Abstract:
A semiconductor memory is disclosed, which comprises a plurality of memory cells, at least one high voltage power supply (CVDD) line coupled to the plurality of memory cells for supplying power to the same, and at least one controllable discharging circuit coupled between the CVDD line and a complementary low voltage power supply (ground), wherein only during a write operation the controllable discharging circuit is turned on for discharging the CVDD line.

Description:
BACKGROUND 
   The present invention relates generally to integrated circuit designs and more particularly to write circuit design for improving write margins in static random access memory (SRAM). 
   SRAM is typically used for storing data needed to be speed accessed by processing units. A conventional 6-T SRAM cell comprises two cross-coupled inverters forming a data latch and two pass-gate NMOS transistors for controlling accesses to the data latch by a bit-line-true (BLT) and a bit-line-complementary (BLC). During a read operation, the data latch drives the BLT or BLC to develop a differential voltage between the BLT and BLC, therefore a higher supply voltage provides a greater read margin. During a write operation, it is the BLT or BLC that forces the data latch to flip, therefore, given a fixed BLT and BLC voltage level, a lower supply voltage provides a greater write margin. 
   There have been many attempts to increase write margins in the SRAM, such as floating a column of SRAM cells that is being written. However, power supply lines of the SRAM may have large capacitances, even though the outside power supply is switched off, the large capacitances may store charges that prevent the power supply line voltage from dropping any significant amount during the write cycle. Especially when the SRAM speed becomes very fast with a very short write cycle, the floating power supply method may not be very effective in improving a write margin of the SRAM. 
   As such, what is desired is a SRAM cell power supply system that discharges the power supply lines during a write operation. 
   SUMMARY 
   The present disclosure provides a semiconductor memory, which comprises a plurality of memory cells, at least one high voltage power supply (CVDD) line coupled to the plurality of memory cells for supplying power to the same, and at least one controllable discharging circuit coupled between the CVDD line and a complementary low voltage power supply (ground), wherein only during a write operation the controllable discharging circuit is turned on for discharging the CVDD line. 
   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 
     The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
       FIG. 1  is a schematic diagram illustrating a SRAM cell column with a power supply switching transistor. 
       FIG. 2  is a schematic diagram illustrating the SRAM cell column with a power supply line discharging circuits according to an embodiment of the present invention. 
       FIGS. 3A and 3B  are schematic diagrams illustrating two implementations of the power supply line discharging circuit according to the embodiment of the present invention. 
   

   DESCRIPTION 
   The present invention discloses a write circuit that discharges power supply lines during a write operation. 
     FIG. 1  is a schematic diagram illustrating a SRAM cell column  100  with a power supply switching transistor  150 . SRAM cells  102 [0:n−1] are conventional 6-T SRAM cells with four transistors  110 ,  115 ,  120  and  125  forming two cross-coupled inverters and two pass-gate NMOS transistors  130  and  135  couples the cross-coupled inverters to a bit-line-true (BLT) and a bit-line-complementary (BLC), respectively. Sources of two PMOS transistors  110  and  120  are coupled to a positive voltage power supply (VDD) through the power supply switch  150 , which is implemented as a PMOS transistor. A node CVDD on a drain side of the PMOS transistor  150 , which is also coupled to the sources of the PMOS transistors  110  and  120 , designates a power supply line in the SRAM column  100 , which may possess a substantial amount of capacitance if the SRAM column  100  has many cells, i.e., the number n is large. During a write operation, in order to make flipping the cross-coupled inverters easier, a signal YSW is asserted a logic HIGH voltage to turn off the PMOS transistor  150 , then the node CVDD is effectively cut off from the VDD or is floating. However, the capacitance on the node CVDD may prevent a voltage at the node CVDD from a significant drop, which in turn may render switching off the VDD less effective in expanding write margins for the SRAM cells  102 [0:n−1]. 
     FIG. 2  is a schematic diagram illustrating a SRAM cell column  200  with a power supply line discharging circuit  210  according to an embodiment of the present invention. The SRAM cell column  200  is formed by adding the discharging circuit  210  and its timing control circuit  220  to the SRAM cell column  100  shown in  FIG. 1 , thus identical reference numbers or labels in both  FIGS. 1 and 2  designate the same elements. 
   Referring to  FIG. 2 , the discharging circuit  210  is coupled to a node CVDD, which designates a power supply line in the SRAM cell column  200 . During a write operation, while a signal YSW turns off a power switching PMOS transistor  150 ; which turns off a conduction of the VDD to the node CVDD, the discharging circuit is also activated which may provide a discharging path between the node CVDD and the ground so that charges stored on the power supply lines, i.e., the node CVDD, can be quickly discharged and allows a voltage at the node CVDD to drop faster than without the discharge path. Dropped voltage at the node CVDD may improve the write margin of the SRAM cells  102 [0:n−1]. However, if the node CVDD voltage drops too much, such as to only a half of the VDD, data retention may become an issue for those SRAM cells on the same column but not selected for being written. The timing control circuit  220  is designed to address this issue and others. 
   Since ΔV=ΔQ/C, where ΔV is the voltage drop, ΔQ is the discharged amount of charges stored at the node CVDD, which has a capacitance of C, and ΔQ=∫di/dt, the longer the discharging time, the larger the ΔV. The timing control circuit  220  is to generate a pulsed control signal with a predetermined duration to activate the discharging circuit  210 . The timing of the pulsed control signal may also have to be correlated with the assertion the signal YSW, i.e., the pulsed control signal is activated after the assertion of the signal YSW, and deactivated before the de-assertion of the signal YSW. An overlap between the assertion of the signal YSW and the activation of the discharging circuit  210  creates a short circuit between the VDD and the ground. 
   Although detailed implementation of the timing control circuit  220  is not shown here, people having skills in the art would have no difficulty to devise a circuit for generating the pulsed control signal which synchronizes with the signal YSW. 
   Another aspect of the discharging circuit  210  designs involves its uniformity throughout the entire SRAM chip, as each SRAM cell column employs at least one such discharging circuit  210 , and it is critical for the node CVDD of very SRAM cell column to drop the same amount of voltage. One way to ensure this uniformity of voltage drop is to design the discharging circuit  210  into providing a constant discharging current throughout the discharging process this will be discussed in implementations of the discharging circuit  210  in the following paragraphs. 
     FIGS. 3A and 3B  are schematic diagrams illustrating two implementations of the power supply line discharging circuit  210  according to the embodiment of the present invention. Referring to  FIG. 3A , the discharging circuit  210  is implemented as a NMOS transistor  310  with a source, gate and drain coupled to the ground, the pulsed control signal YSD and the node CVDD, respectively. When the signal YSD is asserted to a logic HIGH voltage, the NMOS transistor  310  is turned on, and creates a conduction path between the node CVDD and the ground for discharging the node CVDD. 
   Referring to  FIG. 3B , the discharging circuit  210  is implemented as a PMOS transistor  320  with a source, gate and drain coupled to the node CVDD, the pulsed control signal YSD and the ground, respectively. When the signal YSD is asserted to a logic LOW voltage, the PMOS transistor  320  is turned on, and creates a conduction path between the node CVDD and the ground for discharging the node CVDD. 
   Note that the logic HIGH and low voltages are traditionally equal to the VDD and ground, respectively, and there are no exceptions here. The VDD voltage at the gate of the NMOS transistor  310  ensures it to operate at a saturation region and provides a constant current for discharging the node CVDD. Similarly, the ground voltage at the gate of the PMOS transistor  320  ensures it to also operate at a saturation region and is to provide a constant current for discharging the node CVDD. 
   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.