Abstract:
Multi-Q FIFO memory systems include a plurality of multi-Q first-in first-out (FIFO) memory chips electrically coupled to a data output bus. The plurality of multi-Q FIFO memory chips, which are responsive to respective identification codes ID and respective read chip select signals (/RCS), are configured to support an enhanced multi-chip expansion mode of operation. This expansion mode of operation uses the read chip select signals to control one-at-a-time access of at least two selected multi-Q FIFO memory chips receiving equivalent ID codes and equivalent read addresses to the output data bus during read operations.

Description:
REFERENCE TO PRIORITY APPLICATION  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/642,776, filed Jan. 10, 2005, the disclosure of which is hereby incorporated herein by reference.  
       CROSS-REFERENCE TO RELATED APPLICATION  
       [0002]     This application is related to commonly assigned U.S. application Ser. No. 10/721,974, filed Nov. 24, 2003, the disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to integrated circuit memory devices and methods of operating same, and more particularly to buffer memory devices and methods of operating buffer memory devices.  
       BACKGROUND OF THE INVENTION  
       [0004]     Semiconductor memory devices can typically be classified on the basis of memory functionality, data access patterns and the nature of the data storage mechanism. For example, distinctions are typically made between read-only memory (ROM) devices and read-write memory (RWM) devices. The RWM devices typically have the advantage of offering both read and write functionality with comparable data access times. Typically, in RWM devices, data is stored either in flip-flops for “static” memory devices or as preset levels of charge on a capacitor in “dynamic” memory devices. As will be understood by those skilled in the art, static memory devices retain their data as long as a supply of power is maintained, however, dynamic memory devices require periodic data refreshing to compensate for potential charge leakage. Because RWM devices use active circuitry to store data, they belong to a class of memory devices known as “volatile” memory devices because data stored therein will be lost upon termination of the power supply. ROM devices, on the other hand, may encode data into circuit topology (e.g., by blowing fuses, removing diodes, etc.). Since this latter type of data storage may be hardwired, the data cannot be modified, but can only be read. ROM devices typically belong to a class of memory devices known as “nonvolatile” memory devices because data stored therein will typically not be lost upon termination of the power supply. Other types of memory devices that have been more recently developed are typically referred to as nonvolatile read-write (NVRWM) memory devices. These types of memory devices include EPROM (erasable programmable read-only memory), E 2 PROM (electrically erasable programmable read-only memory), and flash memories, for example.  
         [0005]     An additional memory classification is typically based on the order in which data can be accessed. Here, most memory devices belong to the random-access class, which means that memory locations can be read from or written to in random order, typically by supplying a read or write address. Notwithstanding the fact that most memory devices provide random-access, typically only random-access RWM memories use the acronym RAM. Alternatively, memory devices may restrict the order of data access to achieve shorter data access times, reduce layout area and/or provide specialized functionality. Examples of such specialized memory devices include buffer memory devices such as first-in first-out (FIFO) memory devices, last-in first-out (LIFO or “stack”) memory devices, shift registers and content addressable memory (CAM) devices.  
         [0006]     A final classification of semiconductor memories is based on the number of input and output ports associated with the memory cells therein. For example, although most memory devices have unit cells therein that provide only a single port which is shared to provide an input and output path for the transfer of data, memory devices with higher bandwidth requirements often have cells therein with multiple input and output ports. However, the addition of ports to individual memory cells typically increases the complexity and layout area requirements for these higher bandwidth memory devices.  
         [0007]     Single-port memory devices are typically made using static RAM cells if fast data access times are requiring, and dynamic RAM cells if low cost is a primary requirement. Many FIFO memory devices use dual-port RAM-based designs with self-incrementing internal read and write pointers to achieve fast fall-through capability. As will be understood by those skilled in the art, fall-through capability is typically measured as the time elapsing between the end of a write cycle into a previously empty FIFO and the time an operation to read that data may begin. Exemplary FIFO memory devices are more fully described and illustrated at section 2.2.7 of a textbook by A. K. Sharma entitled “Semiconductor Memories: Technology, Testing and Reliability”, IEEE Press (1997).  
