Abstract:
A RAM device, such as the type embedded in a programmable logic device, is configurable to alter the depth of the addressable elements and the width of the number of data bits received or produced by the RAM device. The RAM device includes a number of address ports for receiving the read and/or write address signals, but the RAM device may be configured such that the depth requires fewer address signals then there are address ports. Likewise, the RAM device includes a number of input and output data ports for receiving and producing the data bits, but the width of the RAM device may be configured such that the number of data bits actually received or produced are less than the number of data ports. The depth and the width of the RAM device are configured together so that the depth is increased when the width is decreased and vice versa. This permits a number of appropriately configured RAM devices to be combined to produce a deep and wide RAM circuit without requiring the use of additional logic blocks, such as buffers, inverters, and multiplexors that reduce the speed of the circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor memory devices, and in particular to random access memory (RAM) devices having a configurable depth and width. 
     BACKGROUND 
     Conventional RAM devices include memory cells that are arranged in columns and rows. When writing a data bit into a particular memory cell, the data bit is provided on a bit line for an entire column of memory cells. A particular memory cell along the column is then selected for storing the information bit by providing a row selection signal on a particular write word line. The number of selectable memory cells or elements in the RAM device is known as the depth of the device. The depth of the RAM device is a function of the number of address bits received by the RAM device. The width of the RAM device is the number of data bits that can be stored in the RAM device per address location. Conventionally, the depth and width of a RAM device is fixed. 
     FIG. 1 shows a simplified view of a conventional RAM device  10 . RAM device  10  is shown as having a six bit address port  12  and a two bit wide data word input port  14  and output port  15 . RAM device  10  is also shown as having write and read select ports  16  and  18 , respectively. 
     Because conventional RAM device  10  has a six bit address port  12 , RAM device  10  has a depth of 64 elements (2 6 =64). Because RAM device  10  has a two bit data word input and output ports  14  and  15 , RAM device  10  has a width of 2. Thus, RAM device  10  has a size of 64×2, which is fixed. 
     Blocks of RAM devices are sometimes used in programmable logic devices as building blocks to generate larger RAM devices. When a user desires a RAM device having a greater depth and/or width, the user programs the programmable logic device to combine multiple RAM devices. The combination of multiple RAM devices, where each individual device has a fixed depth and width, results in a greater depth and/or width. 
     A user conventionally increases the width of a RAM device by combining multiple RAM devices as shown in FIG.  2 . FIG. 2 shows a simplified view of two 64×2 RAM devices  10  and  30  combined to form a circuit  28  with an increased width of 64×4. Only the address ports  12 ,  32  and the data output ports  15 ,  35  are shown on RAM devices  10 ,  30 , respectively, for the sake of simplicity. As shown in FIG. 2, the address ports of the two RAM devices  10  and  30  are combined in pairs. The total number of address bits received by the address ports  15  and  35  of the RAM devices  10  and  30  remains the same and, thus, the total depth remains  64 . As shown in FIG. 2, the combination of RAM devices  10  and  30  results in four data output ports  15  and  35 . Thus, by combining RAM devices  10  and  30  as shown in FIG. 2, the total width of the data bits that may be stored is increased by 2. 
     If a user desires a RAM device with an increased number of addressable elements, the user conventionally combines multiple RAM devices as shown in FIGS. 3A and 3B. FIGS. 3A and 3B show a simplified view of two 64×2 RAM devices  10  and  30  combined to form circuits  38  and  39  with an increased depth of 128×2 for write operations and read operations, respectively. The respective address ports  12  and  32  of RAM devices  10  and  30  are combined as in FIG.  2 . As shown in FIG. 3A, for write operations, circuit  38  increases the depth from 64, i.e., 6 address ports, to 128, i.e., 7 address ports, with logic gates such as AND gate  40  and AND gate  42  coupled to enable control ports  11  and  31 , respectively. As shown in FIG. 3A, AND gate  42  has a inverter at the write address  6  input terminal, i.e., the seventh address port. The input terminals of logic gates  40  and  42  are coupled together. As shown in FIG. 3B, for read operations, circuit  39  has the data output ports  15 ,  35  coupled to input terminals of a multiplexor (MUX)  44 , shown as having two multiplexors, which selects the desired output ports. The read address  6  input terminal, i.e., the seventh address port, is coupled to the select terminal of MUX  44 , which selects the appropriate data output ports based on the select signal. Thus, circuits  38  and  39  have effective RAM device sizes of 128×2. The depth of circuits  38  and  39  may be further increased by combining additional RAM devices in a similar manner. Unfortunately, each increase in depth requires the use of additional logic gates. The use of logic gates, however, slows the speed of the RAM device and requires additional power. 
