Method and apparatus for communication within a programmable logic device using serial transceivers

[0001] One or more aspects of the invention relate generally to programmable logic devices, and more particularly, to communication within a programmable logic device using serial transceivers.

[0002] Programmable logic devices (PLDs) exist as a well-known type of integrated circuit (IC) that can be programmed by a user to perform specified logic functions. There are different types of programmable logic devices, such as programmable logic arrays (PLAs) and complex programmable logic devices (CPLDs). One type of programmable logic device, called a field programmable gate array (FPGA), is popular because of a superior combination of capacity, flexibility, time-to-market, and cost.

[0003] An FPGA typically includes an array of configurable logic blocks (CLBs) surrounded by a ring of programmable input/output blocks (IOBs). The CLBs and IOBs are interconnected by a programmable interconnect structure. The CLBs, IOBs, and interconnect structure are typically programmed by loading a stream of configuration data (bitstream) into internal configuration memory cells that define how the CLBs, IOBs, and interconnect structure are configured. The configuration bitstream can be read from an external memory, conventionally an external integrated circuit memory such as EEPROM, EPROM, PROM, or the like, though other types of memory may be used. The collective states of the individual configuration memory cells then determine the function of the FPGA. In some cases, the designs implemented on an FPGA can be organized into modules, each module comprising a related group of programmable resources (e.g., CLBs, IOBs, and interconnect structure).

[0004] As feature size within an FPGA becomes smaller, especially for dense routing in sub-quarter-micron fabricated integrated circuits, globally synchronous communication at high clock frequencies over long-haul routing within an FPGA (such as FPGA internal module-to-module routing) is becoming more problematic. This difficulty is due at least in part to an increase in resistance-capacitance (RC) time constants. In some instances, none of the available paths of the interconnect structure from a source circuit to a sink circuit has a signal delay that is compatible with the desired clock period. That is, as clock frequencies increase, and delays in the interconnect structure also increase, a signal may not be able to propagate from one circuit to another within a target number of clock cycle(s). Thus, completely synchronous designs can become impractical, or even impossible.

[0005] Therefore, there exists a need in the art for point-to-point communication between respective portions of a programmable logic device integrated circuit, such as an FPGA, that facilitates module-to-module communication for an integrated circuit.

[0006] Additionally, it is desirable to facilitate communication over an interconnect structure with less delay to allow for “dynamic” or “on-the-fly” module configuration. Heretofore, at an interface, buffers were tri-stated to electrically isolate a module from one or more other modules while the isolated module was undergoing removal, initial or subsequent programming of programmable logic, or was not in use. This allowed such other modules to continue to operate, while taking a module undergoing configuration off-line.

[0007] Method and apparatus for communication within a programmable logic device using serial transceivers is described. In one embodiment, an integrated circuit includes a first module and a second module. The first module and the second module each include a transceiver coupled to a serial/parallel interface, with each transceiver configured to have at least one signal conductor for serial communication between the first module and the second module. The first module and the second module are configured to communicate with one another asynchronously. Each transceiver is configured to communicate with each of the serial/parallel interfaces in a synchronous time domain.

[0013] Method and apparatus for communication within a programmable logic device are described in terms of field programmable gate arrays (FPGAs). While specific reference is made to FPGAs, those skilled in the art will appreciate that one or more aspects of the invention may be used in other types of programmable logic devices, such as complex programmable logic devices (CPLDs).

[0014] FIG. 1 depicts a block diagram of an FPGA 100 with which the invention can be used coupled to a program memory 112. FPGA 100 illustratively includes CLBs 107, I/O routing ring 106A (“programmable interconnect”), memory 111, such as random access memory, delay lock loop (DLL) blocks 109, multiply/divide/de-skew clock circuits 110, and programmable IOBs 106B. DLL blocks 109 and clock circuits 110 collectively provide well-known digital clock management (DCM) circuits for managing clock signals within FPGA 100. Those skilled in the art understand that FPGA 100 may include other types of logic blocks and circuits, such as multipliers and processors, in addition to those described herein.