         [0008]     In particular, dual-port SRAM-based FIFOs typically utilize separate read and write pointers to advantageously allow read and write operations to occur independently of each other and achieve fast fall-through capability since data written into a dual-port SRAM FIFO can be immediately accessed for reading. Since these read and write operations may occur independently, independent read and write clocks having different frequencies may be provided to enable the FIFO to act as a buffer between peripheral devices operating at different rates. Unfortunately, a major disadvantage of typical dual-port SRAM-based FIFOs is the relatively large unit cell size for each dual-port SRAM cell therein. Thus, for a given semiconductor chip size, dual-port buffer memory devices typically provide less memory capacity relative to single-port buffer memory devices. For example, using a standard DRAM cell as a reference unit cell consuming one (1) unit of area, a single-port SRAM unit cell typically may consume four (4) units of area and a dual-port SRAM unit cell typically may consume sixteen (16) units of area. Moreover, the relatively large unit cells of a dual-port SRAM FIFO may limit the degree to which the number of write operations can exceed the number of read operations, that is, limit the capacity of the FIFO.  
         [0009]     To address these limitations of dual-port buffer memory devices, single-port buffer memory devices have been developed to, among other things, achieve higher data capacities for a given semiconductor chip size. For example, U.S. Pat. No. 5,546,347 to Ko et al. entitled “Interleaving Architecture And Method For A High Density FIFO”, assigned to the present assignee, discloses a memory device which has high capacity and uses relatively small single-port memory cells. However, the use of only single port memory cells typically precludes simultaneous read and write access to data in the same memory cell, which means that single-port buffer memory devices typically have slower fall-through time than comparable dual-port memory devices. Moreover, single-port buffer memory devices may use complicated arbitration hardware to control sequencing and queuing of reading and writing operations.  
         [0010]     U.S. Pat. No. 5,371,708 to Kobayashi also discloses a FIFO memory device containing a single-port memory array, a read data register for holding read data from the memory array and a write data register for holding write data to the memory array. A bypass switch is provided for transferring data from the write data register to the read data register so that the memory array can be bypassed during testing of the FIFO to detect the presence of defects therein. However, like the above-described single-port buffer memory devices, simultaneous read and write access to data is not feasible.  
         [0011]     Commonly assigned U.S. Pat. Nos. 5,978,307, 5,982,700 and 5,999,478 disclose memory buffers having fast fall-through capability. These memory buffers contain a tri-port memory array of moderate capacity having nonlinear columns of tri-port cells therein which collectively form four separate registers, and a substantially larger capacity supplemental memory array (e.g., DRAM array) having cells therein with reduced unit cell size. The tri-port memory array has a read port, a write port and a bidirectional input/output port. The tri-port memory array communicates internally with the supplemental memory array via the bidirectional input/output port and communicates with external devices (e.g., peripheral devices) via the read and write data ports. Efficient steering circuitry is also provided by a bidirectional crosspoint switch that electrically couples terminals (lines IO and IOB) of the bidirectional input/output port in parallel to bit lines (BL and BLB) in the supplemental memory array during a write-to-memory time interval and vice versa during a read-from-memory time interval. Commonly assigned U.S. Pat. No. 6,546,461 also discloses FIFO memory devices that use multiple multi-port caches to support high rate reading operations.  
         [0012]     In order to increase the capacity of FIFO memory devices, multiple FIFO memory devices may be cascaded in a depth expansion configuration. As illustrated by  FIG. 1A , a pair of FIFO memory devices may be configured to provide a higher capacity FIFO system  10 . In this system  10 , both devices operate in a conventional first-word fall-through (FWFT) mode. When disposed in the FWFT mode (pin FWFT=Vdd), the output ready pin (/OR) is used to indicate whether or not there is valid data at the data outputs (On) and the input ready pin (/IR) is used to indicate whether or not a FIFO memory device has any free space to support a writing operation. In the FWFT mode, the first word written to an empty FIFO memory device goes directly to the corresponding data outputs (On) after three rising edges of the read clock (RCLK) and any requirement that the read enable signal (/REN) be low to produce output data is not necessary.  
         [0013]     The FIFO memory device on the left side of  FIG. 1A  has a write interface and a read interface. The write interface receives a write clock signal WCLK, a write enable signal (/WEN) and input data (Dn) and generates the input ready flag (/IR). The read interface receives a read clock signal RCLK and a read enable signal (/REN) and generates an output ready flag (/OR) and output data (On). This output ready flag (/OR) may be used as the write enable input signal (/WEN) to the next stage in the cascaded arrangement. The read interface of the left FIFO memory device is electrically coupled to a write interface of the FIFO memory device on the right side of  FIG. 1A  and the read and write clock signal pins at these interfaces receive a transfer clock (TRANSFER CLOCK). This transfer clock may be an independent clock signal or may constitute the write clock signal or read clock signal. A transfer clock signal operating a maximum frequency is preferred. However, if the write or read clock signal is used in place of the transfer clock signal, then the read or write clock signal having the higher frequency should be used. The read interface of the right FIFO memory device can be electrically coupled to a downstream peripheral device (not shown) or other device or system.  
