clocks r_design_clk  439  of the receiver chip  115  and design_clk  403  of the sender chip may be asynchronous. In some embodiments, clocks r_design clk  439  and design_clk  403  are of a common frequency. 
     The receiver chip  115  includes a receiver counter  425  and a sync detection (or synchronization) logic  429  coupled with the de-multiplexer  431  to align counter values of receiver counter  425  with time slots (or transmission slots), such as assigning time slots for received logic data to the target logic block  435 . For example, the sync detection logic  429  may recognize synchronization signals sent from the sender chip  101  to indicate a start of a sequence of time slots assigned for logic data  433 . Multiplexed logic data received via interconnects  117  may be de-multiplexed according to a count counted by receiver counter  425  via the de-multiplexer  431 . In one embodiment, the receiver counter is configured to share a common saturation count with the sender counter  409 . Synchronization signals transmitted from the sender chip  101  may be forwarded to the sync detection logic  429  as sync signals  437  via the de-multiplexer  431  when the receiver counter  425  is saturated. 
     The sync detection logic  429  may send a reset signal  427  to reset the receiver counter on detection of synchronization signals received via the de-multiplexer  431 . In one embodiment, the next time slot immediately following when the receiver counter  425  is reset starts the sequence of time slots for the de-multiplexed logic data. The receiver counter may be counted by clock r_tdm_clk  423  for the de-multiplexer  431 . In one embodiment, clocks r_tdm_clk  423  and tdm_clk  405  share the same clock source as the transmission clock driving the chip interconnects  117 . The sync detection logic  429  may recognize a synchronization pattern, such as a signal level change from logic low to logic high from synchronization signals received. The sync detection logic  429  and the corresponding sync logic  411  may be configured with a common synchronization pattern. In some embodiments, the receiver counter  425  and the sender counter  409  are aligned during a training period, during which logic data sent from the sender chip  101  are of a constant know value, such as logic low (0). 
       FIG. 5  is a flow diagram illustrating an embodiment of a control technique to multiplex data pseudo synchronously based on asynchronous clocks according to one embodiment of the present invention. For example, control flow  500  may be performed by a digital design  100  as shown in  FIG. 1 . In one embodiment, at block  501 , the logic of control flow  500  generates synchronization signals per clock cycle of a design clock, such as design clock  109  of  FIG. 1 , driving a logic device. Multiple logic data from the logic device may share a single physical connection, such as interconnect  117  of  FIG. 1 , for coupling with another logic device, such as chip  115  of  FIG. 1 . Each logic data may pass through the physical connection per clock cycle of the associated design clock. 
     In one embodiment, the logic of control flow  500  may generates synchronization signals based on a state of a state machine, such as according to a counter counted by a counter logic. The logic of control flow  500  may determine a state for generating the synchronization signals according to an edge of the design clock (such as a rising edge). Thus, the synchronization signals are generated to align with each clock of the design clock. Synchronization signals may include predetermined patterns which can by unambiguously identified, for example, within a period corresponding to a clock period (or cycle) of the design clock. 
     At block  503 , according to one embodiment, the logic of control flow  500  transmits the synchronization signals over a physical interconnect according to a transmission clock. The transmission clock and the design clock may be asynchronous (e.g. without a known phase or frequency relationship), such as clocks  309  and  303  of  FIG. 3 . Each clock cycle of the design clock may be long enough to include a predetermined number of clock cycles of the transmission clock (e.g. based on the length of the synchronization signals and the number of logic data to be transmitted for each clock cycle of the design clock). In one embodiment, the logic of control flow  500  counts a counter according to each clock of the transmission clock to determine a state of a state machine to generate the synchronized signals. 
     At block  505 , the logic of control flow  500  may transmit data signals to multiplex logic data from a logic device to a target device according to a transmission clock. The logic of control flow  500  may perform data signal transmission subsequent to transmitting the synchronization signals, for example, starting in the next clock of the transmission clock transmitting the synchronization signals or based on other timing relationships. In one embodiment, the logic of control flow  500  updates a state machine to determine when all data values have been transmitted during a clock cycle of a design clock. The logic of control flow  500  may start synchronization signals subsequent to when the transmission of logic data is complete during a clock cycle of the design clock. 