[0015] As is well known in the art, CLBs 107 are programmably connectable to each other, and to I/O routing ring 106A, for performing various types of logic functions. Each of CLBs 107 can include one or more “slices,” and programmable interconnect circuitry (not shown). Each CLB slice in turn includes various circuits, such as flip-flops, function generators (e.g., a look-up table (LUT)), logic gates, memory, and other well-known circuits.

[0016] Programmable IOBs 106B are configured to provide input to, and receive output from, one or more of CLBs 107. Configuration information for FPGA 100, including CLBs 107, I/O routing ring 106A, and programmable IOBs 106B, is stored in memory 111. Briefly stated, a configuration bitstream produced from program memory 112 is coupled to a configuration port of FPGA 100 to implement a desired circuit therein. The configuration process of FPGA 100 is also well known in the art. CLBs 107, I/O routing ring 106A, and programmable IOBs 106B are collectively referred to herein as “FPGA fabric.”

[0017] FIG. 2 depicts a block diagram showing an exemplary embodiment of a portion of FPGA 100 configured for internal communication in accordance with one or more aspects of the invention. FPGA 100 includes a portion 201 in communication with a portion 203. Portion 201 includes a logic circuit 202, a transceiver 208, and interface logic 206. Portion 203 includes a logic circuit 204, a transceiver 210, and interface logic 212. Portion 201 is coupled to portion 203 via signal routing 216.

[0018] Logic circuit 202 is a circuit configured within FPGA 100 using the FPGA fabric as described above. Logic circuit 202 includes N I/O terminals 205, where N is an integer greater than zero. I/O terminals 205 of logic circuit 202 are coupled to interface logic 206. Interface logic 206 in turn is coupled to an I/O port 207 of transceiver 208. Likewise, logic circuit 204 is a circuit configured within FPGA 100 using the FPGA fabric. Logic circuit 204 includes M I/O terminals 211, where M is an integer greater than zero. I/O terminals 211 of logic circuit 204 are coupled to interface logic 212. Interface logic 212 is in turn coupled to an I/O port 209 or transceiver 210.

[0019] In one embodiment, transceiver 208 and transceiver 210 are asynchronous serial transceivers, such as multi-gigabit transceivers (MGTs), with differential inputs and outputs. Transceiver 208 and transceiver 210 are hardwired circuits within FPGA 100. Transceiver 208 is coupled to transceiver 210 via signal routing 216. In one embodiment, signal routing 216 is independent from the programmable interconnect of FPGA 100, namely, no programmable interconnect fabric of FPGA 100 is used for signal routing 216. For example, signal routing 216 may comprise a pair of signal paths, electrical or optical. For clarity, signal routing 216 is described in terms of circuit traces 216A and 216B. Circuit traces 216A and 216B couple transceiver 208 and transceiver 210 in a “cross-over” fashion. A transmit port of transceiver 208 is coupled to a receive port of transceiver 210 via circuit trace 216A. Likewise, a transmit port of transceiver 210 is coupled to a receive port of transceiver 208 via circuit trace 216B. In one embodiment, circuit traces 216A and 216B are formed within FPGA 100. Alternatively, circuit traces 216A and 216B may be formed on a printed circuit board (PCB), as described below with respect to FIG. 3.

[0020] Interface logic 206 provides an interface between logic circuit 202 and transceiver 208. Likewise, interface logic 212 provides an interface between logic circuit 204 and transceiver 210. Interface logic 206 and interface logic 212 serialize signals input from I/O terminals 205 and I/O terminals 211, respectively. Interface logic 206 and interface logic 212 respectively provide serialized bitstreams to transceiver 208 and transceiver 210.

[0021] Interface logic 206 and interface logic 212 also deserialize bitstreams input from I/O port 207 and I/O port 209, respectively. Interface logic 206 and interface logic 212 respectively provide deserialized signals to logic circuit 202 and logic circuit 204, which as indicated above, are parallel transmissions with N and M signal channels, respectively. Interface logic 206 and interface logic 212 can be implemented using FPGA fabric, or can be hardwired circuits within FPGA 100. Those skilled in the art will appreciate that interface logic 206 and interface logic 212 can be part of transceiver 208 and transceiver 210, respectively.