     To avoid the use of many logic gates, large RAM devices may be manufactured. However, large RAM devices are expensive and utilizes a large amount of space on the chip. If the user desires only a small amount of RAM, the cost and space of the large RAM will be wasted. 
     SUMMARY 
     A RAM device, such as the type embedded in a programmable logic device, is configurable to alter the depth of the addressable elements and the width of the number of data bits stored in the RAM device per address. The RAM device includes a number of address ports for receiving the read and/or write address signals. However, for a shallow configuration of the RAM device, the RAM device receives fewer address signals then there are address ports. Consequently, in shallow configurations, a number of address ports will not be used. Likewise, the RAM device includes a number of input and output data ports, but the width of the RAM device may be configured such that the number of data bits actually stored in the RAM device is less than the number of data ports. Thus, in narrow configurations, a number of the input and output data ports will not be utilized. 
     The depth and the width of the RAM device are configured together so that the depth is increased when the width is decreased and vice versa. This permits the user to configure a number of RAM devices to the desired depth. The RAM devices may then be combined easily to increase the width. Consequently, the user can configure the RAM devices to be deep, wide, or both without the use of logic gates that would reduce the speed of the device. Further, because the RAM device is configurable, the user can use the amount of RAM desired and does not unnecessarily waste the RAM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified view of a conventional RAM device. 
     FIG. 2 shows a simplified view of two RAM devices combined to form a RAM device with an increased width. 
     FIGS. 3A and 3B show a simplified views of two RAM devices combined to form a RAM device with an increased depth for write operations and read operations, respectively. 
     FIG. 4 shows a schematic view of a programmable logic circuit that includes a plurality of programmable logic cells and a plurality of configurable RAM devices in accordance with an embodiment of the present invention. 
     FIG. 5 shows a simplified view of a configurable RAM device. 
     FIG. 6, including FIGS. 6A,  6 B,  6 C, and shows a schematic view of a configurable RAM device that has a configurable depth and width in accordance with the present invention. 
     FIG. 7 shows a detailed schematic view of the mode decoder shown in FIG.  6 . 
     FIG. 8 is a truth table showing the mode designated by the mode decoder based on the input signals on the two mode input ports. 
     FIG. 9, including FIGS. 9A and 9B, shows a detailed schematic view of a multiplexor shown in FIG.  6 . 
     FIG. 10 shows a detailed schematic view of a decoder shown in FIG.  9 . 
     FIG. 11, including FIGS. 11A and 11B, shows a detailed schematic view of the input/output multiplexor shown in FIG.  9 . 
     FIG. 12 is a table indicating the data busses used in the input/output multiplexor in the different modes. 
     FIG. 13 shows a detailed schematic view of a memory cell. 
     FIG. 14 shows a detailed schematic view of a precharge circuit. 
     FIG. 15 shows a detailed schematic view of a precharge control circuit. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a schematic view of a programmable logic circuit  100  that includes a plurality of programmable logic cells  102  and that is embedded with a plurality of RAM devices  110 . Each logic cell is a collection of logic gates with associated interconnections and programming devices, such as antifuses. An interface  104  is disposed between the programmable logic cells  102  and the plurality of RAM devices  110 . The programmable logic cells  102  and interface  104  may be conventional, while RAM devices  110  have a configurable depth and width in accordance with the present invention. 
     FIG. 5 shows a simplified view of a configurable RAM device  110 . RAM device  110  includes a configurable number of address ports  112  and a configurable number of data input ports  122  and output ports  114 . The number of address ports  112  is represented by an “x”, while the number of input ports  122  and output ports  114  are represented by a “y”. RAM device  110  also includes a mode port  116 , which may have multiple ports, e.g., 2 ports, represented by a “a”. Mode port  116  receives mode signals indicating the desired depth and width of RAM device  110 , i.e., the signals at the mode port  116  is used to determine the number (x) of address ports  112  and the number (y) of data input ports  122  and output ports  114 . As shown in FIG. 5, RAM device  110  also includes other ports, such as the write control port  118  and read control port  120 , both of which include enable and clock ports for respective write and read operations. RAM device  110  also includes the data input port  122 . RAM device  110 , of course, includes other conventional ports. 
