PLD architecture for flexible placement of IP function blocks

0001This application claims priority to provisional patent application number 60/289,311, filed May 6, 2001.

0002The present invention is in the field of programmable logic devices (PLD's) and, more particularly, relates to a flexible architecture by which logic elements (LE's) of a PLD having an array of logic elements are replaced with intellectual property (IP) function blocks.

0003Conventional programmable logic devices (PLD's) comprise one or more arrays of logic elements (LE's), and the base signal routing architecture between LE's is designed such that LE-to-LE routing (typically performed by software programs that convert hardware design language program instructions into such routing) is optimized. Examples of PLD's having more than one array include PLD's in the APEX family of devices, by Altera Corporation of San Jose, California. It is sometimes desired to add one or more IP function blocks among an LE array. For the purposes of this disclosure, an LE is considered to be a basic -- or most common -- logic element that functions, for example, with respect to look-up table or macrocell logic. The LE's of an array are nominally connected by a base signal routing architecture. An IP function block is a functional block that is not constrained by the function of the most common logic element. For example, by way of illustration and not by limitation, an IP function block may be a high-speed serial interface (HSSI), a digital signal processor or other microprocessor, ALU, memory, or a multiplier.

0004Conventionally, when an IP function block is added to an LE array, the IP function block is placed at an edge of the LE array and spanning the entire edge of the LE array. This minimizes the disruption in the base routing. Furthermore, by placing the IP function block at an edge of the array, the performance degradation that would otherwise be caused by routing over the IP function block is minimized. A disadvantage of placing the IP function block at an edge of the LE array, however, is that the input/output (I/O) cells (for communication off the LE array) are conventionally located at the edge of the LE array. The IP function block may not even require access to the I/O cells at that edge but, nonetheless, impedes access by other elements (LE's, or even other IP function block) to the I/O cells at the edge where the IP function block is placed. In some cases, such as is described in US Patent No. 5,550,782, a block such as an embedded array block (EAB) completely replaces a logic array block (LAB) of LE's. In this case, the routing connects to the EAB in much the same way that the routing interfaces with the conventional logic block and, thus, there is no corresponding performance degradation.

0005Clearly, placement of an IP function block within an LE array of a PLD has commonly been an afterthought such that, typically, an IP function block was merely placed where it could best fit to minimize the disruption to the base signal routing architecture. What is desired is a PLD architecture by which the placement of an IP function block is not dictated by the goal of minimizing the disruption to the base signal routing architecture.

0006In accordance with one aspect of the invention, a "hole"is formed within an LE array of a PLD by interrupting the base signal routing architecture such that a hole is left for the IP function block to be incorporated. An interface region is provided for interfacing the remaining base signal routing to the IP function block. This provides for flexible placement of IP function block within the regular LE-routing structure of the PLD. The invention will be more clearly understood from the following description, given by way of example only, with reference to the accompanying drawings, in which:

0007Figure 1 illustrates possible placements of "big" IP function blocks within a PLD and not adjacent to an edge of the LE array, for a "big" IP function block such as a memory for which direct I/O access is not a requirement.

0008Figures 2A, 2B and 2C illustrate possible placements of big IP function blocks within a PLD and adjacent to an edge of the LE array such that direct I/O access is provided.

0009Figure 3 illustrates a "donut hole" within the context of a PLD, for placement of a "big" IP function block within a PLD and not adjacent to an edge of the LE array.

0010Figure 4 illustrates, in greater detail, a "big block" interface region as used in Figures 1, 2 and 3.

0011Figure 5 illustrates the horizontal interface to a "big block" that is a large random access memory ("MRAM").

0012Figure 6 illustrates the vertical interface to the MRAM.

0013Figure 7 is a table illustrating connectivity details of the MRAM_LIM

0014Figure 8 is a table illustrating the MegaRAM_Out signal connections .

0015Figure 9 is a table illustrating the Mega-RAM interface specifications.

0016Figure 10 illustrates an embodiment of a staggered routing architecture, in which the partial lines at the boundary of the staggered routing architecture are driven.

