Abstract:
A method of serially reading and writing random access memory arrays is provided. Although the read/write inputs continually change as programming data are clocked into the input buffers, a read/write control circuit prevents the constantly changing read/write inputs from causing undesired reading and writing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the serial programming of random access memory arrays, and particularly random access memory arrays contained within larger devices such that the number of input/output pins available for programming is limited. 
     Typically, in the programming of random access memory arrays, one provides the data to be stored and address information that indicates where in the array the data are to be stored. To read the array, the address of the locations desired would be provided. In addition, Output Enable and read/write control signals are typically provided. Typically, the programming data, address data, and control signals are fed in parallel to a buffer. Change of the read/write control signal to a write state then causes the programming data in the buffer to be written to the appropriate places in the array as determined by the address data. 
     A programming scheme such as that just described ordinarily is performed in parallel, requiring a large number of input/output (I/O) lines. For example, in the case of a 64×32-bit array, eleven (11) address lines are required—i.e., six lines for the 64-bit dimension (2 6 =64) and five lines for the 32-bit dimension (2 5 =32). In addition, the programming data are normally transferred in 8-bit bytes, necessitating another eight (8) lines, and the clock and control signals require at least two additional lines, for a total in that case of at least twenty-one (21) lines required for writing. The device is similarly read out into an output buffer, preferably under control of the same address lines as are used for input, when the read/write control signal changes to a read state. The output of the buffer is then read in parallel using, typically, eight lines. Thus, twenty-one lines are required for reading as well. 
     If the random access memory is a discrete memory device, the number of lines required for reading or writing is not of much concern, because the available I/O pins on the device are not required for other functions. However, random access memory arrays are frequently embedded in larger devices. For example, copending commonly-assigned U.S. patent application Ser. No. 08/442,795, filed May 17, 1995, which is hereby incorporated by reference in its entirety, describes a programmable logic device having a large number of interconnected programmable logic regions. In addition, there are a smaller number of random access memory regions embedded in the device (in that example, the random access memory is static random access memory). In that device, there is heavy competition for I/O pins, which must be shared by logic inputs and outputs, programming inputs, testing inputs and outputs, etc. 
     One way of reducing the number of pins required to access a large array device is to use serial techniques. For example, commonly-assigned U.S. Pat. No. 4,930,107 shows a method for serially programming an EPROM-type programmable logic device. However, where the array device is a random access memory, special considerations arise that may make serial programming more difficult. 
     For example, in the random access memory arrangement described above, programming, address and control data are input into a buffer, and written from the buffer into the random access memory on a change of state of the read/write control bit in the buffer. To serially program such a random access memory, one would serially load the input buffer, then flip the read/write control bit to a write state. If there is more than one array, their input buffers would be chained together and loaded in a single serial loading operation. The serially loaded data bits would be arranged so that at the end of the serial loading, the read/write control position of each buffer is in the write state, to cause writing into each array. However, the status of the read/write control bit in each buffer (except the last one) would change continually as the programming data are clocked in. Each time a bit representing the write state passed through the read/write control position of each buffer, the corresponding array would be written with unintended data. Even in the case of a single array, if the read/write control position is not the last position in the buffer, erroneous writing can occur. And even if the read/write control position is the last position, it still may not be desirable for writing to occur as soon as the intended write state bit reaches that read/write control position. 
     Reading of the random access memory occurs in a similar way. Specifically, address data are written into the input buffers and the read/write control bit or bits are changed to a read state, causing the data in the array that is identified by the address in the buffer to be written to output buffers which can then be read either serially or in parallel (again, to optimize pin allocations, serial reading would be preferred). Here again, as the address data are clocked in, unintended reading or writing may occur as the read/write control bit changes state. 
     It would be desirable to be able to provide for serial reading and writing of random access memory while preventing unintended reading and writing of the memory as data are clocked into the input buffers. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide for serial reading and writing of random access memory while preventing unintended reading and writing of the memory as data are clocked into the input buffers. 