     RAM device  110  has a maximum number of address ports  112  that may be used, e.g., x≦9, and a maximum number of data input ports  122  and output ports  114  that may be used, e.g., y≦18. Thus, RAM device  110  has a maximum depth of addressable elements of 512 (2 9 =512) and a maximum width of data bits stored is 18. However, a RAM device having a depth and width of 512×18 would be a relatively large block of RAM, which would be expensive to manufacture in terms of space on the silicon chip and would be slow. Moreover, a 512×18 RAM device would often be under utilized. Consequently, in accordance with an embodiment of the present invention, RAM device  110  is configurable into a number of smaller sizes, or modes. Depending on the desired mode of RAM device  110 , certain of the nine address ports  112  and of the eighteen data output ports  114  and input ports  122  will be unused. 
     By way of example, RAM device  110  will be described as configurable into four different modes of operation having a depth and a width of 64×18 (“×18”), 128×9 (“×9), 256×4 (“×4”), and 512×2 (“×2”). The ×18 mode has a depth of 64 and thus uses six of the address ports  112  (2 6 =64). The other modes use seven, eight, and nine of the address ports  112 , respectively. The ×18 mode has a width of 18 and thus uses all of the input ports  122  and output ports  114 . The other modes use nine, four, and two of the input ports  122  and output ports  114 , respectively. Thus, as can be seen, RAM device  110  advantageously trades off the depth and width of the RAM device  110  so that RAM device  110  can support either a deep but narrow configuration or a shallow but wide configuration, as well as in between configurations. 
     Because RAM device  110  has a configurable depth and width, multiple RAM devices may be coupled together in programmable logic circuit  100  to create many different sizes of RAM, which is particularly useful in creating FIFO&#39;s (first-in-first-out) circuits. The design requirements for FIFOs are usually very specific, i.e., either deep RAM devices or shallow RAM devices are required. Advantageously, the present invention permits the user to easily configure the depth and width of RAM to the desired size, without losing speed of the device or wasting areas of the RAM. If, for example, a user desires a deep RAM with 32 bits of data, RAM device  110  can be configured into a deep but shallow mode, e.g., 512×2. The user can couple sixteen RAM devices  110  together, in a manner similar to that shown in FIG. 2, resulting in a combined 512×32 RAM circuit. Consequently, the user is not required to use logic gates in the front and back of the RAM devices, as shown in FIG. 3, to produce a deep RAM. 
     As shown in FIG. 4, there are four columns of logic cells  102  for every RAM device  110 . Each individual column of logic cells contains a finite number of channels, e.g., 38. RAM devices  110 , however, requires a large number of input signals, e.g., 64. By using four columns of logic cells  102  per RAM device  110 , there is a much greater number of channels per RAM device  110 , e.g., 152, than if only one column is used. Thus, it is assured that RAM device  110  will have access to an adequate number of channels, while each logic cell will have an adequate number of remaining channels for routing. 
     FIGS. 6 through 15 show detailed schematics, in various detail, and associated tables, of a RAM device  200 , which has a configurable depth and width in accordance with the present invention. RAM device  200  may be used in programmable logic circuit  100  (FIG.  4 ). Conventional areas of RAM device  200  are shown in FIGS. 6 through 15 in block form in order to avoid unnecessarily obfuscating the present invention. 
     As shown in FIG. 6, RAM device  200  is a dual port RAM having a conventional read address register  202  and a separate conventional write address register  204 . Because RAM device  200  is a dual port RAM, many of the elements in RAM device  200  are duplicated, one set of elements for writing and another set for reading. It should be understood, however, that a single port RAM embodying the present invention may be easily designed by one of ordinary skill in light of the present disclosure, for example, by combining any duplicate elements into a single element. 