       FIG. 6  is a flow diagram illustrating an embodiment of a control technique to de-multiplex data pseudo synchronously based on asynchronous clocks according to one embodiment of the present invention. For example, control flow  600  may be performed by a digital design  100  as shown in  FIG. 1 . In one embodiment, at block  601 , the logic of control flow  600  detects synchronization signals clocked by a transmission clock to an input port of a logic device, such as receiver chip  115  of  FIG. 1 . The logic device receiving the synchronization logic may be clocked by a design clock asynchronous to the transmission clock. Each logic data may pass through the physical connection per clock cycle of the associated design clock. 
     The logic of control flow  600  may determine when to start detecting the synchronization signals during each clock cycle of a design clock based on a state of a state machine, such as according to a counter counted by a counter logic  425  of  FIG. 4 . The state machine may be driven by the transmission clock clocking the synchronization signals. In one embodiment, detection of synchronization signals may start when the counter indicates a predetermined number of logic data has been received during a clock cycle of the design clock. In one embodiment, the logic of control flow  600  identifies synchronization signals according to patterns of transitions of signals received. A particular transition, such as from logic 1 to logic 0, may indicate the end of synchronization signals during a clock cycle of the design clock. 
     In one embodiment, subsequent to the detection of the synchronization signals, at block  603 , the logic of control flow  600  stores data signals received into separate buffers to de-multiplex logic data transmitted via an input port of a logic device clocked by a design clock. The logic of control flow  600  may count the number of logic data values received according to the transmission clock driving the transmission of the logic data values through to the input port. In one embodiment, the logic of control flow  600  starts counting the number of logic data values received according to the transmission clock (e.g. counting the number of clock cycles of the transmission clock). The logic of control flow  600  may wait for synchronization signals after a predetermined number of logic data values have been received. In one embodiment, the logic of control flow  600  repeats detecting the synchronization signals and de-multiplexing the predetermined number of logic data for each clock of a design clock asynchronous to the transmission clock. 
       FIG. 7  is a flow diagram illustrating a process to create a design across multiple chips communicated pseudo synchronously according to one embodiment of the present invention. Exemplary process  700  may be performed by a processing logic that may comprise hardware (circuitry, dedicated logic, etc.), software (such as is run on a dedicated machine), or a combination of both. For example, a software tool may be configured to automatically implement a design across multiple chips communicated pseudo synchronously according to process  700 . In one embodiment, at block  701 , the processing logic of process  700  groups data items (or signals) of a design by edges of design clocks driving the design. The processing logic of process  700  may identify an edge of a design clock by traversing the design from data items being transmitted back to a flip-flop of the design. Each edge or clock edge of a design clock may correspond to a clock cycle of the design clock. In one embodiment, a design may be based on an HDL (Hardware Description Language) description. All data items of the same group corresponding to a clock edge may arrive within a clock cycle of the clock edge. In one embodiment, at block  703 , the processing logic of process  700  may perform timing analysis to generate timing requirements for assigning a data signal/value to a channel/slot. The processing logic of process  700  may calculate arrival time for transmitting each signal of the same group relative to the clock edge for a clock cycle corresponding to the group. 
     In one embodiment, at block  705 , the processing logic of process  700  inserts a circuit for each design clock to multiplex signals (or logic data) driven by the clock in a pseudo synchronous manner. The time period for a clock cycle of the design clock may include one or more time slots (or channels), such as in slot sequence  305  for a design clock  301  of  FIG. 3 . A transmission clock which is asynchronous to the design clock may allocate the time slots based on clock cycles of the transmission clock. The circuit may multiplex logic data corresponding to a group of data items according to the number of slots within each clock cycle of the design clock. For each data item of the same group, at block  707 , the processing logic of process  700  may assign a separate time slot within a clock cycle according to satisfy the timing constraints generated from a timing analysis. For example, a time slot (or transmission slot) for a data item or signal value may be assigned based on the arrival time of the data value calculated. In one embodiment, the assignment of time slots is based on the earliest time logic data could be sampled for the corresponding data item. 
     At block  709 , the processing logic of process  700  may create an implementation of the design, such as generating a netlist including pseudo synchronous time domain multiplexer circuits. In one embodiment, the processing logic of process  700  loads (or installs) the design implementation (e.g. netlists stored in a memory) into one or more hardware devices, such as PLDs (programmable logic device) or FPGAs, to configure the design. The processing logic of process  700  may emulate (prototype) the design including pseudo synchronous multiplexing logic data across multiple chips according to the configured programmable devices. For certain embodiments, the processing logic of process  700  may also be based on application-specific integrated circuits (ASICs). 