     In accordance with the invention, there is provided serially progammable random access memory having at least one random access memory array. Each array includes a plurality of random access memory bits, a plurality of data input lines for entering data to be stored in the random access memory array, a plurality of address input lines and an address decoder for entering address information indicating in a write mode in which of the plurality of random access memory bits the data to be stored is to be stored and for indicating in a read mode which of the plurality of random access memory bits are to be output from the random access memory array, and a read/write control input. The random access memory array is in a read mode when a read/write control signal applied to the read/write control input is in a first logic state and is in a write mode when the read/write control signal applied to the read/write control input is in a second logic state. For each of the at least one random access memory array, a set of input/programming registers is connected to the data input lines, the address input lines and the read/write control input, for inputting programming data for the random access memory array, which programming data include the address information and a registered control signal to be applied to the read/write control input, as well as the data to be written when the input/programming registers are used for writing. The input/programming registers are chained together such that the data to be stored, the address information and the registered control signal can be entered serially into the registers. A read/write control circuit allows selective operation of the at least one random access memory array in one of (a) a first mode in which the random access memory array is in one of (i) a read mode, and (ii) a write mode, regardless of the registered control signal, and (b) a second mode in which the random access memory array is in one of (i) a read mode, and (ii) a write mode, under control of the registered control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 is a high-level diagram of a programmable logic device including embedded random access memory configured according to the present invention; 
     FIG. 2 is a block diagram of a plurality of random access memory arrays configured according to the present invention; 
     FIG. 3 is a block diagram of an example of a serial register configuration according to the present invention; and 
     FIG. 4 is a block diagram of a read/write control arrangement according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention allows for serial reading and writing of random access memory without unintended reading and writing as the state of the read/write control bit changes during the serial clocking of programming data. (As used herein, “programming data” means the data that are clocked into the random access memory input registers for reading or writing the random access memory, including address and control data which are used for either reading or writing, as well as data to be stored when in the write mode.) The invention accomplishes that result by interposing, between the read/write control register and the random access memory array, a read/write control circuit that allows the user to override the read/write control register during serial loading of the input registers. 
     The read/write control circuit preferably is a multiplexer having two data inputs and a control input. One data input is the output of the read/write control register. The other data input is a read/write mode selection signal which is applied to an I/O pin of the device by the user (or by some other device or circuit according to the design of the user) during the clocking of programming data into the random access memory input registers (and possibly at other times as discussed below). The control input is a read/write option signal applied to an I/O pin by the user (or by some other device or circuit according to the design of the user) during the clocking of programming data into the random access memory input registers (and possibly at other times as discussed below). 
     When data are to be clocked into the input registers to read or write the random access memory, the user would apply (or cause to be applied), to the read/write option input of the read/write control circuit, a signal that causes the read/write control circuit to select, as the read/write control input to the random access memory, the read/write mode selection signal rather than the contents of the read/write control register. The user would then apply (or cause to be applied) to the read/write mode selection input a signal that will keep the random access memory in read mode. 
     In a preferred embodiment of the invention, the read/write control input to the random access memory array is a Read/Not-Write input. That is, the array is in a read state when the Read/Not-Write input is high (a logic “1”), and in a write state when the Read/Not-Write input is low (a logic “)”). In that embodiment, the read state of the signal applied to the read/write mode selection input would be a logic “1.” It should be understood, however, that the read/write control input could be implemented in other ways, such as a Not-Read/Write input in which a logic “0” is the read state and a logic “1” is the write state. Other arrangements for read/write control might also be used. 
     There is normally no reason why the user would want the read/write mode selection signal to keep the random access memory in write mode. Therefore, the read/write mode selection input could be kept permanently in a read state—e.g., it could be tied high in a Read/Not-Write embodiment or tied low in a Not-Read/Write embodiment. The read/write option input alone would then determine whether the random access memory is in a read state, or in a state determined by the read/write control register. However, there may be applications where the user would want to force the random access memory into a write state, and such applications can be accomplished by applying the desired signal (in the Read/Not-Write example, a logic “0”) to the read/write mode selection input while selecting that input using the read/write option signal. 
