Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C8/00—Arrangements for selecting an address in a digital store
  • G11C8/08—Word line control circuits, e.g. drivers, boosters, pull-up circuits, pull-down circuits, precharging circuits, for word lines
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11C—STATIC STORES
  • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
  • G11C11/21—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using electric elements
  • G11C11/34—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using electric elements using semiconductor devices
  • G11C11/40—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using electric elements using semiconductor devices using transistors
  • G11C11/41—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using electric elements using semiconductor devices using transistors forming static cells with positive feedback, i.e. cells not needing refreshing or charge regeneration, e.g. bistable multivibrator or Schmitt trigger
  • G11C11/413—Auxiliary circuits, e.g. for addressing, decoding, driving, writing, sensing, timing or power reduction

Definitions

• This invention relates generally to data processing systems and more particularly to a word line driver circuit for a static random access memory (SRAM) and method therefor.
• SRAM static random access memory
• Static random access memories are generally used in applications requiring high speed, such as a cache memory in a data processing system.
• a SRAM is usually implemented as an array of memory cells organized in rows and columns. Each SRAM cell stores one bit of data and is implemented as a pair of inverters having their inputs and outputs cross-coupled at differential storage nodes.
• the SRAM cell is "bistable", that is, it is stable at one of two possible logic levels. The logic state of the cell is determined by whichever of the two inverter outputs is a logic high, and can be made to change states by applying a voltage of sufficient magnitude and duration to the appropriate cell input.
• FIGURE illustrates, in partial schematic diagram form and partial block diagram form, a data processing system in accordance with the present invention.
• the present invention provides a data processing system having an embedded SRAM.
• the power supply voltage provided to the word line driver circuits of the memory is reduced by a predetermined voltage below the power supply voltage that is provided to the memory cells.
• the static noise margin of the memory array is improved for low voltage operation.
• Data processing system 10 includes a central processing unit (CPU) 12, a memory 14, a word line driver power supply control circuit 36, fuse block circuit 64, and registers 72.
• the memory 14 includes a memory array 15, a row decoder 46, word line driver circuits 48, and column logic 62.
• the memory array 15 has representative SRAM cells 16, 18, 20, and 22.
• Memory cell 16 is a conventional six transistor cell and includes P-channel pull-up transistors 24 and 26, N- channel pull-down transistors 28 and 30, and N-channel access transistors 32 and 34. All of the memory cells of array 15 are identical to memory cell 16.
• the word line driver power supply control circuit 36 includes a plurality of transistors including a P-channel transistor 38, and N-channel transistors 40, 42, and 44, fuse block circuit 64, and registers 72.
• Word line driver circuits 48 include a word line driver 50 and a word line driver 52. Note that the data processing system 10 may include other circuits not shown in the FIGURE.
• a column of memory cells includes a bit line pair and all of the memory cells coupled to the bit line pair.
• the bit line pair labeled "BLO" and “BLO*” and cells 16 and 20 comprise one column.
• bit lines BLM and BLM* and memory cells 18 and 22 comprise another column in a memory array having M + 1 columns, where M is an integer.
• a signal name having an asterisk (*) is a logical complement of a signal having the same name but lacking the asterisk.
• a row of memory array 15 comprises a word line and all of the memory cells coupled to the word line.
• a word line labeled "WLO" and memory cells 16 and 18 comprise one row.
• word line WLN and memory cells 20 and 22 comprise another row in a memory array having N + 1 rows, where N in an integer.
• column logic 62 includes, for example, column decoders, sense amplifiers and bit line loads.
• the read/write enable signal labeled "R/W ENABLE” is provided as an input to column logic 62 and determines whether the memory array will be written to, or read from.
• the column address labeled "COLUMN ADDRESS” selects which of the columns of the memory array will receive the write data at data terminals labeled "I/O DATA”.
• the COLUMN ADDRESS selects which of the columns will provide read data to the FO DATA terminals.
• Row decoder 46 has a plurality of inputs for receiving a row address labeled "ROW ADDRESS".
• the row decoder In response to receiving the ROW ADDRESS, the row decoder provides an address signal Ao - AN to select one of the word lines during an access (read or write) to the memory array 15.
• Each of the address signals A 0 through AN is provided to a corresponding word line driver represented by word line driver circuits 50 and 52.
• the word line drivers are implemented as inverters, hi other embodiments, the word line drivers may be a different circuit, such as for example, a non- inverting buffer circuit. Note that read and write operations of memory 14 are conventional, and will not be described further.
• Word line driver 50 includes a P-channel pull-up transistor 54 and an N-channel pull- down transistor 56.
• Word line driver 52 includes a P-channel pull-up transistor 58 and an N- channel pull-down transistor 60.
• the source terminal of the N-channel transistor 56 is coupled to a power supply voltage terminal labeled Vss-
• the commonly connected drains of transistors 54 and 56 provide the output terminal for the word line voltage WLO.
• the transistors 58 and 60 of the word line driver circuit 52 are coupled together in the same way.
• the word line driver power supply control circuit 36 provides a power supply voltage for each of the word line driver circuits that is lower than the power supply voltage V DD by a predetermined voltage.