0017Figures 1 and 2 illustrate possible floorplans for a PLD architecture in accordance with an aspect of the invention. The illustrations in Figure 1 are examples only, and the invention is not limited to the examples shown in Figure 1. In accordance with the invention, a method of creating the Figure 1 floorplans (and other floorplans within the scope of the invention) is now described.

0018The base signal routing architecture is defined and optimized for LE's. For example, an array of LE's is created for a particular target die size. For variants of the created LE array, as discussed in the Background, it is desired to place the IP function block within the LE array. In some embodiments, the IP function block is added as IP function blocks at some desired uniform density, although the density of IP function blocks need not be uniform. For IP function blocks added to the LE array, LE's are replaced. Thus, there is a tradeoff between LE's and the amount of IP added to the die. The array of LE's for which a particular base signal routing architecture is optimized may occupy substantially an entire target die. Alternately, a base signal routing architecture may be optimized for an array of LE's that coexists on a die with other circuitry, including other LE's.

0019Figures 1A, 1B and 1C illustrate IP function block 110 incorporated within the interior of an LE array, for an IP function block that does not require direct I/O pad 112 access. Figures 2A and 2B illustrate an IP function block 120 incorporated at the edge of the LE array but not spanning the entire edge of the LE array, such that I/O pad 112 access is provided to the IP function block 120. Furthermore, while not shown in Figures 2A and 2B, the IP function block can even displace I/O pads 112, if desired.

0020Figure 3 illustrates how an IP function block is incorporated as a "donut hole" 302 as applied to the Figure 1 examples. That is, within the region where the IP function block is incorporated, the base signal routing architecture is interrupted, and a hole 302 is left (at least in one layer of the metal) for the IP function block to be incorporated. In addition, an interface region 304 is provided completely surrounding the hole 302 for interfacing the remaining base signal array routing to the IP function block. A very specific example of one such interface block is described later with reference to Figures 5 and 6. It should be noted that some of the lines of the base signal routing architecture may be routed around the hole 302 by being raised from the interface region 304 at one side of the hole 302 to a different layer of the metal not occupied by the hole 302 and lowered back into the interface region 304 at another side of the hole 302, typically the opposite side of the hole 302. To generalize, signal routing lines for driving shorter distances are terminated, and routing lines for driving longer distances are raised to a different layer of metal, buffered across the hole, and lowered into the interface region at the opposite side of the donut hole. What is "shorter" and "longer" is variable for any particular embodiment and is discussed later with respect to the size of the interruption ― "small" or "large." Typically, those routing lines buffered across a hole do so without being connected to the IP function block in the hole region.

0021An interface region is provided even when the IP function block is not to be bordered on all four sides by the base signal routing architecture as illustrated in the Figure 3 embodiment but, rather, is to be bordered on two sides (an.example of which is shown in Figure 2A), three sides (an example of which is shown in Figure 2B), or even one side (an example of which is shown in Fig 2C). The interface region is typically provided on each side of the hole that borders the base signal routing architecture.

0022Figure 4 illustrates an embodiment of the interface region 304. That is, in accordance with the Figure 4, it is shown that, for some level of granularity, the interface region 408 includes up to 24 signal lines into the hole 402 and up to 12 signal lines out of the hole 402. The interface region 408 is built modularly, at a particular level of granularity. In one embodiment, the particular level of granularity is one width or height of the logic array block (LAB) of the LE array.

0023A design consideration for the placement of a hole is the number of signal lines in and out of a hole that would result from a particular placement, primarily as a result of the extent to which the hole would border the base signal routing architecture. This can be seen with reference again to Figures 2A and 2B.