     The invention thus provides a way to serially clock programming data into the input registers of a random access memory without the random access memory being rewritten every time a bit in the “write” state (a logic “0” in the example) is clocked through the read/write control register. 
     The invention can be further described with reference to FIGS. 1-4. 
     FIG. 1 shows, as an example of a device with which the present invention can be used, a programmable logic device  10  having embedded random access memory. It should be recognized, however, that the present invention can be used with random access memory alone, or with random access memory that is part of some other kind of device. 
     As can be seen, device  10  has a plurality of random access memory arrays  11 , which are arranged in one column on device  10 . The remaining columns of device  10  are populated by programmable logic regions  12 . Each row has two groups of horizontal interconnection conductors  13  that extend the entire length of the row, one group of conductors  13  being above the row and the other group being below the row. Conductors  13  are sometimes referred to as full-horizontal conductors or as global horizontal conductors. Each row also has four groups of horizontal interconnection conductors  14  that extend along half the length of the row. Two of these groups extend respectively along the top and bottom of the left half of the row. The other two groups of these conductors  14  extend respectively along the top and bottom of the right half of the row. The conductors  14  associated with each half of a row are preferably not directly connectable to the conductors  14  associated with the other half of the row. Conductors  14  are sometimes referred to as half-horizontal conductors. Each column of regular logic regions  12  has a group of vertical interconnection conductors  15  that extend continuously or substantially continuously along the entire length of the column. 
     In order to feed logic signals to each region  12 , each regular logic region has an associated plurality of region feeding conductors  16  that can bring signals to the logic region from the horizontal conductors  13  and  14  associated with that region. Each region  12  also has eight associated local feedback conductors  17 . Each conductor  17  makes the output signal of a respective one of the logic modules  18  in the associated region  12  available as an input to any of the other logic modules in that region without having to use any interconnection resources that are not exclusively associated with the region. 
     Each region  12  also has output conductors  19  for conveying the output logic signals of the logic modules  18  in that region to the associated conductors  13  and  14 . Programmable logic connectors (“PLCs”) (not shown) are associated with each regular logic region  12  for making connections from the vertical ( 15 ) to the horizontal ( 13  and  14 ) conductors associated with the region. A plurality of output networks  100  connect the various conductors  13 - 15 ) to input/output pins  101 . 
     As can be seen, many resources are competing for access to I/O pins  101 , hence the desire, as discussed above, to minimize the number of I/O pins  101  required in order to carry out serial reading and writing of random access memory arrays  11 . That minimization is achieved as discussed below. 
     In FIG. 2, N random access memory arrays  11  are depicted schematically as 64×32-bit arrays, arranged in N rows of a single column. Preferably, arrays  11  are static random access memory arrays, but the invention applies equally to dynamic random access memory. Representative bits  20  are shown schematically scattered throughout the uppermost one of arrays  11  (in Row  0 ). Associated with each array  11  is an input buffer  21  made up of a plurality of registers. In the illustrative embodiment shown, input buffer  21  includes twenty-one registers—eight data registers D 0 -D 7 , eleven address registers A 0 -A 10 , an Output Enable (OE) register and a read/write control register implemented as a Read/Not-Write (RNW) register. Buffer  21  is connected to array  11  by eight data inputs  22 , eleven address inputs  23 , and an Output Enable input  24 . Buffer  21  also has a read/write control output  25  which is switched by read/write control circuit  26 . Read/write control circuit  26  is also fed by read/write mode selection input  27  and read/write option input  28 . The output of read/write control circuit  26  feeds read/write control input  25  of array  11 . 
     Each array  11  preferably also has an associated output buffer  29  preferably made up of eight output registers Reg  0 -Reg  7 , each of which is preferably connected to array  11  by one of preferably eight outputs  200 . 
