Abstract:
A method of configuring a programmable integrated circuit device with a user logic design includes analyzing the user logic design to identify unidirectional logic paths within the user logic design and cyclic logic paths within the user logic design, assigning the cyclic logic paths to logic in a first portion of the programmable integrated circuit device that operates at a first data rate, assigning the unidirectional logic paths to logic in a second portion of the programmable integrated circuit device that operates at a second data rate lower than the first data rate, and pipelining the unidirectional data paths in the second portion of the programmable integrated circuit device to compensate for the lower second data rate. A programmable integrated circuit device adapted to carry out such method may have logic regions operating at different rates, including logic regions with programmably selectable data rates.

Description:
FIELD OF THE INVENTION 
     This invention relates to heterogeneous programmable integrated circuit devices such as field-programmable gate array (FPGAs) or other types of programmable logic devices (PLDs), and to configuration software adapted for configuring such heterogeneous devices. 
     BACKGROUND OF THE INVENTION 
     Early programmable devices were one-time configurable. For example, configuration may have been achieved by “blowing”—i.e., opening—fusible links. Alternatively, the configuration may have been stored in a programmable read-only memory. Those devices generally provided the user with the ability to configure the devices for “sum-of-products” (or “P-TERM”) logic operations. Later, such programmable logic devices incorporating erasable programmable read-only memory (EPROM) for configuration became available, allowing the devices to be reconfigured. 
     Still later, programmable devices incorporating static random access memory (SRAM) elements for configuration became available. These devices, which also can be reconfigured, store their configuration in a nonvolatile memory such as an EPROM, from which the configuration is loaded into the SRAM elements when the device is powered up. These devices generally provide the user with the ability to configure the devices for look-up-table-type logic operations. 
     While it may have been possible to configure the earliest programmable logic devices manually, simply by determining mentally where various elements should be laid out, it was common even in connection with such earlier devices to provide programming software that allowed a user to lay out logic as desired and then translate that logic into a configuration for the programmable device. With current larger devices, it would be impractical to attempt to lay out the logic without such software. 
     Some user logic designs would be able to operate at higher clock speeds if the designs could be pipelined. However, certain user logic designs may include portions that are amenable to pipelining as well as portions that are not amenable to pipelining. Configuration software has not heretofore been well-adapted to deal with pipelining in such types of user logic designs. 
     SUMMARY OF THE INVENTION 
     Unidirectional or “feed-forward” logic paths are amenable to pipelining. On the other hand, cyclic logic paths (i.e., loops) are less amenable to pipelining. If a user logic design includes both feed-forward and cyclic logic paths, and only the feed-forward logic can be pipelined, then a way to speed up the cyclic logic paths, so that they can operate at the same clock speed as the pipelined feed-forward logic paths, should be provided. Otherwise, the cyclic logic paths will fall behind the pipelined feed-forward logic paths, and then execution of the feed-forward logic paths will have to stop and wait for the cyclic logic paths, defeating the purpose of pipelining the feed-forward logic paths. 
     One way to accomplish such a result is to provide a programmable device in which most of the programmable logic resources are in a first group operable up to a first data rate, with a smaller amount of programmable logic resources being in a second group operable up to a second data rate faster than the first data rate. The feed-forward logic paths which can be pipelined may be implemented in the slower programmable logic resources in the first group, while the cyclic logic paths that cannot be pipelined may be implemented in the faster programmable logic resources in the second group. 
     When configuring such a device according to a user logic design, the configuration software would have to be able to distinguish the cyclic paths from the feed-forward paths, so that it can implement the cyclic paths in the faster programmable logic resources and implement the feed-forward paths, with pipelining, in the slower programmable logic resources. 
     Therefore, in accordance with the present invention there is provided a method of configuring a programmable integrated circuit device with a user logic design. The method includes analyzing the user logic design to identify unidirectional logic paths within the user logic design and cyclic logic paths within the user logic design, assigning the cyclic logic paths to logic in a first portion of the programmable integrated circuit device that operates at a first data rate, and assigning the unidirectional logic paths to logic in a second portion of the programmable integrated circuit device that operates at a second data rate lower than the first data rate, and pipelining the unidirectional data paths in the second portion of the programmable integrated circuit device to compensate for the lower second data rate. 