0024Figures 5 and 6 illustrate a situation where the IP function block is a RAM block (designated in Figures 5 and 6 as a "Mega-RAM" 502). Figure 5 illustrates the situation from the horizontal routing perspective, and Figure 6 illustrates the situation from the vertical routing perspective. The Mega-RAM block 502 spans multiple columns and multiple rows of the LE array, and therefore interfaces with many lines ("channels") of routing. The general routing does not cross the Mega-RAM 502. Rather, the general routing "dead ends" at the Mega-RAM Interface (a specific instance of the Figure 3 interface region) to form the donut hole for the Mega-RAM. The H24 (Figure 5) and V16 (Figure 6) routing lines are buffered across the Mega-RAM block. The Mega-RAM horizontal interface 504 (Figure 5) allows the Mega-RAM to interface to logic to the left or to the right of the Mega-RAM. The Mega-RAM vertical interface 604 (Figure 6) allows the Mega-RAM to interface to logic above and below the Mega-RAM. In the Mega-RAM vertical interface (Figure 6), there are two groups of fifteen MRAM_LIM's (Not all are shown). Each of the two groups is fed by a different V-Channel. One Mega-RAM interface is provided on each of the left and right hand edges of the MegaRAM for every LAB row it spans, and one Mega-RAM interface is provided on each of the top and bottom edges of the Mega-Ram for every pair of LAB columns it spans.

0025Driving into the Mega-RAM 502 is now described. H and V routing lines in a typical embodiment connect into MRAM_LIM's 506, 606a and 606b (LAB input multiplexers). The MRAM_LIM 506, 606a and 606b is a two stage 4-way sharing multiplexer. Of the portion of the routing that terminates at the boundaries of the Mega-RAM 502, only the routing able to carry signals toward the Mega-RAM 502 feeds the MRAM_LIM's 506, 606a and 606b. Therefore, if the routing is unidirectional (i.e., each line can carry a signal in one direction), then routing able to carry signals away from the MRAM will not be coupled to the input interface. In another embodiment, bi-directional lines are used in addition to, or in place of, unidirectional lines.

0026Connectivity details of the MRAM_LIM 506, 606a and 606b are listed in the table of Figure 7. Briefly, Figure 7 lists the number of "ways in per line" and the number of "connections per MRAM_LIM" for each of the Horizontal MegaRAM interface (Figure 5) and the Vertical MegaRAM interface (Figure 6). At the bottom of the Figure 7 table, the listed totals include the total number of MRAM_LIM fan in signals, and the total number of MRAM_LIM multiplexers, for each of the Horizontal MegaRAM interface and the Vertical MegaRAM interface.

0027Clock inputs 524 are taken into the Mega-RAM block 502 from the global clock network at the side of the Mega-RAM block 502 through the Mega-RAM horizontal interface 504 in (Figure 5). The MRAM_CLOCK MUX 526 chooses one of the eight LABCLK's that are feeding through the adjacent LABs. There is one clock input to the Mega-RAM 502 per row, although the Mega-RAM 502 typically would not use every clock input available to it.

0028The Mega-RAM input mux ("MRIM") is a fully populated 4-way mux-sharing mux that connects thirty LAB lines onto twenty-four I/O block inputs.

0029Driving out of the Mega-RAM 502 is now described. At the edge of the Mega-RAM, routing lines driving into the core do not have LAB's to drive them and are left as partial length lines. The Mega-RAM interface uses the full-length and partial length (i.e., length four and length eight lines, in this embodiment) to connect to the core via the MRAM_DIM. The Mega-RAM interface provides similar resources as are provided for a LAB to drive onto the core routing. For example, H4 lines extending four LAB's into the core are driven, and H4 lines extending three LAB's in or less are not driven. These partial length lines are driven to Vcc. In another embodiment, the partial length lines connect to the MRAM_LIM's as described below with reference to Fig 10.

0030The Mega-RAM horizontal interface can also drive signals out onto the adjacent V-channel routing. Ten partial length sneak paths (H4, H8, V16, H24) (e.g., as collectively designated by line 528) are driven directly into adjacent LAB's by ten of the twelve MegaRAM_Out signals for a "quick" path to logic.

0031Each MRAM driver input multiplexer ("MRAM DIM") 612a, 612b supports the V-channel at the edge of the core and the half H-channel able to carry signals from the MRAM in the direction of the core. The Mega-RAM vertical interface 604 drives the full-length routing resources of two full V-channels. These drivers are dedicated to the MegaRAM_Out signals and do not support turns from other routing resources. The DIM's 612a and 612b associated with the V-line drivers in the Mega-RAM vertical interface 604 are used to choose between MegaRAM_Out signals. Each DIM 612a, 612b in the vertical interface is a 4:1 mux that can be implemented in one or more stages, and each input to the DIM is a MegaRAM_Out signal. The connection pattern from the MegaRAM_Out signals to the DIM 612a, 612b is typically spread equally between the two V-channels.