     The registers of buffer  21  and the registers of buffer  29  preferably are positive edge trigger D-type flip-flops  30  connected serially as shown in FIG.  3 . Buffers  21  are preferably connected together serially as indicated by arrows A, with the last register in each buffer  21  connected to the first register in the next buffer  21  in the same manner as the individual registers of each buffer  21 . Similarly, buffers  29  are preferably connected together serially as indicated by arrows B, with the last register in each buffer  29  connected to the first register in the next buffer  29  in the same manner as the individual registers of each buffer  29 . 
     FIG. 4 shows detail of read/write control circuit  26 . As can be seen, read/write control circuit  26  is preferably a multiplexer  40 , fed by read/write control output  25  of the RNW register of buffer  21 ( as a first data input), by read/write mode selection input  27 ( as a second data input), and by read/write option input  28 ( as a control input), the latter two preferably being fed by appropriate ones of pins  101 . 
     In operation, the user, or a circuit or software under the direction or control of the user, would arrange a string of programming bits that are clocked into buffers  21  serially under the control of clock signal CLK 1  from input DATA-SIN, both preferably fed from appropriate pins  101 . The string of bits would be arranged so that after being clocked into buffers  21 , the desired bit would be present in each of the registers of buffers  21  to accomplish the desired read or write operation. For a read operation in the embodiment shown, in addition to arranging the data and address bits, the user would arrange that each bit in each RNW register is a logic “1,” while for a write operation in that embodiment, the user would arrange that each bit in each RNW register is a logic “0.” As the programming data are clocked into buffers  21 , the user would activate read/write control circuit  26  by applying a signal on read/write option input  28  (e.g., from an appropriate pin  101 ) to cause multiplexer  40  to feed read/write mode selection signal  27 , rather than read/write control output signal  25 , to read/write control input  25 ′ of array  11 . The user would also apply a logic “1” to read/write mode selection input  27  (e.g., from an appropriate pin  101 ). As stated above, in the embodiment shown the user most likely will never want input  27  to be anything other than a logic “1,” and so it could be permanently held high, with all control based on input  28 . After the data have been clocked into buffers  21 , the user will change input  27  to a state that causes multiplexer  40  to select output  25 . For a write operation in the embodiment shown the logic “0&#39;s” in registers RNW will then be applied to inputs  25 ′, and the data in respective sets of registers D 0 -D 7  will be written to the locations specified by the data in respective sets of registers A 0 -A 11 , which are decoded by an address decoder  230  shown schematically as being part of random access memory array  11 , although it could be a separate circuit between registers A 0 -A 10  and random access memory array  11 . 
     For a read operation, the logic “1&#39;s” in registers RNW would be applied by circuit  26  to inputs  25 ′, and the data in array locations specified by the data in respective sets of registers A 0 -A 11  will be written to respective sets of registers Reg  0 -Reg  7  in output buffers  29 . The data can then be read by clocking them out on line RAMSOUT under control of clock signal CLK 2 . 
     In a programmable logic device, a write operation would most likely be used to load random access memory arrays  11  to function as look-up tables or other logic devices, which during normal operation would probably be accessed in a way that directs individual outputs to different parts of the device. Therefore, the serial output or read mode just described would probably be used only for testing (by the manufacturer or the user), although other uses would be possible. And if the random access memory were being used alone or in some other application, the serial read and write modes can be used as desired. 
     As described, the present invention allows serial reading and writing of relatively large random access memory arrays using preferably only six pins—CLK 1 , CLK 2 , DATASIN, RAMSOUT, read/write mode selection input  27  and read/write option input  28 . Indeed, if read/write mode selection input  27  is permanently held high (or low) as discussed, the number of pins needed can be reduced to five. 
     Thus it is seen that serial reading and writing of random access memory is provided, while preventing unintended reading and writing of the memory as data are clocked into the input buffers. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.