Abstract:
An improved macrocell for sum-of-products logic allows independent selection of D or T flip-flop operation, inverted or non-inverted register input, and use of a product term in register input control. The macrocell circuitry for providing this enhanced functionality can be implemented using only a small number of transistors greater than the number typically used to implement less flexible prior art macrocells.

Description:
This application claims the benefit of United States provisional patent application No. 60/086,432, filed May 22, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to programmable logic devices, and more particularly product term (“p-term”) macrocells for programmable logic devices. 
     Pederson U.S. Pat. No. 5,121,006 (“the &#39;006 patent”) shows a p-term-based macrocell for use in programmable logic devices. The &#39;006 patent macrocell is capable of providing the logical OR of N p-terms. In the &#39;006 patent N is up to five, but N can be any number smaller or larger than five as desired. Also in he &#39;006 patent the sum-of-products output of the above-mentioned logical OR can be optionally subjected logical EXCLUSIVE OR combination with any one of the following signals: (1) a p-term, (2) a feedback output (“Q”) from a register in the macrocell, (3) the logical inverse (“Q-bar”) of the just-mentioned feedback, (4) fixed logic 1 potential (“VCC”), or (5) fixed logic 0 potential (“GND”). The ability to select one of the EXCLUSIVE OR inputs (sometimes referred to herein as the “XOR Control” or “XOR_CTRL” input) from among Q-bar, Q, VCC, or GND allows the &#39;006 patent macrocell to implement D or T flip-flops, and also allows inversion control on the input to the register. However, when the XOR Control input is tied to a product term, the &#39;006 patent macrocell loses the ability to select between the D or T flip-flop. Inversion control on the input to the register is also Lost. 
     In view of the foregoing, it is an object of this invention to provide improved p-term macrocells for programmable logic devices. 
     It is a more particular object of this invention to provide p-term macrocells for programmable logic devices which allow both inversion control and D/T flip-flop selection even when a p-term is used as a contributor to the XOR Control input. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are accomplished in accordance with the principles of the invention by providing a p-term macrocell of the general type shown in the &#39;006 patent but with modification of circuitry for providing the XOR Control input. A preferred embodiment of the XOR Control input circuitry of this invention includes NAND circuitry configured to provide the logical NAND of Q and the output signal of a first programmable function control element (“FCE”). This embodiment further includes first EXCLUSIVE OR circuitry configured to provide the logical EXCLUSIVE OR of the output signal of the above mentioned NAND circuitry and the output signal of a second FCE. The embodiment being described still further includes second EXCLUSIVE OR circuitry configured to provide the logical EXCLUSIVE OR of the output signal of the first EXCLUSIVE OR circuitry and a signal which is selectable (typically programmably selectable) as either a p-term or GND. The output signal of the second EXCLUSIVE OR circuitry is the XOR Control signal. 
     If desired, the modified XOR Control input circuitry of this invention can be implemented using a number of transistors which is only slightly greater than the number of transistors needed for the &#39;006 patent XOR Control input circuitry. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic block diagram of macrocell circuitry of the type shown in the &#39;006 patent. 
     FIG. 2 is a simplified schematic block diagram showing illustrative modification of the FIG. 1 circuitry in accordance with this invention. 
     FIG. 3 is a table summarizing various possible operating conditions of the FIG. 2 circuitry. 
     FIG. 4 is a more detailed but still simplified schematic block diagram of a typical implementation of a portion of the FIG. 1 circuitry. 
     FIG. 5 is a more detailed but still simplified schematic block diagram of an illustrative implementation of a portion of the FIG. 2 circuitry in accordance with the invention. 
     FIG. 6 is a simplified block diagram of an illustrative system employing a programmable logic device incorporating macrocell circuitry in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Typical circuitry for a p-term macrocell  10  of the type shown in the &#39;006 patent is shown in FIG.  1 . Macrocell  10  is assumed to have two p-term inputs  20   a  and  20   b , but more p-terms (like  20   a  and therefore feeding OR circuitry  40 ) can be added if desired. In the particular embodiment shown in FIG. 1 OR circuitry  40  is implemented by a NOR gate  42  followed by an inverter  44 , but in the interest of brevity these elements will sometimes be referred to collectively as OR circuitry  40 . 
