Abstract:
A programmable cell comprises an externally loadable electrically erasable (EE) transistor cell that is configured to be independent of the currently active state of the programmed cell. When all of the EE cells are loaded with a new configuration, the contents of all of the EE cells are loaded into the corresponding programmable cells, preferably within one clock cycle. Because the entirety of the programmable cells can be pre-loaded with the new configuration, the time to effect a reconfiguration is one clock cycle. Because an EE cell is significantly smaller than a conventional four to six transistor storage cell, the area required to implement this single-clock-cycle reconfiguration capability is substantially less than traditional dynamically reprogrammable memory configurations. In an alternative embodiment, multiple EE cells can be associated with each programmable cell, thereby allowing a multiple-configuration capability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of integrated circuit design, and in particular to programmable devices. 
     2. Description of Related Art 
     Programmable integrated circuits are common in the art, wherein the operation of the circuit is based on the contents of memory cells. Typically, the programming comprises a sequence of instructions, or a set of data, or a combination of both. The instructions may be, for example, object code for an embedded processor, and the data may be, for example, the values for a look-up table in a programmable logic device, or, the desired state of switch elements in a field-programmable gate array, and so on. For ease of reference, the terms “program”, “program data” and “code” as used herein includes instructions and data, and any combination thereof. 
     The program is typically “downloaded” from a programming device, such as a desktop computer. The programming device asserts a control signal to place the programmable device into a reception mode, and communicates the content of the program as a sequence of location/code pairs. The location identifies the individual programmable element within the programming device that is being programmed, and the code identifies the value that the programmable element receives. In the reception mode, the programmable device places the received code at the specified location. In some devices, the location argument can be implicit: upon receipt of the control signal, the programmable device places the code at a default start location, and at sequential locations thereafter. 
     Depending upon the architecture of the programmable device, the reprogramming of the programmable device can be incremental, allowing for select locations to be reprogrammed, or total, requiring all locations to be reprogrammed. In most cases, additional information is also provided to facilitate the programming or reprogramming, such as checksums, error correcting sums, and so on. These and other programming techniques are common in the art. 
     The downloading of a program onto a programmable device typically renders the device inoperative while the program is being downloaded. A variety of techniques are available to minimize the inoperative time associated with the download of the program. The device and program may be partitioned into independent blocks, and the individual device blocks are loaded by the corresponding program block while that device block is not being utilized. This approach requires safeguards to assure that conflicts between the prior program and the new program do not arise among related blocks. An alternative approach is to provide multiple “planes” of programmable elements within the system. In this approach, for example, a select-bit is used to select one of two planes of memory as an “active” plane for system operation, the other plane being inactive, from the system&#39;s perspective. Programming is effected by loading the inactive plane, then toggling the select-bit, making the newly programmed plane the currently active plane. The use of two programmable planes, however, effectively doubles the size of the area consumed by the programmable elements in the design. U.S. Pat. No. 5,778,439 discloses the use of multiple storage cells per memory element. In the referenced patent, incorporated herein by reference, one of the cells in each memory element is designated as an active storage, and the remaining cells (nominally 7) are inactive storage elements. Each of the inactive storage elements form a “virtual” memory, that can be dynamically designated as the active storage, replacing the currently active storage. Each storage cell of each memory device in the referenced patent, however, comprises four to six transistors, thereby substantially increasing the size of the area consumed by these virtual memory device. Hybrid approaches are also viable. For example, buffering can be provided within the programmable device to minimize the effects of the relatively slow process of transferring the program from an external programming device by delaying the commencement of the actual programming of the programmable elements until a significant portion of the program is received. Because an internal transfer of the program can be effected more efficiently than a transfer from an external source, this buffering approach significantly reduces the overall inoperable duration, but a significant amount of buffering, and corresponding circuit area, must be provided to realize this gain. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a programmable logic device that can be programmed, and reprogrammed, quickly. It is a further object of this invention to provide a programmable logic device that can be reprogrammed quickly that consumes minimal additional circuitry and circuitry area. It is a further object of this invention to provide an area efficient architecture for multi-plane programming applications. 
     These objects and others are achieved by providing a programmable cell having an externally loadable electrically erasable (EE) transistor cell that is configured to be independent of the currently active state of the programmed cell. When all of the EE cells are loaded with a new configuration, the contents of all of the EE cells are loaded into the corresponding programmable cells, preferably within one clock cycle. Because the entirety of the programmable cells can be pre-loaded with the new configuration, the time to effect a reconfiguration is one clock cycle. Because an EE cell is significantly smaller than a conventional four to six transistor storage cell, the area required to implement this single-clock-cycle reconfiguration capability is substantially less than traditional dynamically reprogrammable device configurations. In an alternative embodiment, multiple EE cells can be associated with each programmable cell, thereby allowing a multiple-configuration capability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
     FIG. 1 illustrates an example block diagram of a programmable device containing programmable cells in accordance with this invention. 
     FIG. 2 illustrates an example flow diagram for programming a programmable device in accordance with this invention. 