         [0014]     Unfortunately, the ability to increase the capacity of FIFO memory devices operating in the FWFT mode of operation does not translate to FIFO memory devices that are configured to operate in standard mode, which is another conventional mode of operation. This is because an empty flag (/EF) generated at an output of a FIFO memory device in standard mode may not be used as a write enable signal (/WEN) to the next stage in a cascaded arrangement. This is because there is a one cycle difference between the empty flag (/EF) and the output ready flag (/OR) when a FIFO memory device is disposed in the standard mode and FWFT mode, respectively. This one cycle difference in flag generation precludes reliable operation of a depth expansion arrangement of FIFO memory devices when they are disposed in the standard mode. Thus, as illustrated by  FIG. 1B , a FIFO memory device  12  that is disposed in a conventional standard mode (pin FWFT=GND) cannot be arranged in a depth expansion configuration.  
         [0015]      FIG. 2  illustrates a conventional multi-Q FIFO memory system  100  having a plurality of multi-Q first-in first-out (FIFO) memory chips therein. These chips, which are identified by the labels Device  1 , Device  2 , . . . , Device n, are responsive to respective ID codes, which are shown as 3-bit codes ID 1 [ 2 ; 0 ], ID 2 [ 2 : 0 ], . . . , IDn[ 2 : 0 ]. Because these 3-bit codes are unique to each device, the multi-Q FIFO memory system  100  is limited to a maximum of 2 3 =8 devices, which provides for a queue expansion of up to a maximum of 256 queues for the case where the write and read addresses (WRADD, RDADSD) are 8-bits wide and a maximum of 32 queues (2 (8−3) ) can be allocated within each device. These and other aspects of the FIFO memory system  100  are illustrated and described at page 79 and elsewhere in the aforementioned U.S. Provisional Application Ser. No. ______, filed Jan. 10, 2005.  
       SUMMARY OF THE INVENTION  
       [0016]     Multi-Q FIFO memory systems according to embodiments of the present invention include a plurality of multi-Q first-in first-out (FIFO) memory chips electrically coupled to a data output bus. The plurality of multi-Q FIFO memory chips, which are responsive to respective identification codes ID and respective read chip select signals (/RCS), are configured to support an enhanced multi-chip expansion mode of operation. This expansion mode of operation uses the read chip select signals to control one-at-a-time access of at least two selected multi-Q FIFO memory chips receiving equivalent ID codes and equivalent read addresses to the output data bus during read operations. This one-at-a-time access is achieved because the read chip select signals dispose all but a selected one of the plurality of multi-Q FIFO memory chips in a high impedance output mode (to the output bus) during a read operation. The multi-Q FIFO memory chips are also configured so that a most significant portion of a read address is compared to the ID codes associated with the plurality of multi-Q FIFO memory chips to detect multiple equivalencies therebetween during a read operation. This allowance for multiple equivalencies supports greater queue expansion relative to FIFO memory systems requiring uniqueness between an ID code of a chip and a most significant portion of the read address.  
         [0017]     Still further embodiments of the present invention include methods of operating a depth-expanded system of multi-Q FIFO memory chips coupled to a common data output bus. These methods include comparing bits (e.g., most significant bits) of an applied read address to ID codes associated with the multi-Q FIFO memory chips to thereby identify a plurality of equivalencies (i.e., identify a plurality of the multi-Q FIFO memory chips as candidates to undergo a read operation). A step is then performed to deselect all but one of the candidate multi-Q FIFO memory chips using a plurality of read chip select signals to dispose data output ports of the deselected multi-Q FIFO memory chips in high impedance states that preclude competing use of the output bus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1A  illustrates a pair of conventional FIFO memory devices that are arranged in a depth expansion configuration and support conventional first-word fall-through (FWFT) mode operation.  
         [0019]      FIG. 1B  illustrates a conventional FIFO memory device that is disposed in a conventional standard mode operation.  
         [0020]      FIG. 2  illustrates a plurality of first-in first-out (FIFO) memory devices arranged as an expanded FIFO memory system that utilizes ID codes to identify devices being addressed during write and read operations, according to the prior art.  
         [0021]      FIG. 3A  illustrates a plurality of first-in first-out (FIFO) memory devices arranged as an expanded FIFO memory system that utilizes ID codes and chip select signals to identify devices being addressed during write and read operations, according to embodiments of the present invention.  
         [0022]      FIG. 3B  is a timing diagram that illustrates operation of the FIFO memory system of  FIG. 3A .  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     The present invention now will be described more fully herein with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout and signal lines and signals thereon may be referred to by the same reference characters. Signals may also be synchronized and/or undergo minor boolean operations (e.g., inversion) without being considered different signals. The suffix B (or prefix symbol “/”) to a signal name may also denote a complementary data or information signal or an active low control signal, for example.  
         [0024]      FIGS. 3A-3B  illustrate operations performed by an expandable multi-queue FIFO memory system  200  that utilizes ID codes and chip select signals to select devices being addressed during write and read operations. The FIFO memory system  200  of  FIG. 3A  includes a plurality of FIFO memory devices  202   a - 202   c  that receive write data from a common input bus (D_BUS) and output read data to a common output bus (Q_BUS) during write and read operations, respectively. Each of these FIFO memory devices  202   a - 202   c  may include a packaged integrated circuit chip(s) that performs the FIFO operations described herein. Each of the integrated circuit chips may include cache memory and high capacity supplemental memory in the form of embedded memory. This memory may be configured with control logic to support multiple queues within each chip. In alternative embodiments, high capacity supplemental memory may be provided by separate integrated circuit chips (e.g., DRAM memory chips). Examples of cache and supplemental memories that may be used in FIFO memory devices are more fully disclosed in commonly assigned U.S. Pat. Nos. 6,546,461 and 6,754,777 and in commonly assigned U.S. application Ser. Nos. 10/721,974, filed Nov. 24, 2003, the disclosures of which are hereby incorporated herein by reference.  
         [0025]     The write side of each of the FIFO memory devices  202   a - 202   c  is responsive to at least a write clock signal (WCLK), an active low write enable signal (/WEN), a write “queue” address (WRADD) and an active low write chip select signal (/WCS). The data input terminals (DIN) of the FIFO memory devices  202   a - 202   c  are commonly connected to the input bus (D_BUS), which supplies write data during FIFO write operations. As will be understood by those skilled in the art, additional write side signals and flags, such as those illustrated in  FIG. 1 , may be received and generated by the FIFO memory devices  202   a - 202   c . Similarly, the read side of each of the FIFO memory devices  202   a - 202   c  is responsive to at least a read clock signal (RCLK), an active low read enable signal (/REN), a read “queue” address (RDADD) and an active low read chip select signal (/RCS). The data output terminals (QOUT) of the FIFO memory devices  202   a - 202   c  are commonly connected to the output bus (Q_BUS), which receives read data during FIFO read operations. As will be understood by those skilled in the art, additional read side signals and flags, such as those illustrated in  FIG. 1 , may be received and generated by the FIFO memory devices  202   a - 202   c.    
         [0026]     The FIFO memory devices  202   a - 202   c  may be responsive to respective ID codes, which are shown as ID 1 [ 2 : 0 ], ID 2 [ 2 : 0 ], . . . , IDn[ 2 : 0 ]. In contrast to the ID codes illustrated by  FIG. 1 , the values of the ID codes illustrated by  FIG. 3A  need not be unique to each FIFO memory device  202   a - 202   c . For example, the first and second FIFO memory devices  202   a - 202   b  may belong to the same class of queues, which occurs when ID 1 [ 2 : 0 ]=ID 2 [ 2 : 0 ]. The ID codes ID 1 -IDn are used in combination with the write chip select signals /WCS 1 -/WCSn to identify a unique one of the FIFO memory devices  202   a - 202   c  during an operation to write data to an addressed queue specified by the write address WRADD. In alternative embodiments of the present invention, the write chip select signals may be independently used to uniquely identify a FIFO memory device during a write operation. The write address WRADD may serve to identify a class of queues associated with a FIFO memory device as well as a specific queue within the class. For example, if the write address WRADD is an 8-bit address, the three most significant bits of WRADD (i.e., WRADD[ 7 : 5 ]) may be used to specify one of eight possible classes and the five least significant bits of WRADD (i.e., WRADD[ 4 : 0 ]) may be used to specify one of 32 possible queues within a class. In this case, the three most significant bits of WRADD may be compared with the ID code associated with each FIFO memory device  202   a - 202   c  to identify which FIFO memory device(s) has been selected for a respective write operation. Because this comparison may identify more than one of the FIFO memory devices  202   a - 202   c  as a candidate for a respective write operation, the write chip select signals /WCS 1 -/WCSn are used to select a unique FIFO memory device based on the constraint that only one of the write chip select signals /WCS 1 -/WCSn may be asserted (i.e., set low) at a time. Moreover, if ID codes are not used, the entire write address WRADD may be used to specify one of 256 queues within a FIFO memory device.  
         [0027]     The same ID codes ID 1 -IDn are also used in combination with the read chip select signals /RCS 1 -/RCSn to identify a unique one of the FIFO memory devices  202   a - 202   c  during an operation to read data from an addressed queue specified by the read address RDADD. However, in alternative embodiments of the present invention, the use of ID codes may be eliminated and the read chip select signals may be used to uniquely identify one of a plurality of FIFO memory devices during a read operation. The read address RDADD may serve to identify a class of queues associated with a FIFO memory device as well as a specific queue within the class. As described above with respect to the write address WRADD, the read address RDADD can be an 8-bit address. The three most significant bits of RDADD (i.e., RDADD[ 7 : 5 ]) may be used to specify one of eight possible classes and the five least significant bits of RDADD (i.e., RDADD[ 4 : 0 ]) may be used to specify one of 32 possible queues within a class. The three most significant bits of RDADD may be compared with the ID code associated with each FIFO memory device  202   a - 202   c  to identify which FIFO memory device(s) has been selected for a respective read operation. Because this comparison may identify more than one of the FIFO memory devices  202   a - 202   c  as a candidate for a respective read operation, the read chip select signals /RCS 1 -/RCSn are used to select a unique FIFO memory device based on the constraint that only one of the read chip select signals /RCS 1 -/RCSn may be asserted (i.e., set low) at a time. If ID codes are not used, the entire read address RDADD may be used to specify one of 256 queues within a FIFO memory device.  
         [0028]     The timing diagram of  FIG. 3B  illustrates the reading of data from various queues within the FIFO memory devices  202   a - 202   c  to the output data bus Q_BUS during the read cycles A-J. This timing diagram illustrates a read clock signal RCLK and an active low read enable signal /REN, which are received by each of FIFO memory devices  202   a - 202   c . The read enable signal /REN controls whether any of the FIFO memory devices are active during a read cycle and the read chip select signals /RCS 1 , /RCS 2  and /RCSn control which one of the FIFO memory devices  202   a - 202   c  is active when the read enable signal /REN is also active. Only one of the read chip signals can be asserted (e.g., low) at a time. When a read chip select signal is inactive at a high level (i.e., /RCS=1), the output terminals (QOUT) of a respective FIFO memory device are disposed in a high impedance state.  
         [0029]     As illustrated by read cycles A-B, setting /REN and /RCS 1  low and setting /RCS 2  and /RCSn high will operate to select the first FIFO memory device  202   a  during a FIFO read operation. This selection will cause the FIFO read data Q 1 _A and Q 1 _B (from a queue identified by RDADD) to be passed from the output terminals (QOUT 1 ) of the first FIFO memory device  202   a  to the output bus Q_BUS. The read data Q 1 _B is then held on the output bus Q_BUS during read cycle C, because the read enable signal is inactive (/REN=1) when read cycle C commences. Thereafter, during read cycle D, additional read data is transferred from the output terminals QOUT 1  of the first FIFO memory device  202   a  to the output bus Q_BUS. This transfer occurs because the read enable signal /REN and the read chip select signal /RCS 1  are both low when read cycle D commences (e.g., rising edge of RCLK for cycle D is received).  
         [0030]     During read cycle E, read data Q 2 _A is transferred from the output terminals QOUT 2  of the second FIFO memory device  202   b  to the output bus Q_BUS. This transfer occurs because the read enable signal /REN is low and the read chip select signal /RCS 2  is low when the read cycle E commences. During read cycle F, read data Q 1 _D is transferred from the output terminals QOUT 1  of the first FIFO memory device  202   a  to the output bus, in response to the condition that /REN and /RCS 1  are both low at the commencement of the read cycle F. This read data Q 1 _D is held on the output bus Q_BUS during read cycle G because the read enable signal /REN is inactive when the read cycle G commences and this inactive state overrides the fact that the read chip select signal /RCSn for the last FIFO memory device  202   c  is low when the read clock signal RCLK switches low-to-high during cycle G. When read cycle H commences, the read chip select signal /RCS 2  and the read enable signal /REN are both low. This condition causes read data Q 2 _B to be transferred from the output terminals QOUT 2  of the second FIFO memory device  202   b  to the output bus Q_BUS. Likewise, during read cycle  1 , the read chip select signal /RCSn and the read enable signal /REN are both low, which causes read data Qn_A to be transferred from the output terminals QOUTn of the last FIFO memory device  202   c  to the output bus Q_BUS. Finally, during read cycle J, read data Q 1 _E is transferred from the output terminals QOUT 1  to the output bus Q_BUS because /RCS 1  and /REN are both low when the leading edge of RCLK for cycle J is received.  
         [0031]     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.