0032The number of MegaRAM_Out signal connections per DIM for each of the Mega_RAM Horizontal Interface (Figure 5) and the Mega_RAM Vertical Interface (Figure 6) are set forth in Figure 8. Figure 9 is a summary of the Mega_RAM interface specifications for each of the Mega_RAM Horizontal Interface and the Mega_RAM Vertical Interface, and lists the total number of the various interface components described above.

0033It is noted that, typically, not all IP function blocks need be incorporated into an LE array using the hole concept. For example, the IP function block may be of two types -- small and large. In general, the terms small and large as used here can be thought of as indicating size. One actual design consideration, however, in determining whether to consider particular IP function block as small or large is a consideration of how much disruption to the timing of signal routing is to be tolerated. For example, in accordance with one embodiment, a small block is an IP function block whose layout can be drawn at a width on the order of an LE width. In accordance with this embodiment, the width of small blocks may be wider than an LE so long as the timing of signal routing over the block does not get significantly larger than for routing over an LE. For example, in one 0.13 µm architecture, it has been deemed that the timing of the signal routing over a block of roughly 5 LE widths does not get significantly larger than for routing over an LE. Typically, additional inputs and/or outputs may be added that exceed the width of an LE, so long as the base signal routing architecture across the IP function block is maintained with the LE's surrounding the small block. Another consideration for determining whether an IP function block is large (implemented using the hole concept) or small is the size of the IP function block relative to the overhead associated with employing an interface region.In one embodiment, small blocks include MEAB's (medium sized embedded array blocks), SEAB's (small sized embedded array blocks) and a DSP block. By contrast, large blocks are IP function blocks that typically have dimensions much larger than that of an LE. Extending the base signal routing architecture across these blocks without modification would cause routing over these blocks to be significantly larger than routing over an LE, forming a boundary in the PLD timing model. Such large blocks may be inserted into the LE array as holes in the base signal routing architecture, as described above. In some sense, what occurs at the boundary between the base signal routing architecture and a hole is similar to the base signal routing architecture ending at the edge of an LE array.

0034In some embodiments, shown with reference to Figure 10, the LE routing lines are part of a staggered, uni-directional routing architecture. As a result of the staggering, as can be seen from Figure 10, there are partial lines 1002 at boundaries formed by, for example, the I/O interface 1008 at the edge of the PLD 1001 or by an inserted IP function block (designated by reference numeral 1004). In accordance with some such staggered architecture embodiments, routing drivers 1006 are included in the interface region 1008 (whether the I/O interface region or the hole interface region) to drive both the full and partial lines. The output ports 1010 of the logic block 1004 connect to the drivers of both the "full" lines and the "partial" lines. Signal selection muxes may be used in front of the drivers to add routing flexibility. As described above, the routing may also be implemented with segmented bi-directional lines.

0035The partial lines 1002 driving out of the PLD core 1001 feed an input selection mux 1012 to drive into the logic block 1004. These partial lines 1002 impose a smaller load on the drivers 1014 than do full lines 1016, and having a small load makes the partial line 1002 a faster path into the PLD core 1001. If area is a concern, drivers 1018 for partial lines 1002 may be smaller than drivers 1020 for full lines 1016, and still not be at a speed disadvantage due to the smaller load.

0036Furthermore, by driving even the partial lines 1002, additional routing flexibility is provided for signals from the PLD core 1001 to the PLD boundaries. Allowing the partial lines 1002 headed out of the PLD 1001 to drive into an IP function block 1004 increases the routability from the PLD core 1001 to the logic block 1004. In addition, the additional drivers 1018 may be used to provide the core 1001 access to more signals, or the signals may be used to provide more paths into the PLD core 1001 for a given signal. Thus, quite simply, lines that would have otherwise been unused are utilized to provide needed access to the PLD core 1001.

0037While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the claims. For example, the techniques described herein may be applied to other types of fixed blocks or routing structures.