     Read address register  202  includes nine ports to receive the read address (ra&lt; 0 : 8 &gt;) and the write address register includes nine ports to receive the write address (wa&lt; 0 : 8 &gt;). Because there are physically nine ports to receive the read address (ra&lt; 0 : 8 &gt;) and the write address (wa&lt; 0 : 8 &gt;), the RAM device  200  can support a deep configuration, i.e., the 512×2 configuration. In shallower configurations, e.g., the 256×4, 128×9, and 64×18, some of the ports will not be used. As shown in FIG. 6, read address register  202  and write address register  204  receive independent control signals including their own clocking signals (rdclk and wdclk, respectively) from a RAM control  208 . This advantageously permits RAM device  200  to read and write data simultaneously and at different clock frequencies. Of course, these functions may be combined into one address register such that RAM device  200  acts as a single port RAM, as is well understood by those of ordinary skill in the art. 
     RAM device  200  also includes a conventional data register  206 , which receives write data (wd&lt; 0 : 17 &gt;), which is the input data to the RAM device  200 . There are physically eighteen ports into data register  206  for receiving write data (wd&lt; 0 : 17 &gt;) so that a wide configuration, i.e., ×18, is supported by RAM device  200 . However, in narrow configurations, e.g., ×2, ×4, and ×9, some of the ports will not be used. 
     Also included in RAM device  200  is the RAM control  208 . RAM control  208  controls the operation of RAM device  200 . As can be seen in FIG. 6, RAM control  208  receives a number of input signals, including a read clock signal (rclk) and a separate write clock signal (wclk), and other necessary input signals, which RAM control  208  uses to independently control read address register  202  and write address register  204 . RAM control  208  is a conventional RAM control circuit except that it independently controls read operation on one port and a write operation on the other port. 
     RAM device  200  also includes a mode decoder  210 , which decodes the mode control signals (mode&lt; 0 : 1 &gt;) used to define the size configuration of RAM device  200 . Mode decoder  210  is shown in greater detail in FIG.  7 . As shown in FIG. 7, mode decoder  210  receives two mode input ports (mode 0  and mode 1 ), which are logically converted by NOR logic gates  212 ,  214 , and  216  and NAND logic gate  218  into configuration signals used to configure the depth and width of RAM device  200 , i.e., ×2, ×4, ×9, or ×18. In addition, mode decoder  210  receives a control signal (con&lt; 0 &gt;) and a scan signal (scan). Control signal (con&lt; 0 &gt;) is an internal disable signal that is used to facilitate testing of the RAM device  200  during production. Scan signal (scan) is used in power up loading of the RAM. Both control signal (con&lt; 0 &gt;) and scan signal scan are logic “0” during normal operation of the RAM device  200  and are not relevant to the operation of the present invention. FIG. 8 is a truth table showing the mode designated by mode decoder  210  based on the input signals on the two mode input ports (mode 0  and mode 1 ), which are fixed during programming. 
     The read address register  202  and write address register  204  produce read address signals (radd&lt; 0 : 8 &gt;) and write address signals (wadd&lt; 0 : 8 &gt;), respectively. The first four least significant bits of read address (radd&lt; 0 : 3 &gt;) and write address (wadd&lt; 0 : 3 &gt;) are siphoned off and received by a conventional ×16 word line decoder  220 , which provides write word line signals (wlw&lt; 0 : 15 &gt;) and read word line signals (wlr&lt; 0 : 15 &gt;) to an array of memory cells  230 . 
     The remaining bits of the read address signal (radd&lt; 4 : 8 &gt;) and write address signal (wadd&lt; 4 : 8 &gt;) are received by multiplexor  240 . Multiplexor  240  also receives the output signals from mode decoder  210  designating the size configuration of RAM device  200  and the write data (wd&lt; 0 : 17 &gt;) from the data register  206  to be written into the array of memory cells  230 , during write operations. Multiplexor  240  produces the bit line write (blw&lt; 0 : 71 &gt;) to the memory cells  230 , and receives the bit line read (blr&lt; 0 : 71 &gt;) from memory cells  230 . Multiplexor  240  also produces the read data (m&lt; 0 : 17 &gt;) from the array of memory cells  230  during read operations. 
     FIG. 9 shows a detailed schematic of multiplexor  240 . As shown in FIG. 9, multiplexor  240  includes two decoders  242 ,  244  which receive some of the bits from the read address (ra&lt; 6 : 8 &gt;) and the write address (wa&lt; 6 : 8 &gt;), respectively, as well as the size configuration. FIG. 10 shows a detailed schematic of decoder  242 . It should be understood that decoder  244  is substantially the same as decoder  242 . Decoder  242  logically converts the mode and the address signals it receives into output signals. 