     FIG. 3 illustrates an example block diagram of an alternative programmable cell in accordance with this invention. 
    
    
     Throughout the drawings, same reference numerals indicate similar or corresponding features or functions. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an example block diagram of a programmable device  100  containing programmable cells  101 - 103  in accordance with this invention. The programmable device  100  includes a system  190  of logic that performs a function and produces an output in dependence upon the contents of the programmable cells  101 - 103 . Although only three programmable cells  101 - 103  are illustrated in FIG. 1, for ease of understanding, the principles of this invention are scalable to arbitrarily larger quantities of programmable cells. 
     Each programmable cell  101 - 103  includes a first memory element  151 - 153 , a transfer gate  141 - 143 , and a second memory element  161 - 163 . The first memory elements  151 - 153  receive program data  111 - 113  from an external source (not shown). The transfer gates  141 - 143  isolate the first memory elements  151 - 153  from the second memory elements  161 - 163 , respectively, until the system load signal  140  is asserted. In accordance with this invention, the first memory elements  151 - 153  comprise a electrically erasable (EE) transistor cell  254 , which is significantly smaller in area than a conventional four to six transistor RAM cell. As would be evident to one of ordinary skill in the art, EE cells are substantially slower than conventional RAM cells, requiring a longer time to store a new logic value, and a longer time to retrieve that value. This invention is premised on the observation that if the first memory elements  151 - 153  are used for relatively infrequent reconfiguration tasks, the slow speed of access of an EE device can be tolerated, in return for a significant reduction of circuit area. 
     The program data  111 - 113  is provided to the first memory element by selectively loading  121 - 123  each first memory element  151 - 153  when the appropriate program data value  111 - 113  is provided to each cell. As discussed above with regard to conventional programmable devices, the loading of each cell is typically effected via a sequential load of the program data. In a preferred embodiment, the program data is provided on a single bus to all cells, and loaded to the individual cells using a location address associated with each cell  101 - 103  to selectively assert each cell load signal  121 - 123  when the program data corresponds to that cell. 
     After all of the first memory elements  151 - 153  of each programmable cell  101 - 103  are loaded with the program data  111 - 113 , the contents of each of the first memory elements  151 - 153  are transferred to the second memory elements  161 - 163  via the transfer gate  141 - 143  of each programmable cell  101 - 103 . As illustrated in FIG. 1, the transfer gates  141 - 143  have a common control signal, the system load signal  140 . When the system load signal  140  is asserted, the contents of all of the first memory elements  151 - 153  are simultaneously transferred to all of the second memory elements  161 - 163 . 
     The system  190  interacts exclusively with the second memory elements  161 - 163 . Thus, immediately after the system load signal  140  is asserted, and the program data is transferred from the first memory elements  151 - 153  to the second memory elements  161 - 163 , the system  190  will operate in accordance with the newly loaded program data  111 - 113 . In a preferred embodiment, the system load  140  is asserted in synchronization with a master clock signal having a period that is sufficiently long to allow the transfer of the program data from the first memory elements  151 - 153  to the second memory elements  161 - 163 . In a preferred embodiment, the system  190  is placed into an inoperative state for a single clock cycle while the system load  140  is asserted. Thereafter, the system  190  interacts in dependence upon the newly loaded program data  111 - 113  in the second memory elements  161 - 163 , via the outputs  181 - 183  of each cell  101 - 103 . 
     When the system load  140  is de-asserted, the second memory elements  161 - 163  are isolated from the first memory elements  151 - 153 , thereby allowing the first memory elements  151 - 153  to be reloaded with new program data  111 - 113  without affecting the operation of the system  190 . When all of the first memory elements  151 - 153  contain the new program data  111 - 113 , and the system  190  is placed in an inoperative mode, the system load  140  is asserted, as discussed above, to load the second memory elements  161 - 163  with the new program data  111 - 113  that is contained in the first memory elements  151 - 153 . 
     Thus, in accordance with this invention, the programmable device  100  can be reconfigured from one program to another program within a single clock cycle, regardless of the size of the program, and regardless of the time required to load the program data into the first memory elements  151 - 153 . 
     Because the first memory elements  151 - 153  need only transfer a logic value to the second memory elements  161 - 163 , the first memory elements  151 - 153  can be minimally sized, compared to the second memory elements  161 - 163  that may have loading demands and speed requirements that require larger sized components. A sense amplifier  258  is used to provide the current required to effect the transfer of the state of the EE cell  254  to the corresponding second memory elements  161 - 163 . The electrically erasable transistor comprising the EE cell  254  is loaded with the program data  110  upon assertion of the cell load signal  121 . 
     The second memory elements  161 - 163  of each of the programmable cells  101 - 103  comprise two inverters  262 ,  264  that are configured as a conventional static latch, as typically used in an SRAM device. When the transfer gates  141 - 143  are in a conductive state, the corresponding sense amplifiers  258  provide sufficient current to drive the corresponding inverters  262  to the appropriate state, and the inverters  264  maintain that state after the transfer gates  141 - 143  isolate the sense amplifiers  258  from the inverters  262 . Drivers  268  transfer this stored state to the cell outputs  181 - 183 . Alternative memory devices may be used for the second memory elements  161 - 163 , including dynamic latches and the like. 