     A machine-readable data storage medium encoded with instructions for carrying out such a method also is provided, as is a programmable integrated circuit device adapted to carry out such method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the invention, its nature and various advantages will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  shows an overview of the floor plan of one example of a heterogeneous programmable integrated circuit device  100  useful in embodiments of the present invention; 
         FIG. 2  shows an example of pipelining of a system including cyclic and acyclic logic; 
         FIG. 3  is a flow diagram of an example of a programmable device configuration method according to embodiments of the invention; 
         FIG. 4  is a cross-sectional view of a magnetic data storage medium encoded with a set of machine-executable instructions for performing the method according to the present invention; 
         FIG. 5  is a cross-sectional view of an optically readable data storage medium encoded with a set of machine executable instructions for performing the method according to the present invention; and 
         FIG. 6  is a simplified block diagram of an illustrative system employing a programmable logic device incorporating the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an overview of the floor plan of one example of a heterogeneous programmable integrated circuit device  100  useful in embodiments of the present invention. Device  100  is a field-programmable gate array (FPGA), but other types of programmable logic devices (PLDs) or other programmable integrated circuit devices may be used in embodiments of the invention. 
     In addition to input/output regions  101 , optional memory (RAM) regions  102  and optional digital signal processing (DSP) regions  103 , one or more regions  104  of device  100  may contain lower (or “normal”) speed programmable logic resources. These resources are designated in the drawing as “LS LABs,” or low-speed logic array blocks, referring to a unit of programmable logic resources in devices provided by Altera Corporation, of San Jose, Calif. However, the invention is applicable to programmable devices from any source. In addition to low-speed resources  104 , device  100  also contains high-speed resources in regions  105 , designated in the drawing as “HS LABs,” or high-speed logic array blocks. 
     As depicted schematically in  FIG. 1 , there is less area devoted to high-speed resources  105  than there is area devoted to low-speed resources  104 . That is because high-speed resources  105  are more expensive than low-speed resources  104 . For example, high-speed resources  105  may be implemented using wider transistor widths, lower threshold voltages, shorter gate lengths, higher drive voltages, wider wire widths, or other circuit techniques that consume more device overhead per unit of logic. 
     For example, providing a threshold voltage 30 mV lower than nominal may provide 20% higher speed (1.2 times f max  or 0.8333 times the delay) than a nominal device speed, at a cost of doubling the static power consumption. 
     Alternatively, using a drive voltage 20% higher than nominal in the high-speed resources  104  may provide 20% higher speed at 50% total (static+dynamic) power consumption. At the same time, a drive voltage 20% lower than nominal in the low-speed resources  105  may provide 20% lower speed (0.8 times f max =1.25 times the delay) at 50% lower static power or 40% lower total power. Thus the total device power consumption is comparable to nominal power consumption, but high-speed resources  104  offer 20% higher than nominal speed. 
     Assuming that 30% of the programmable resources are high-speed resources  104  and 70% of the programmable resources are low-speed resources  105 , then using threshold voltage to increase speed results in a static power adjustment factor of 0.3×2+0.7×0.6=1.02, while using the drive voltage to increase speed results in a static power adjustment factor of 0.3×1.5+0.7×0.6=0.87. Thus it is expected that the performance gain may be achieved at a power increase as low as 2%, or even a decrease in power of at least 13%. 
     As noted above, decisions on which regions to assign to particular portions of a user logic design can be based on whether or not those portions can be pipelined. One factor which may indicate whether a particular portion of logic can be pipelined is whether that particular portion of logic is cyclic or not. Unidirectional or feed-forward logic can be assigned low-speed logic regions  105  with little impact on speed because the logic can be pipelined to a depth needed to compensate. Cyclic logic can be assigned to high-speed regions  105 , so that the overall logic design operates at a desired speed. 
     For example, considering a user logic design that can be pipelined to 1000 MHz—e.g., using a pipelined FPGA such as that described in copending, commonly-assigned U.S. patent application Ser. No. 13/630,925, with a depth of five registers, or 5 ns latency in the unidirectional logic. In embodiments of the present invention, the cyclic logic, which cannot be pipelined, can be placed in a high-speed region  104  and run at, e.g., 1200 MHz. However the unidirectional logic will now exhibit latency of 5×1.2=6 ns, because the low speed logic will now have a clock period of 1.2 ns instead of the original 1 ns, but will need to run at 1200 MHz to match the clock rate of the cyclic logic. Therefore, the unidirectional logic would need to be pipelined to a depth of 6 ns×1200 MHz=7.2 stages, so at least eight pipeline stages will be required to meet this speed. In a pipelined FPGA fabric, with sufficient registers available, this should not be difficult. Therefore the overall performance of the device can improved by 20% compared to a known device, at essentially no power cost as described above. 