• the predetermined voltage is a threshold voltage (V T ) drop of a transistor.
• V T threshold voltage
• the FIGURE illustrates three N-channel transistors 40, 42, and 44 in the word line driver power supply circuit 36. hi the illustrated embodiment, each of these transistors is implemented with a different V ⁇ to provide a different voltage drop when selected.
• the transistors 40, 42, and 44 are coupled in parallel between VD D and an internal power supply node 37.
• the source terminals of the P-channel transistors 54 and 58 are also coupled to the internal power supply node 37.
• registers 72 includes a plurality of programmable bits that are read and written to by CPU 12.
• CPU 12 has a serial terminal labeled "SI" for programming the bits of registers 72 and a clock terminal for providing a clock signal labeled "CLK" to the registers 72.
• SI serial terminal labeled
• CLK clock signal labeled "CLK”
• a fuse block 64 having a plurality of fuse circuits is also coupled to the gates of the N- channel transistors 40, 42, and 44.
• the type of fuse circuit is not important for describing the present invention and can be a conventional fuse circuit such as one used to implement redundancy in a memory.
• the fuse block circuit may include a laser blowable fuse (not shown) having one terminal coupled to VD D and a second terminal coupled to the drain of an N-channel transistor (not shown).
• the gate of the N-channel transistor is biased high at power up of the data processing system.
• the drain of the N- channel transistor is coupled to an input of a cross-coupled latch circuit (not shown).
• the fuse circuit may be implemented in another way.
• the corresponding fuse circuit 66, 68, or 70 couples the power supply voltage V D D to the gate of the corresponding transistor 40, 42, or 44, respectively.
• the enable signal labeled "LOW V D D" is provided at a logic low voltage to cause transistor 38 to be conductive.
• the power supply voltage for the word line driver circuits 48 is provided through the P-channel transistor 38 and is substantially equal to V DD - When one of the word lines WLO - WLN is selected, the word line driver circuit provides a logic high word line voltage substantially equal to VDD- The unselected word lines are held at about ground potential (Vss).
• Vss ground potential
• the power supply voltage is lowered to conserve power. The lower power supply voltage can have detrimental effects on the operation of an SRAM. For example, the static noise margin of the memory cells may be lowered to the extent that memory operation becomes unreliable.
• the illustrated embodiment reduces the word line voltage by a predetermined voltage below the power supply voltage, for example, a V T below VD D - This will increase the static noise margin and thus allow reliable memory operation at lower power supply voltages.
• the registers 72 are used to select each of the transistors 40, 42, and 44 in turn to provide different power supply voltages below V DD to the word line drivers.
• the low V DD operation of the memory cells of the array 15 is determined for each of the word line driver voltages provided by transistors 40, 42, and 44 in turn.
• the fuse circuit 66, 68, or 70 that corresponds to the gate of the transistor 40, 42, or 44 which provides the best low V D D operation is blown.
• the fuse of the fuse circuit can be electrically blown or may be laser blown. The blown fuse circuit then permanently biases the gate of that transistor when a power supply voltage is present.
• VD D is asserted as a logic high voltage to cause P-channel transistor 38 to be substantially non-conductive.
• the enable signal LOW VD D is provided by a source external to data processing system 10.
• the enable signal LOW V DD may be provided by another circuit on data processing system 10 such as the CPU 12, or a different circuit not shown in the FIGURE.
• the power supply for the word line driver circuits is provided via the selected one of the N-channel transistors 40, 42, and 44. By way of example, if the power supply voltage of data processing system 10 is normally 1.2 volts nominal, the power supply voltage provided to V D D during a low power mode may be 0.8 volts or lower.
• the threshold voltages of the N-channel transistors 40, 42, and 44 may be, for example, selected to be about 40 millivolts (mV), 60 mV, and 80 mV, respectively. If, for example, the voltage provided by N-channel transistor 40 provides the best low power operation, then the fuse circuit 66 is blown and the word line voltage WLO is equal to about VDD minus 4OmV.
• the beta ratio of a memory cell is the width/length (W/L) ratio of the pull-down transistor to the W/L ratio of the access transistor.
• the beta ratio has an effect on access speed and on cell stability. In general, for a given cell size, a higher beta ratio improves cell stability at the expense of lower access speed. Lowering the word line voltage has the effect of increasing the beta ratio.
• the word line voltage during low voltage operation may be controlled in different ways.
• the transistors 40, 42, and 44 may be replaced by voltage dividers.
• a plurality of series-connected transistors may be used in place of transistors 40, 42, and 44.
• the power supply voltage may be provided externally.
• the present invention allows the memory to operate with a word line voltage equal to about VDD for high speed operation during a normal operating mode.

Landscapes

• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Computer Hardware Design (AREA)
• Static Random-Access Memory (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=36386078

Family Applications (1)

Country Status (5)

Cited By (3)

Families Citing this family (30)

Citations (4)

Family Cites Families (16)

• 2004
  • 2004-11-18 US US10/991,910 patent/US7085175B2/en not_active Expired - Lifetime
• 2005
  • 2005-10-25 KR KR1020077011213A patent/KR101227291B1/ko not_active Expired - Lifetime
  • 2005-10-25 WO PCT/US2005/038468 patent/WO2006055190A1/en not_active Ceased
  • 2005-10-25 JP JP2007543071A patent/JP4988588B2/ja not_active Expired - Lifetime
  • 2005-10-25 CN CN2005800347232A patent/CN101040343B/zh not_active Expired - Lifetime

Patent Citations (4)

Also Published As

Similar Documents

Legal Events