     Assuming that logical AND circuitry  30   b  is enabled by the applied output signal of programmable function control element (“FCE”) RI, OR circuitry  40  can provide an output signal which is the logical OR of p-terms  20   a  and  20   b . OR circuitry  40  also receives the output of logical AND circuitry  30   c , which can be a Cascade In signal from another similar macrocell if the FCE R 2  in that other macrocell is programmed to provide an AND-enabling Cascade Control input signal to AND circuitry  30   c . The output signal of OR circuitry  40  can therefore be any of the following: (1) p-term  20   a  alone (if FCE R 1  disables AND circuitry  30   b  and the Cascade Control input to AND circuitry  30   c  disables that circuitry), (2) the logical OR of p-terms  20   a  and  20   b  (if AND circuitry  30   b  is enabled but AND circuitry  30   c  is not), (3) the logical OR of p-term  20   a  and the Cascade In signal (if AND circuitry  30   c  is enabled but AND circuitry  30   b  is not), or (4) the logical OR of p-terms  20   a  and  20   b  and the Cascade In signal (if both of AND circuitries  3   b  and  30   c  are enabled). 
     The output signal of OR circuitry  40  is applied to one input terminal of programmable logic connector (“PLC”)  50  (e.g., a multiplexer) and is also the Cascade Out signal of the depicted macrocell. The Cascade Out signal of one macrocell is the Cascade In signal of another macrocell. PLC  50  is programmably controlled by FCE R 2  to apply either the output signal of OR circuitry  40  or a fixed logic 0 (VSS or ground (“GND”)) signal to one input terminal of EXCLUSIVE OR circuitry  60 . The other input to EXCLUSIVE OR circuitry  60  is an XOR Control signal output by PLC  70 . PLC  70  is programmably controlled by FCEs R 3  and R 4  to select one of the following four signals as the XOR Control signal: (1) the Q output signal of register circuitry  80 , (2) the logical inverse of Q (“Q-bar”) output by inverter  82 , (3) fixed logic 1 (VCC), or (4) the output signal of PLC  22 . PLC  22  is programmably controlled by FCE R 1  to output either p-term input  20   b  or fixed logic 0 (VSS or GND). Thus PLCs  22  and  70  collectively allow the XOR Control signal to be any one of Q, Q-bar, VCC, p-term  20   b , or GND. PLC  90  is programmably controlled by FCE R 5  to select as the primary output signal of the macrocell either Q or the unregistered output signal of EXCLUSIVE OR circuitry  60 . 
     As the &#39;006 patent says, by properly selecting the inputs to EXCLUSIVE OR circuitry  60 , register circuitry  80  can be used to implement D or T flip-flops with inversion control on the D or T inputs. Considering first implementation of a D flip-flop, register circuitry  80  responds to a clock signal pulse by storing the signal that is then being applied to its D input terminal. This signal is the output of EXCLUSIVE OR circuitry  60  and can be any of several possibilities such as (1) the sum-of-products output signal of OR circuitry  40 , either inverted (XOR Control signal logic 1) or not inverted (XOR Control signal logic 0), or (2) p-term  20   b  (selected via PLCs  22  and  70  and passed uninverted by EXCLUSIVE OR circuitry  60  due to PLC  50  being programmed to apply logic 0 to the other input terminal of EXCLUSIVE OR circuitry  60 ). In case (1) immediately above, the XOR Control signal can be logic 1 either as a result of PLC  70  being programmed to output VCC, or as a result of PLCs  22  and  70  being programmed to pass p-term  20   b  and that p-term having a logic 1 value. Similarly in case (1) immediately above, the XOR Control signal can be logic 0 either as a result of PLCs  22  and  70  being programmed to pass GND, or as a result of those PLCs being programmed to pass p-term  20   b  and that p-term having a logic 0 value. Thus the inversion control described above can be either “programmed” (or fixed) (i.e., due to selection of VCC or GND) or dynamic (i.e., due to selection of p-term  20   b ). 
     For register circuitry  80  to implement a T flip-flop, PLC  70  is programmed to feed back either Q or Q-bar. If Q is fed back, register  80  will toggle in response to each clock pulse as long as the output signal of PLC  50  is logic 1. Register  80  will not toggle while the output signal of PLC  50  is logic 0. If Q-bar is fed back, register  80  will toggle in response to each clock pulse while the output signal of PLC  50  is logic 0, but not when that signal is logic 1. 
     From the foregoing, it will be seen that if p-term  20   b  must be applied to EXCLUSIVE OR circuitry  60 , the T flip-flop option is no longer available. Also, if p-term  20   b  must be applied to EXCLUSIVE OR circuitry  60 , programmed inversion control (i.e., based on selection of VCC or GND) on the input to register circuitry  80  is no longer available. 