     For example,  FIG. 2  shows an example of a system  200  containing cyclic logic  201  and acyclic (i.e., unidirectional) logic  202 , including registers  203 . Acyclic logic  202  in cycles may be decomposed into more pipeline stages  212 , by adding additional registers  213 , decreasing the depth of logic between each stage, and increasing its performance and latency of resulting system  210 , but not changing its behavior in any other way. 
     According to other implementations, the differentiation between high-speed resources and low-speed resources could be made user-configurable. For example, that differentiation might be accomplished by body bias or by providing a programmable drive voltage for some or all of the programmable logic regions. In such a device, the number of high-speed regions can be configured based on the fraction of the design that is cyclic, which determines the number of regions needed to implement the cyclic logic. Based on that number and an allowable power budget, the drive voltages of the high-speed and low-speed logic regions can be selected to minimize total power while meeting desired performance goals. 
     In one example of such an implementation, the drive voltage in the low-speed region may be reduced as far as possible until one of two limits is met. A first limit is that at very low speed, the individual circuit elements may not be able to operate at the required f max —i.e., at sufficiently low drive voltage, even a single routing wire may not be fast enough. A second limit is that as the drive voltage is lowered, the latency increases, and may reach an unacceptable value at some point. 
     In another implementation, the high-speed regions can include logic elements based on six-input lookup tables (6-LUTs), while the low-speed regions can include logic elements based on four-input lookup tables (4-LUTs). 
     A heterogeneous logic fabric of the type described may present challenges to the configuration software used to implement a user logic design. For example, a mix of different logic element types may create difficulty in matching the user logic design to the exact ratio of logic element types provided on the device. However, where the design includes a unidirectional portion that can be pipelined, because pipelining can be accomplished late in the configuration process (as described in more detail below), the heterogeneity of the device may be less of a challenge than might otherwise be expected. 
     For example, in an architecture as described above, the high-speed logic elements are based on 6-LUTs for performance, and the low-speed logic elements are based on 4-LUTs for low cost. The configuration software may synthesize the entire netlist of a user logic design into 6-LUTs—i.e., as a single high-speed region. Next, the cyclic logic portion of the user logic design are placed in that high-speed region. The unidirectional part of the user logic design may then be placed into the unused portion of the high-speed region, approximating a low-speed region by pretending that the 6-LUTs in the user logic design are actually 4-LUTs. Finally, all of the LUTs that are placed in the approximated low-speed region can be resynthesized into 4-LUTs and placed in the actual hardware. 
     It would be more efficient to synthesize the cyclic logic into high-speed logic elements and the unidirectional logic into low-speed logic elements in the first instance, but only if the amounts of each type of logic element are known in advance. Otherwise, attempting to synthesize different types of logic elements would be less efficient. Therefore, it is desirable to be able to determine in advance how much high-speed logic and how much low-speed logic will be required for a particular user logic design. 
     There are a number of different ways to determine how much high-speed logic and how much low-speed logic will be required. One approach would be to run a cycle detection algorithm on the user logic design. If one assumes that all cyclic logic paths will be placed in high-speed logic and all feed-forward logic would be placed in low-speed logic, then the relative amounts of high-speed logic and low-speed logic could be determined from the results of the cycle detection algorithm. 
     There are some well-known algorithms for cycle detection. For example, Tarjan, R. E., “Depth-first search and linear graph algorithms,”  SIAM Journal of Computing  1(2): 146-60 (1972) describes a strongly-connected-components algorithm that discovers all nodes that are within cycles. Use of such an algorithm would be acceptable when the amount of available high-speed resources is large relative to the amount of logic in the user logic design that is in cycles. However, in a situation in which the amount of available high-speed resources is insufficient to accommodate all of the logic that is in cycles, the assignment of logic to the high-speed resources preferably should be more intelligent. 
     For example, if the target period is ins, a loop that has a total delay of 5 ns and ten registers in it is not likely to be critical because it could potentially be retimed by repositioning registers to obtain a 5 ns/10=500 ps period. However, a loop that has a total delay of 5 ns and five registers in it is likely to be critical. Thus, it would be better to be able to determine the estimated delay of each loop, and the number of registers in each loop, to determine the nodes that are most likely to benefit from being assigned to the high-speed logic resources, rather than simply basing the assignment on whether or not the node is in a loop. 