     FIG. 2 illustrates an example flow diagram for programming a programmable device in accordance with this invention. The loop  310 - 350  effects the loading of the program data into the first memory elements of each programmable cell in the programmable device, based on the set of location/data pairs in the program data set  110 . As in a conventional programmable device, only the programmable cells that have a different value from their current value need be changed, but the explicit loading of all cells is often the simpler process. In a preferred embodiment, the data is placed at the input of all the cells, at  320 , and the cell that is identified by the program data location parameter is loaded, at  330 . 
     After all of the programmable cells are loaded with their appropriate logic values, the system is placed in an inoperative mode, at  360 , and the system load signal is asserted, at  370 , and held in that state for a sufficient duration to transfer the contents of all of the first memory elements to the second memory elements, typically one clock cycle. At  380 , the system is placed back into an operative mode, and resumes operation in accordance with the newly loaded program. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, FIG. 3 illustrates an example block diagram of an alternative programmable cell  400 , as may be used for each of the programmable cells  101 - 103  in FIG. 1 to allow for the storage of multiple alternative programs. 
     As illustrated in FIG. 3, a first multiplexer  410  selects which memory element  450 A,  450 B, . . .  450 X receives the current program data  110 . In accordance with this invention, each memory element  450 A,  450 B, . . . is an EE cell, as discussed above. In a preferred embodiment of a programmable device  100 , each first multiplexer of the programmable cells  400  is controlled by the same memory-load-select signal  415 , so that one program can be loaded into memory element  450 A of each cell  400  when the load-select  415  is in a first state, another program can be loaded into memory element  450 B of each cell  400  when the load-select  415  is in a second state, and so on. A second multiplexer  420  selects, via a system-load-select signal  425 , which memory element  450 A,  450 B, . . .  450 X is loaded into the second-stage memory element  460  when the system load signal  140  is asserted to the transfer gate  440 , corresponding to the transfer gates  141 - 143  of FIG.  1 . The system-load-select signal  425  in a preferred embodiment is also commonly connected to all cells  400 . 
     The configuration of FIG. 3 is particularly well suited for applications wherein the programmable device  100  performs a sequence of functions, each function being effected via a program that is loaded in each set of memory elements  450 A,  450 B, . . .  450 X. 
     Note that not every programmable element in a programmable device  100  need be a multi-memory programmable cell  101 - 103 . For example, a programmable device  100  may be a processing chip that includes a program segment and a data segment of programmable elements. In such an application, as discussed above, the need for a rapid access to the program segment may not exist, but a rapid access to update and retrieve data within the data segment may be required. In such an embodiment, the data segment may comprise conventional prior-art multi-state memory cells, while the program segment, may comprise the smaller, albeit slower, EE cell based memory cells in accordance with this invention. Other memory elements may be conventional single-state memory cells. In like manner, some of the programmable elements of the programmable device may be the multi-state programmable cells  400  in accordance with this invention. 
     The structure and architecture presented in the figures are presented for illustration purposes, and alternative embodiments will be evident to one of ordinary skill in the art in view of this invention. For example, although the programmable cells  101 - 103 ,  400  are presented as single logic entities in the illustrations, the individual components of the cells  101 - 103 ,  400  may be physically distinct from each other. For ease of layout, manufacture, or testing, for example, all of the EE cells  254  may be located in a contiguous area, and the second memory elements  161 - 163  distributed throughout the device  100 ; or, some programmable cells may be physically integrated, while others have physically partitioned components; and so on. 
     Note also that, depending upon the particular design criteria for the programmable device  100 , other optimizations may also be employed. For example, the function of the transfer gate  440  in FIG. 3 can be integrated into the second multiplexer  420 , wherein the system load select signal  425  includes a “select none” mode that isolates all of the first memory elements  450 A- 450 X from the second memory element  460 , and effects a transfer to the second memory element  460  when one of the first memory elements  450 A- 450 X are selected. 
     In like manner, although the system load  140  is preferably asserted in synchronization with a master clock, while the system  190  is in an inoperative state, an asynchronous system load, while the system  190  is operative, can be effected, provided that the potential anomalies caused during the transition from one program to another are determined to be acceptable. Similarly, although the system load  140  is presented as the sole means for programming the second memory element  161 - 163 ,  460 , the second memory element  161 - 163 ,  460  may also be programmed via the system  190 . That is, the second memory element  161 - 163 ,  460  may operate as a conventional two-port RAM, with input from either the first memory elements  151 - 153 ,  450 A- 450 X, or from the system  190 , via  181 - 183 . In such an embodiment, the entire contents of the RAM can be replaced within one clock cycle, then subsequently processed and updated by the system  190  while new data is being loaded into the first memory elements  151 - 153 ,  450 A- 450 X. 
     These and other configuration and operation modifications will be evident to one of ordinary skill in the art in view of this invention, and are included within the scope of the following claims.