       FIG. 8  is a flow diagram illustrating a process to perform timing analysis for a design according to one embodiment of the present invention. Exemplary process  800  may be performed by a processing logic that may comprise hardware (circuitry, dedicated logic, etc.), software (such as is run on a dedicated machine), or a combination of both. For example exemplary process  800  may perform timing analysis operations included in block  703  of  FIG. 7 . In one embodiment, at block  801 , the processing logic of process  800  identifies an earliest time slot (or transmission slot) for transmitting data value (or a logic signal) among time slots within a period (or cycle) of a design clock, such as design clock  310  of  FIG. 3 . The processing logic of process  800  may compute the earliest possible sampling time relative to the design clock edge for each transmission slot. For example, the processing logic of process  800  may take into account the earliest possible reset of a transmission counter associated with a synchronization logic for transmitting the data, such as slot counter  213  of  FIG. 2 . 
     At block  803 , the processing logic of process  800  may identify the latest possible time slot for transmitting the data within the period of the design clock. In one embodiment, the processing logic of process  800  computes the latest possible delivery time relative to a receiving design clock edge. The processing logic of process  800  may take into account the latest possible time of reset of a transmission counter associated with the synchronization logic for transmitting the data. Subsequently at block  805 , the processing logic of process  800  can generate timing requirements based on the identified earliest and latest transmission slots for the data. The timing requirement may include the earliest sample time and the latest delivery time for the data. 
       FIG. 9  shows an example of a data processing system which may be used with one embodiment of the present invention. Note that while  FIG. 9  illustrates various components of a computer system, it is not intended to represent any particular architecture or manner of interconnecting the components as such details are not germane to the present invention. It will also be appreciated that network computers and other data processing systems which have fewer components or perhaps more components may also be used with the present invention. The computer system of  FIG. 9  may, for example, be an Apple Macintosh computer. 
     As shown in  FIG. 9 , the computer system  901 , which is a form of a data processing system, includes a bus  902  which is coupled to a microprocessor  903  and a ROM  907  and volatile RAM  905  and a non-volatile memory  906 . The microprocessor  903 , which may be a G3 or G4 microprocessor from Motorola, Inc. or IBM is coupled to cache memory  904  as shown in the example of  FIG. 9 . The bus  902  interconnects these various components together and also interconnects these components  903 ,  907 ,  905 , and  906  to a display controller and display device  908  and to peripheral devices such as input/output (I/O) devices which may be mice, keyboards, modems, network interfaces, printers, scanners, video cameras and other devices which are well known in the art. Typically, the input/output devices  910  are coupled to the system through input/output controllers  909 . The volatile RAM  905  is typically implemented as dynamic RAM (DRAM) which requires power continually in order to refresh or maintain the data in the memory. The non-volatile memory  906  is typically a magnetic hard drive or a magnetic optical drive or an optical drive or a DVD RAM or other type of memory systems which maintain data even after power is removed from the system. Typically, the non-volatile memory will also be a random access memory although this is not required. While  FIG. 1  shows that the non-volatile memory is a local device coupled directly to the rest of the components in the data processing system, it will be appreciated that the present invention may utilize a non-volatile memory which is remote from the system, such as a network storage device which is coupled to the data processing system through a network interface such as a modem or Ethernet interface. The bus  902  may include one or more buses connected to each other through various bridges, controllers and/or adapters as is well known in the art. In one embodiment the  110  controller  909  includes a USB (Universal Serial Bus) adapter for controlling USB peripherals, and/or an IEEE-1394 bus adapter for controlling IEEE-1394 peripherals. 
     It will be apparent from this description that aspects of the present invention may be embodied, at least in part, in software. That is, the techniques may be carried out in a computer system or other data processing system in response to its processor, such as a microprocessor, executing sequences of instructions contained in a memory, such as ROM  907 , volatile RAM  905 , non-volatile memory  906 , or a remote storage device. In various embodiments, hardwired circuitry may be used in combination with software instructions to implement the present invention. Thus, the techniques are not limited to any specific combination of hardware circuitry and software nor to any particular source for the instructions executed by the data processing system. In addition, throughout this description, various functions and operations are described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by a processor, such as the microprocessor  903 . 
     A machine readable storage medium can be used to store software and data which when executed by a data processing system causes the system to perform various methods of the present invention. This executable software and data may be stored in various places including for example ROM  907 , volatile RAM  905 , or non-volatile memory  906  as shown in  FIG. 9 . Portions of this software and/or data may be stored in any one of these storage devices. 
     Thus, a machine readable storage medium includes any mechanism stores information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine readable medium includes recordable/non-recordable media (e.g., read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.). 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.