     One alternative may be to determine critical and near-critical cycles, where a critical cycle is one whose delay cannot be increased without increasing the period achievable by retiming, and a near-critical cycle is one whose delay can be increased slightly before it increases the period achievable by retiming. Preferential treatment could then be given to nodes on critical and near-critical cycles during the assignment of nodes as between high-speed logic resources and low-speed logic resources. One way to make such a determination is to use the timing analysis techniques described in commonly-assigned, concurrently-filed U.S. patent application Ser. No. 13/733,982, now U.S. Pat. No. 8,677,298. 
     Another alternative uses timing analysis techniques also described in the above-identified application Ser. No. 13/733,982, now U.S. Pat. No. 8,677,298, to analyze the timing of a user logic design under the assumption that registers will be moved to minimize the clock period. Given an estimate of the delays on each circuit element, that timing analysis produces a “cycle slack” value and a “cycle slack ratio” value for each circuit element. Cycle slack indicates how much delay can be added to the circuit element before it impacts the optimal clock period. Cycle slack ratio is a normalization of the slack ratio, resulting in a number between 0 and 1. As the cycle slack ratio for an element gets closer to 0, that element is considered more critical, while as the cycle slack ratio for an element gets closer to 1, that element is considered less critical. Preferential treatment may be given to those nodes with less cycle slack or with cycle slack ratios closer to 0 when determining which portion of the user logic design to implement in high-speed logic resources. 
     Where the hardware, as described above, uses regions of different technologies for the high-speed and low-speed resources (e.g., larger and smaller LUTs as described), the configuration software can be designed so that the technology mapping step maps logic elements into a heterogeneous mixture of LUTs according to their criticality, as determined, e.g., by cycle slack or cycle slack ratio. Many known technology mapping techniques use an iterative flow, such as that described in Manohararajah, V., et al., “Heuristics for Area Minimization in LUT-Based FPGA Technology Mapping,”  IEEE Transactions on CAD , vol. 25, no. 11, pp. 2331-40 (November 2006), which during a current pass uses information from previous passes to bias the current pass. Technology mapping algorithms are also capable of using cycle slack and cycle slack ratios, provided by timing analysis, to bias the selection of larger LUTs for those nodes with low cycle slack or low cycle slack ratios. 
     Those two features of mapping algorithms can be used together to map a user logic design into a fabric of heterogeneous logic resources. For example, on a first pass, the cost function of the larger LUTs in the high-speed region can be made low so that the mapping algorithm puts a large portion of the user logic design into the high-speed region. However, if the actual high-speed resources are overused, or the resulting ratio of the high-speed resources to low-speed resources does not mimic what is present in the actual device, the cost function of the larger LUTs can be gradually increased in subsequent iterations—which would have the effect of reassigning less critical nodes to the low-speed resources—until an appropriate distribution of logic between the high-speed resources and the low-speed resources is achieved. 
     The distribution of logic between the high-speed resources and the low-speed resources by technology mapping will frequently result in an acceptable solution. However, there may be situations when the subsequent clustering step may find inaccuracies in the actual amounts of high-speed and low-speed resources required. In such situations, some of the logic mapped into high-speed resources could be resynthesized into low-speed resources. As in the original technology mapping, the logic that would be selected for resynthesis from high-speed resources to low-speed resources would be that logic, among the logic originally synthesized in high-speed resources, with the highest cycle slack or cycle slack ratio as reported by timing analysis. 
       FIG. 3  shows an example of a programmable device configuration method  300 , which may be implemented in software, according to embodiments of the invention. Method  300  starts at  301  and proceeds to RTL and gate-level synthesis at  302 , which may be conventional. Method  300  ends at  308  with the outputting of a configuration bitstream for a programmable device such as an FPGA. Outputting  308  is preceded at  307  by retiming, which, as is conventional, redistributes registers to reduce latency and maximize operating frequency f max . Each of the intervening portions  303 - 306  is carried out based on a timing analysis  309  that takes into account that retiming  307  ultimately will be carried out. 
     Iterative technology mapping is carried out at  303  in the manner described above. As noted, an initial distribution between high-speed resources and low-speed resources is made based on timing analysis  309 . That distribution is compared to the actual distribution of high-speed resources and low-speed resources available on the physical device, adjusting the relative cost of high-speed resources and low-speed resources at  313  to make high-speed resources relatively more costly on each iteration. 
     Next, the mapped logic elements are clustered at  304  into blocks. During clustering, it may be determined, as noted above, that the ratio of high-speed resources to low-speed resources does not correspond to the actual distribution of resources on the device to be configured, and in particular that more logic has been mapped to high-speed resources than there are available high-speed resources on the device. If so, then as described above, some of the logic that had been mapped to high-speed resources will be resynthesized at  314  to low-speed resources. 