[0022] In operation, portion 201 of FPGA 100 communicates with portion 203 of FPGA 100 using transceiver 208 and transceiver 210. In one embodiment, transceiver 208 and transceiver 210 establish an asynchronous serial communication channel over signal routing 216 between portion 201 and portion 203. This allows portion 201 to communicate with portion 203, namely, FPGA portion-to-portion (“internal”) communication, without using FPGA fabric.

[0023] For example, each of logic circuit 202 and logic circuit 204 can be designed to operate synchronously within a range of operating clock frequencies using FPGA fabric. Due to the clock frequency and/or the propagation delay, however, synchronous communication between logic circuit 202 and logic circuit 204 using FPGA fabric may not be achievable. As such, logic circuit 202 can be designed to communicate with logic circuit 204 using transceiver 208 and transceiver 210.

[0024] Using an asynchronous communication channel over signal routing 216, logic circuit 202 can transmit signals output from I/O terminals 205 to I/O terminals 211 of logic circuit 204. Likewise, logic circuit 204 can transmit signals output from I/O terminals 211 to I/O terminals 205 of logic circuit 202. In order to employ asynchronous communication more effectively between logic circuit 202 and logic circuit 204, interaction between logic circuit 202 and logic circuit 204 should be somewhat latency tolerant. By moving signals from a synchronous domain to an asynchronous domain, increased bandwidth from high-speed, asynchronous serial transmission is used to facilitate communication at the expense of some latency.

[0025] Thus, it should be appreciated that signal routing 216 is for module-to-module communication, or more generally point-to-point communication, where multiple asynchronous serial communication channels are employed. Physical connectivity of signal routing 216 for each communication channel can exist as a separate routing structure from FPGA fabric routing resources. Additionally, serial communications should have substantially uniform communication characteristics with respect to bandwidth, throughput and latency for enhanced effectiveness.

[0026] As mentioned above, it is desirable to support dynamic module configuration with communication within a programmable logic device integrated circuit, such as an FPGA, that facilitates module-to-module communication for an integrated circuit.

[0027] FIG. 3 depicts a block diagram of an exemplary embodiment of at least a portion of FPGA 100 configured for internal communication and dynamic module configuration in accordance with one or more aspects of the invention. FPGA 100 includes a plurality of logic circuits (“modules”), illustratively modules 306A through 306D (generically referred to as modules 306). Each of modules 306 is a circuit configured within FPGA 100 using FPGA fabric as described above. Each of modules 306 includes a transceiver 308A through 308D (generically referred to as transceivers 308). Transceivers 308 are used for communication between modules 306 as described below. Each of modules 306 can include a transceiver 310A through 310D (generically referred to as transceivers 310). Transceivers 310 are coupled to I/O pins 314A through 314D (generically referred to as I/O pins 314), respectively, and can be used for communication with various external devices or systems, including a computer network 399, such as a private network or a public network like the Internet. I/O pins 314 are dedicated, namely, not part of IOBs of FPGA fabric.

[0028] Transceiver 308A is coupled to an I/O pin 312A of FPGA 100, and transceiver 308B is coupled to an I/O pin 312B of FPGA 100. I/O pin 312A is coupled to I/O pin 312B via signal routing 302. Likewise, transceiver 308C is coupled to an I/O pin 312C, and transceiver 308D is coupled to an I/O pin 312D. I/O pin 312C is coupled to I/O pin 312D via signal routing 304. I/O pins 312A through 312D are dedicated, and are not part of IOBs of FPGA fabric. Signal routing 302 and signal routing 304 are also independent from the FPGA fabric and can include a pair of circuit traces substantially as described above with respect to signal routing 216 of FIG. 2. In one embodiment, FPGA 100 is mounted to a PCB 300 and signal routing 302 is provided by traces formed on PCB 300. In this manner, module 306A can communicate with module 306B, and module 306C can communicate with module 306D, substantially as described above with respect to logic circuit 202 and logic circuit 204 of FIG. 2.