     FIG. 2 shows an illustrative embodiment of modification of the FIG. 1 circuitry in accordance with this invention so that when it is desired to use p-term  20   b , the T flip-flop option and programmed inversion control are not lost. In FIG. 2 elements  70  and  82  used in FIG. 1 are effectively replaced by elements  110 ,  120  and  130 . In all other respects the circuitry of FIG. 2 may be the same as the circuitry of FIG.  1 . 
     As shown in FIG. 2 the Q output signal of flip-flop  80  is applied to one input terminal of logical NAND circuitry  110 . The output signal of FCE R 4  is applied to the other input terminal of NAND circuitry  110 . Accordingly, the programmed state of FCE R 4  controls whether or not NAND circuitry  110  passes Q. If FCE R 4  is programmed logic 1, NAND circuitry  110  passes Q with logical inversion (i.e., to Q-bar). If FCE R 4  is programmed logic 0, the output signal of NAND circuitry  110  is fixed logic 1. 
     The output signal of NAND circuitry  110  is applied to one input terminal of logical EXCLUSIVE OR (“XOR”) circuitry  120 . The output signal of FCE R 3  is applied to the other input terminal of XOR circuitry  120 . If the output signal of NAND circuitry  110  is Q-bar, the programmed state of FCE R 3  determines whether the output signal of XOR circuitry  120  is Q or Q-bar (i.e., if FCE R 3  is logic 0, the output of XOR circuitry  120  is Q-bar; but if FCE R 3  is logic 1, the output signal of XOR circuitry  120  is Q). On the other hand, if the output signal of NAND circuitry  110  is fixed logic 1, the output signal of XOR circuitry  120  is either fixed logic 1 (when FCE R 3  is logic 0) or fixed logic 0 (when FCE R 3  is logic 1). 
     The output signal of XOR circuitry  120  is applied to one input terminal of logical XOR circuitry  130 . The other input to XOR circuitry  130  is the XOR_PT/GND output signal of PLC  22  (i.e., either p-term  20   b  or fixed logic 0 (GND)). If the output signal of XOR circuitry  120  is fixed logic 1 and the output signal of PLC  22  is GND, then the output signal of XOR circuitry  130  is fixed logic 1 (VCC). This corresponds to the condition shown in line  1  of FIG.  3 . If the output signal of XOR circuitry  120  is fixed logic 0 and the output signal of PLC  22  is GND, the output signal of XOR circuitry  130  is fixed logic 0 (GND). This corresponds to the condition shown in line  2  of FIG.  3 . If the output signal of XOR circuitry  120  is Q-bar and the output signal of PLC  22  is GND, the output signal of XOR circuitry  130  is Q-bar. This corresponds to the condition shown in line  5  of FIG.  3 . If the output signal of XOR circuitry  120  is Q and the output signal of PLC  22  is GND, the output signal of XOR circuitry is Q. This corresponds to the condition shown in line  6  of FIG.  3 . 
     Lines  7 - 10  of FIG. 3 show that if the output signal of XOR circuitry  120  is Q or Q-bar and the output signal of PLC  22  is p-term  20   b , the logical state of p-term  20   b  can be used to control XOR circuitry  130  to selectively invert the Q or Q-bar signal from XOR circuitry  120 . For example, lines  7  and  8  in FIG. 3 show the effect of the logical state of p-term  20   b  on a Q-bar output from XOR circuitry  120 . Lines  9  and  10  in FIG. 3 show the effect of the logical state of p-term  20   b  on a Q output from XOR circuitry  120 . 
     From the foregoing it will be seen that the FIG. 2 circuitry allows simple programmed control of whether or not XOR circuitry  60  inverts the output signal of PLC  50 . This is shown by lines  1  and  2  of FIG. 3, wherein when the XOR_CTRL output signal of XOR circuitry  130  is VCC, XOR circuitry  60  inverts the output signal of PLC  50 , but when the XOR_CTRL output signal of XOR circuitry  130  is GND, XOR circuitry  60  does not invert the output signal of PLC  50 . 
     The FIG. 2 circuitry also allows the XOR_CTRL input of XOR circuitry  60  to be either p-term  20   b  (line  4  of FIG. 3) or the logical inverse of p-term  20   b  (line  3  of FIG. 3) based on programmed control. Thus programmed inversion control is not lost (as it is in the case of FIG. 1) when the XOR_CTRL signal is derived from p-term  20   b.    
     Lines  5  and  6  of FIG. 3 show the basic T flip-flop options available with the FIG. 2 circuitry. Lines  7 - 10  show that these T flip-flop options are not lost when it is desired to use p-term  20   b  as a logical constituent of the XOR_CTRL signal. In particular, the logical state of p-term  20   b  can be used to invert or not invert either the Q or Q-bar signal coming from XOR circuitry  120 . 