     Multiplexor  240  also includes a conventional predecode unit  250  and conventional read and write multiplexors  252 ,  254 , which respectively receive the bit line read signals (blr&lt; 0 : 71 &gt;) from the array of memory cells  230  and transmit the bit line write signals (blw&lt; 0 : 17 &gt;) to the array of memory cells  230 . 
     As shown in FIG. 9, multiplexor  240  includes two input/output multiplexors (“iomux”)  246 ,  248 , which receive the output signals from respective decoders  242 ,  244 . As can be seen, iomux  248  receives the write data (wdb&lt; 0 : 17 &gt;) to be written into RAM device  200  from data register  206 , while iomux  246  produces the read data (m&lt; 0 : 17 &gt;) to be produced by RAM device  200 . FIG. 11 shows a detailed schematic of iomux  246 . It should be understood, however, that iomux  248  is substantially similar to iomux  246 . Based on the output signals generated by decoder  242 , iomux  246  will select the appropriate data busses. FIG. 12 shows a table indicating the data busses that are used in the different modes. As shown in FIG. 12, in the ×18 mode all eighteen data busses are used, in the ×9 mode data busses d 0 , d 2 , d 4 , d 6 , d 9 , d 11 , d 13 , d 15 , and d 17  are used, in the ×4 mode data busses d 0 , d 4 , d 9 , d 13  are used, and in the ×2 mode only data busses d 0  and d 9  are used. Thus, iomux  246  can support a ×18 wide data register but, depending on the mode, can convert to a narrower data register, such as a ×2, ×4 or ×9. 
     FIG. 13 shows a schematic view of an individual memory cell  231  that is used in the array of memory cells  230 . Memory cell  231  is disposed between a read bit line blr and a write bit line blw. The memory cell  231  includes a latching circuit  232  having a first inverter  233  cross-connected with a second inverter  234  that are connected to the read bit line blr through transistor  236  and to the write bit line blw through transistor  237 . The memory cell  231  also includes a buffer  235  that is used to isolate the latching circuit  232  from any capacitive loading on the read bit line blr. Buffer  235  is larger than either first inverter  233  or second inverter  234 . The gates of transistors  236  and  237  are connected to read word lines wlr and write word lines wlw from word line decoder  220  (shown in FIG.  6 ). 
     The array of memory cells  230  shown in FIG. 6 is an array of 72 columns and 16 rows of memory cells  231  as shown in FIG.  13 . 
     Each write bit line wbl in the array of memory cells  230  is coupled to a precharge circuit  270 . FIG. 14 shows precharge circuit  270 . As shown in FIG. 14, precharge circuit  270  switchably connects write bit line wbl to a ground reference voltage in response to a precharge signal prchg. 
     Precharge circuit  270  is controlled by a precharge control circuit  280 , as shown in FIG.  15 . As shown in FIG. 15, precharge control circuit  280  receives a write pulse signal from the write pulse generator  290  (FIG.  6 ), which produces a delayed write pulse signal wp after receiving the generate write pulse signal genwp from write address register  204 . Precharge control circuit  280  also receives the write enable signal wen and a precharge enable signal enb from the RAM control  208 . The write enable signal wen is conventionally generated when it is desired that RAM device  100  perform a write operation. The precharge enable signal enb is a test signal and is a logic “0” during normal operation of RAM device  100 . 
     It should be understood that if desired, a conventional memory cell and precharge circuit may be used. 
     The read data (m&lt; 0 : 17 &gt;)produced by multiplexor  240  is received by a conventional driver buffer circuit  260 , which produces the read data (rd&lt; 0 : 17 &gt;). Driver buffer circuit  260  is coupled to a conventional enable circuit  261  that receives the mode signals from mode decoder  210  and enables only output ports that are used. Thus, the unused output ports from driver buffer circuit are disabled. 
     While the present invention has been described in connection with specific embodiments, one of ordinary skill in the art will recognize that various substitutions, modifications and combinations of the embodiments may be made after having reviewed the present disclosure. The specific embodiments described above are illustrative only. Various adaptations and modifications may be made without departing from the scope of the invention. For example, the configurable depth and width may be used with single port or dual port RAM devices. The spirit and scope of the appended claims should not be limited to the foregoing description.