     Retiming-aware placement and routing take place at  305  and  306 , respectively, followed by the aforementioned retiming  307 . Method  300  ends with the outputting at  308  of a configuration bitstream for device  100 . 
     Thus it is seen that a programmable device in which most of the programmable logic resources are in a first group operable up to a first data rate, with a smaller amount of programmable logic resources being in a second group operable up to a second data rate faster than the first data rate, in which feed-forward logic paths which can be pipelined may be implemented in the slower programmable logic resources in the first group, while cyclic logic paths that cannot be pipelined may be implemented in the faster programmable logic resources in the second group, has been provided, along with a method of programming or configuring such a device. 
     Instructions for carrying out a method according to this invention for programming a programmable device may be encoded on a machine-readable medium, to be executed by a suitable computer or similar device to implement the method of the invention for programming or configuring PLDs or other programmable devices. For example, a personal computer may be equipped with an interface to which a PLD can be connected, and the personal computer can be used by a user to program the PLD using suitable software tools as described above. 
       FIG. 4  presents a cross section of a magnetic data storage medium  1200  which can be encoded with a machine executable program that can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium  1200  can be a floppy diskette or hard disk, or magnetic tape, having a suitable substrate  1201 , which may be conventional, and a suitable coating  1202 , which may be conventional, on one or both sides, containing magnetic domains (not visible) whose polarity or orientation can be altered magnetically. Except in the case where it is magnetic tape, medium  1200  may also have an opening (not shown) for receiving the spindle of a disk drive or other data storage device. 
     The magnetic domains of coating  1202  of medium  1200  are polarized or oriented so as to encode, in manner which may be conventional, a machine-executable program, for execution by a programming system such as a personal computer or other computer or similar system, having a socket or peripheral attachment into which the PLD to be programmed may be inserted, to configure appropriate portions of the PLD, including its specialized processing blocks, if any, in accordance with the invention. 
       FIG. 5  shows a cross section of an optically-readable data storage medium  1210  which also can be encoded with such a machine-executable program, which can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium  1210  can be a conventional compact disk read-only memory (CD-ROM) or digital video disk read-only memory (DVD-ROM) or a rewriteable medium such as a CD-R, CD-RW, DVD-R, DVD-RW, DVD+R, DVD+RW, or DVD-RAM or a magneto-optical disk which is optically readable and magneto-optically rewriteable. Medium  1210  preferably has a suitable substrate  1211 , which may be conventional, and a suitable coating  1212 , which may be conventional, usually on one or both sides of substrate  1211 . 
     In the case of a CD-based or DVD-based medium, as is well known, coating  1212  is reflective and is impressed with a plurality of pits  1213 , arranged on one or more layers, to encode the machine-executable program. The arrangement of pits is read by reflecting laser light off the surface of coating  1212 . A protective coating  1214 , which preferably is substantially transparent, is provided on top of coating  1212 . 
     In the case of magneto-optical disk, as is well known, coating  1212  has no pits  1213 , but has a plurality of magnetic domains whose polarity or orientation can be changed magnetically when heated above a certain temperature, as by a laser (not shown). The orientation of the domains can be read by measuring the polarization of laser light reflected from coating  1212 . The arrangement of the domains encodes the program as described above. 
     A PLD  140  programmed according to the present invention may be used in many kinds of electronic devices. One possible use is in a data processing system  1400  shown in  FIG. 6 . Data processing system  1400  may include one or more of the following components: a processor  1401 ; memory  1402 ; I/O circuitry  1403 ; and peripheral devices  1404 . These components are coupled together by a system bus  1405  and are populated on a circuit board  1406  which is contained in an end-user system  1407 . 
     System  1400  can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, digital signal processing, or any other application where the advantage of using programmable or reprogrammable logic is desirable. PLD  140  can be used to perform a variety of different logic functions. For example, PLD  140  can be configured as a processor or controller that works in cooperation with processor  1401 . PLD  140  may also be used as an arbiter for arbitrating access to a shared resources in system  1400 . In yet another example, PLD  140  can be configured as an interface between processor  1401  and one of the other components in system  1400 . It should be noted that system  1400  is only exemplary, and that the true scope and spirit of the invention should be indicated by the following claims. 
     Various technologies can be used to implement PLDs  140  as described above and incorporating this invention. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, the various elements of this invention can be provided on a PLD in any desired number and/or arrangement. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow.