Abstract:
A read module for register files includes at least one local I/O module coupled to a memory cell for outputting a value stored in the memory cell; and at least one global bit line driver having an input terminal coupled to the local I/O module, and a output terminal coupled to a global bit line for selectively pre-charging the global bit line at a default voltage in response to a local pre-charge signal, and outputting the value stored in the memory cell to the global bit line when the local pre-charge signal is not asserted.

Description:
BACKGROUND 
   The present invention relates generally to integrated circuit (IC) designs, and more particularly to a single end read module for register files. 
   A register file is a set of registers implemented in a central processing unit (CPU) for temporary data storage. The register file typically contains dedicated read and write ports whereas a memory device, such as random access memory (RAM), usually perform read and write functions through the same port. The register file can be accessed at a speed faster than that of the memory device, and therefore supports the CUP to function at a high speed. 
     FIG. 1  schematically illustrates a conventional single end read module  100  for enabling register files to be accessed from the outside for read operation. The single end read module  100  is comprised of a number of local I/O modules  102 , each of which is connected to a register or memory cell. The local I/O module  102  is connected to a global bit line  106 , which is further connected to an I/O pin (not shown in this figure) for data outputs, though a pull-down driver  104 . 
   The I/O module  102  is comprised of PMOS transistors P 1 , P 2 , P 3  and P 4  and a NAND gate  112 . PMOS transistors P 1  and P 2  have their sources coupled to a voltage supply VDD, and their gates controlled by a local pre-charge signal S 1 . The NAND gate  112  has two input terminals coupled to local bit lines  114  and  116 , and an output terminal coupled to the gate of the NMOS transistor N 1 , which makes up the pull-down driver  104 . 
   A latch  108  comprised of two serially connected inverters  110   a  and  110   b  is coupled to the global bit line  106  for latching a logic state of the signal thereon. A PMOS transistor P 5  has a source coupled to the voltage supply VDD and a drain coupled to the global bit line  106 . The gate of the PMOS transistor P 5  is controlled by a global pre-charge signal S 2 . 
   In the pre-charge stage, the local pre-charge signal S 1  is asserted to turn on PMOS transistors P 1  and P 2 , thereby raising the signals on the local bit lines  114  and  116  to a high logic state. Since both input terminals of the NAND gate  112  receive high signals, the NAND gate  112  outputs a low signal, which, in turn, switches of the NMOS transistor N 1 . In this stage, the global pre-charge signal S 2  is also asserted to turn on the PMOS transistor P 5 , thereby raising the signal on the global bit line  106  to a high state. The high signal on the global bit line  106  is latched by the latch  108 . 
   In read operation, the local and global pre-charge signals S 1  and S 2  are disabled to allow the signals on the global bit line  106  to freely respond to the value stored in the register or memory cell (not shown in the figure) coupled to the local bit lines  114  and  116 . If the voltage on either one of the bit lines  114  and  116  is low, the NAND gate  112  outputs a high signal, which, in turn, switches on the NMOS transistor N 1 . As the source of the NMOS transistor N 1  is coupled to ground or VSS, the voltage on the global bit line  106  is pulled down, thereby causing the latch  108  to flip. 
   One drawback of the conventional single end read module  100  is that it is susceptible to noise-induced failure. Noise present on the global bit line  106  can cause the latch  108  to flip, thereby causing the read operation to fail. This causes reliability issues. Moreover, in a low voltage supply design, the latch  108  is even more prone to the noise-induced failure. Given the trend of low supply voltage in IC designs, failures caused by the latch  108  become a serious reliability issue. 
   As such, what is needed is a single end read module with improved reliability for register files. 
   SUMMARY 
   The present invention is directed to a read module for register files. In one embodiment of the present invention, the read module comprises: at least one local I/O module coupled to a memory cell for outputting a value stored in the memory cell; and at least one global bit line driver having an input terminal coupled to the local I/O module, and a output terminal coupled to a global bit line for selectively pre-charging the global bit line at a default voltage in response to a local pre-charge signal, and outputting the value stored in the memory cell to the global bit line when the local pre-charge signal is not asserted. 
   In another embodiment of the present invention, the read module comprises: at least one local I/O module coupled to a memory cell for outputting a value stored in the memory cell; a first NMOS transistor having a source coupled to ground, a drain coupled to a global bit line, and a gate coupled to the local I/O module; a first PMOS transistor having a drain coupled to the drain of the first NMOS transistor, and a gate controlled by a select signal; and a second PMOS transistor having a source coupled to a voltage supply, a drain coupled to a source of the first PMOS transistor, and a gate coupled to the gate of the first NMOS transistor and the I/O module, wherein the select signal is asserted to turn on the first PMOS transistor when the local I/O module is selected, wherein the first NMOS transistor is turned off and the second PMOS transistor is turned on to pre-charge the global bit line during pre-charge operation, wherein the first NMOS transistor and the second PMOS transistor are selectively turned on and off in response to a value stored in the memory cell during read operation. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically  1  illustrates a conventional single end read module for register files. 
       FIG. 2  schematically  1  illustrates a single end read module for register files in accordance with one embodiment of the present invention. 
   

   DESCRIPTION 
   This invention describes a single end read module with improved reliability against noise for register files. The following merely illustrates various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention. 
     FIG. 2  schematically illustrates a single end read module  200  for enabling register files to be accessed from the outside for read operation in accordance with one embodiment of the present invention. The single end read module  200  is comprised of a number of local I/O modules such as  202   a  and  202   b , each of which is connected to a column of registers or memory cells (not shown in the figure). The local I/O modules  202   a  and  202   b  are connected to a global bit line GBL, which is further connected to an I/O pin (not shown in this figure) for data outputs, through global bit line drivers  204   a  and  204   b , respectively. 
   A pre-charge module  206 , such as a PMOS transistor P 1 , is connected to the global bit line GBL. The PMOS transistor P 1  has a source coupled to a voltage supply VDD, a drain coupled to the global bit line GBL, and a gate controlled by a global pre-charge signal S 1 . Although this embodiment of the present invention utilizes a PMOS transistor as the pre-charge module, other devices, such as NMOS transistors, bipolar transistors, or diodes, can also be employed as the pre-charge module based on various design considerations. 
   The I/O module  202   a , for example, is comprised of PMOS transistors P 4 , P 5 , P 6  and P 7 , and a NAND gate  208 . The NAND gate  208  has two input terminals connected to a first local bit line ULBL and a second local bit line LLBL, and is coupled between an internal voltage supply VDDI and ground or VSS. The NAND gate  208  has an output terminal connected to the global bit line driver  204   a , which will be explained in detail below. 
   The PMOS transistor P 4  has a source coupled to the voltage supply VDD, a drain coupled to the first local bit line ULBL, and a gate controlled by a local pre-charge signal S 2 . The PMOS transistor P 6  has a source coupled to the internal voltage supply VDDI, a drain coupled to the first local bit line ULBL, and a gate coupled to the output terminal of the NAND gate  208 . The PMOS transistor P 5  has a drain coupled to the voltage supply VDD, a source coupled to the second local bit line LLBL, and a gate controlled by the local pre-charge signal S 2 . The PMOS transistor P 7  has a drain coupled to the internal voltage supply, a source coupled to the second local bit line LLBL, and a gate coupled to the gate of the PMOS transistor P 6  and the output terminal of the NAND gate  208 . 
   The global bit line driver  204   a  is comprised of PMOS transistors P 2  and P 3 , and an NMOS transistor N 1 , serially coupled between the internal voltage supply and ground or VSS. The PMOS transistor P 2  has a drain connected to the internal voltage supply VDDI, and a source connected to the drain of the PMOS transistor P 3 . The NMOS transistor N 1  has a drain coupled to the drain of the PMOS transistor P 3 , and a source coupled to ground or VSS. The gates of the PMOS transistor P 2  and the NMOS transistor N 1  are connected together to function as an input terminal of the global bit line driver  204   a . The drain of the PMOS transistor P 3  and the drain of the NMOS transistor N 1  are connected together to the global bit line GBL, to function as an output terminal of the global bit line driver  204   a . The gate of the PMOS transistor P 3  is controlled by a select signal S 3 . 
   In the global pre-charge stage, the global pre-charge signal S 1  turns on the PMOS transistor P 1 , thereby raising the signal on the global bit line GBL to a high state. Once a particular I/O module  202   a  is selected for read operation, the select signal S 3  is asserted to turn on the PMOS transistor P 3 , and the local pre-charge signal S 2  is asserted to turn on PMOS transistors P 4  and P 5 , thereby raising the signals on the local bit lines ULBL and LLBL to a high logic state. Since both input terminals of the NAND gate  208  receive high signals, the NAND gate  208  outputs a low signal, which, in turn, switches on the PMOS transistors P 6  and P 7 . The low signal output from the NAND gate  208  turns off the NMOS transistor N 1  and on the PMOS transistor P 2 , thereby raising the voltage on the global bit line GBL to a high state. 
   It is noted that in the embodiment of the present invention, the pre-charge module  206  is comprised of a PMOS transistor. However, other devices, such as NMOS transistors or bipolar devices can also be used to function as a pre-charge switch, depending on design considerations. 
   In read operation, the global and local pre-charge signals S 1  and S 2  are disabled to turn off the PMOS transistors P 1 , P 4  and P 5 , thereby allowing the signals on the global bit line GBL to freely respond to the value stored in the register or memory cell (not shown in the figure) coupled to the I/O module  202  though the local bit lines ULBL and LLBL. If the voltage on either one of the local bit lines ULBL and LLBL is low, the NAND gate  208  outputs a high signal, which, in turn, switches off the PMOS transistors P 2 , P 6  and P 7 , and on the NMOS transistor N 1 . As the source of the NMOS transistor N 1  is coupled to ground or VSS, the voltage on the global bit line is pulled down in response to the voltages on the local bit lines ULBL and LLBL. If the voltage on both the local bit lines ULBL and LLBL are high, the NAND gate  208  outputs a low signal, which turns on the PMOS transistor P 2  and off the NMOS transistor N 1 , such that the voltage on the global bit line GBL remains high. Thus, the proposed single end read module  202  for register files is able to provide the global bit line GBL with a high voltage as a default state, and produce read signals on the global bit line GBL in response to the inputs on the local bit lines ULBL and LLBL. 
   The proposed single end read module in accordance with one embodiment of the present invention improves the reliability of read operation compared to the conventional single end read module. By virtue of implementing a CMOS device in the global bit line driver, the embodiment of the present invention eliminates the need of a latch for latching the default state of the signal on the global bit line. Because the conventional global bit line latch is particularly susceptible to noise-induced reliability issues, the proposed single end read module without the conventional global bit line latch is able to improve its reliability against noise. Such single end read module is particular suitable for low voltage supply and high noise circuitry environment. For example, this proposed single end read module is particularly suitable for complier-type register file designs. In addition, the proposed single end read module imposes no layout area penalty as opposed to the conventional design. As such, the proposed design is able to achieve higher reliability without compromising on layout areas. 
   The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.