     The foregoing demonstrates that the circuitry of FIG. 2 has all the capabilities of FIG. 1, plus several capabilities that the FIG. 1 circuitry does not have. In particular, the circuitry of FIG. 2 allows the user to independently choose between D and T flip-flops, inverted or non-inverted input, and p-term  20   b.    
     FIGS. 4 and 5 show that the more capable circuitry of FIG. 2 can be constructed using only a small number of transistors greater than the number required to construct the less capable FIG. 1 circuitry. FIG. 4 shows one possible transistor-level implementation of the circuitry A for producing the XOR_CTRL signal in FIG.  1 . Not including the control FCEs R 3  and R 4 , this implementation employs  13  transistors (inverter  210  requires two transistors to implement). FIG. 5 shows one possible transistor-level implementation of the circuitry A′ for producing the XOR_CTRL signal in FIG.  2 . Not including the control FCEs R 3  and R 4 , this implementation employs  16  transistors (NAND circuitry  110  requires four transistors to implement, and inverter  220  requires two transistors to implement). Thus only three more transistors are required to provide circuitry A′ for FIG. 2 than are required to provide circuitry A for FIG.  1 . 
     It will be appreciated that the macrocell circuitry of this invention can be used in a variety of contexts. The above-mentioned &#39;006 patent is an example of sum-of-products programmable logic devices in which the present macrocell circuitry can be used. Heile U.S. Pat. No. 6,020,759 shows an example of a programmable logic device which is basically a look-up table logic device, but which has large blocks of random access memory (“RAM”) or read-only memory (“ROM”) that can be used to perform sum-of-products logic. The macrocell circuitry that augments these RAM or ROM blocks is another example of circuitry that can be implemented as shown herein. The &#39;006 patent and the Heile reference mentioned above are hereby incorporated by reference herein in their entireties. 
     FIG. 7 illustrates a programmable logic device  302  including macrocell circuitry in accordance with this invention in a data processing system  300 . Data processing system  300  may include one or more of the following components: a processor  304 ; memory  306 ; I/O circuitry  308 ; and peripheral devices  310 . These components are coupled together by a system bus  320  and are populated on a circuit board  330  which is contained in an end-user system  340 . 
     System  300  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. Programmable logic device  302  can be used to perform a variety of different logic functions. For example, programmable logic device  302  can be configured as a processor or controller that works in cooperation with processor  304 . Programmable logic device  302  may also be used as an arbiter for arbitrating access to a shared resource in system  300 . In yet another example, programmable logic device  302  can be configured as an interface between processor  304  and one of the other components in system  300 . It should be noted that system  300  is only exemplary, and that the true scope and spirit of the invention should be indicated by the following claims. 
     The PLCs mentioned throughout this specification (which includes the appended claims) can be implemented in any of a wide variety of ways. For example, each PLC can be a relatively simple programmable connector such as a switch or a plurality of switches for connecting any one of several inputs to an output. Alternatively, each PLC can be a somewhat more complex element which is capable of performing logic (e.g., by logically combining several of its inputs) as well as making a connection. In the latter case, for example, each PLC can be product term logic, implementing functions such as AND, NAND, OR, or NOR. Examples of components suitable for implementing PLCs are EPROMs, EEPROMs, pass transistors, transmission gates, antifuses, laser fuses, metal optional links, etc. As has been mentioned, the components of PLCs and other logic circuitry can be controlled by various, programmable, function control elements (“FCEs”). (With certain implementations (e.g., fuses and metal optional links) separate FCE devices may not be required, so that in those cases depiction of FCE devices in the accompanying drawings merely indicates that the circuitry is programmable.) FCEs can also be implemented in any of several different ways. For example, FCEs can be SRAMs, DRAMs, first-in first-out (“FIFO”) memories, EPROMs, EEPROMs, function control registers (e.g., as in Wahlstrom U.S. Pat. No. 3,473,160), ferro-electric memories, fuses, antifuses, or the like. From the various examples mentioned above it will be seen that this invention is applicable both to one-time-only programmable and reprogrammable devices. 
     It will be understood that the foregoing is only illustrative of the principles of this 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 particular signal levels (e.g., logic 0, logic 1, VCC, GND, etc.) mentioned herein are only illustrative, and different signal levels can be used instead if desired. Similarly, the particular types of logic circuits or elements shown herein are only illustrative, and other logically equivalent elements can be used instead if desired.