[0029] In one embodiment, modules 306 can be “dynamic modules” in that logic circuits forming modules 306 are dynamically configurable. That is, module 306A can be configured, including reconfigured, without affecting modules 306B through 306D, which may continue to operate during configuration or reconfiguration of module 306A. A module that is to be dynamically reconfigured is isolated from other modules before reconfiguration using programmable MGTs. In other words, transceivers 308 can be programmable MGTs, and can be programmed not to transmit or receive while a module 306 is to be isolated for reconfiguration. As modules 306 are coupled only through transceivers 308, 310, this arrangement facilitates isolation among modules 306. For example, module 306A is readily isolated from module 306B, since no portion of the FPGA fabric is used to connect module 306A to module 306B. After isolation, module 306A can be configured and then transceivers can be reprogrammed to put module 306A back into communication with one or more other modules 306 when configuration is complete.

[0030] Although four modules 306 have been shown, those skilled in the art will appreciate that fewer or more modules may be employed within FPGA 100. In general, a plurality of modules can be formed within FPGA 100, where any of the modules can be coupled to any other of the modules using dedicated (“static”) signal routing. In addition, a given module can employ more than one transceiver for communicating with more than one other of the modules.

[0031] FIG. 4 depicts a block diagram of an exemplary embodiment of at least a portion of FPGA 100 configured for internal communication and dynamic module configuration and re-routing in accordance with one or more aspects of the invention. FPGA 100 includes a plurality of modules 402A through 402D (generically referred to as modules 402). Each of modules 402 is a circuit configured within FPGA 100 using FPGA fabric as described above. Each of modules 402 respectively includes a programmable transceiver 404A through 404D (generically referred to as transceivers 404). Transceivers 404 are used for communication among modules 402. Transceivers 404 can be coupled to I/O pins 408A through 408D, respectively, for communication with external devices or systems, such as a network of computers.

[0032] Each of transceivers 404 is coupled to a crossbar switch 406 via signal routing 410A through 410D, respectively. Signal routing 410A through 410D is independent from the FPGA fabric and can include a pair of circuit traces substantially as described above with respect to signal routing 216 of FIG. 2. In one embodiment, crossbar switch 406 is a dedicated circuit within FPGA 100, and signal routing 410A through 410D comprises circuit traces formed within FPGA 100. In another embodiment, crossbar switch 406 can be implemented using the FPGA fabric, and signal routing 410A through 410D can be circuit traces formed within FPGA 100, or can be part of the general-purpose programmable routing of FPGA 100. In yet another embodiment, crossbar switch 406 and signal routing 410A through 410D can be formed off-chip on a PCB, such as PCB 300 of FIG. 3, on which FPGA 100 is mounted.

[0033] Crossbar switch 406 allows for programmable interconnection among modules 402. In particular, any one of modules 402 can be coupled to any other of modules 402 through crossbar switch 406. If it is on-chip, crossbar switch 406 can be implemented using FPGA fabric, a hardwired circuit embedded within FPGA 100, or a combination thereof. Notably, a full cross-bar switch need not be employed, and thus not all module-to-module connections may be possible. However, programmable transceivers 404 facilitate use of a full cross-bar switch between on-chip differential outputs of such transceivers 404, allowing all module-to-module combinations of connections for forming high-speed routes. Of course, the number of channels that cross-bar switch 406 can handle affects routing granularity. Additionally, as shown in FIG. 4, this is one-hop module-to-module connectivity; however, topologies other than one-hop may be used. For example, a cross-bar switch can be replaced by a segmented programmable routing architecture, though possibly with less uniformity of communication characteristics (e.g., bandwidth, throughput and latency). Furthermore, it should be understood that data passing through a cross-bar switch can be routed from one module to one or more modules.

[0034] Method and apparatus for communication within a programmable logic device using serial transceivers has been described. Transceivers are embedded within the PLD and establish point-to-point asynchronous serial communication channels between circuits configured within the PLD. The physical connection underlying the asynchronous communication channel can exist as a separate routing structure from the main routing resources of the PLD. Communication between two or more synchronous circuits is thus facilitated, where high clock frequencies employed within the synchronous circuits would have prohibited or made problematic synchronous inter-circuit communication. In addition, one or more on-chip modules can be programmably isolated to provide dynamic module configurable circuits from other circuits within a PLD.

[0035] While the foregoing describes exemplary embodiments in accordance with one or more aspects of the invention, other and further embodiments in accordance with the one or more aspects of the invention may be devised without departing from the scope thereof, which is determined by the claims that follow and equivalents thereof. Claims listing steps do not imply any order of the steps.