Electronic digital logic circuitry – Function of and – or – nand – nor – or not – Decoding
Reexamination Certificate
1999-09-14
2001-08-21
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Function of and, or, nand, nor, or not
Decoding
C326S105000, C326S106000, C365S230060
Reexamination Certificate
active
06278297
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an integrated circuit and more particularly to an integrated circuit with a row decoder having a switched power supply.
BACKGROUND OF THE INVENTION
Present complementary metal oxide semiconductor (CMOS) dynamic random access memory (DRAM) circuits are frequently used for main memory in a variety of applications including desk top and portable computer systems. These dynamic random access memory circuits frequently utilize memory cells formed by a single access transistor and a storage capacitor for storing an electrical charge representing a datum. An increasing demand for greater speed and cell density in dynamic random access memory circuits has been partially satisfied by reducing the operating voltage and the feature sizes of the memory cell. This reduction in operating voltage reduces the total charge that must be transferred to active signal lines. The reduction in feature sizes decreases load capacitance of the signal lines. Large capacitance signal lines such as word lines, however, still require large drive transistors to satisfy speed requirements. Moreover, active word lines of memory circuits organized in banks must be latched. These latched active word lines permit activation of word lines in alternate banks without deactivation of the first bank.
Row decode circuits of the prior art have segmented word lines to reduce load capacitance of each drive circuit. The decode circuit of
FIG. 6
of the prior art includes thirty-six word line segment drive circuits
650
for each global row decode circuit
600
. In operation, global word line signal /WLGS of the global row decode circuit
600
is precharged high by transistor
611
. The global row decode circuit is selected when address signals turn on transistors
615
and
619
, and block select signal /BSEL
2
is driven low. The resulting current path discharges terminal
613
and drives global word line signal /WLGS low at each of the thirty-six word line segment drive circuits
650
. This low level is not latched, and must be maintained, therefore, by the active state of the address signals and the block select signals. The X+ drive circuit of
FIG. 5
selects one of the word line segment drive circuits
650
. This X+ drive circuit includes a decode circuit
500
for producing drive signal /X+ and a buffering inverter
550
for producing complementary signal X+. These drive signals together with a low level of global word line select signal /WLGS produce a high level output signal WLa at word line lead
639
.
Previous memory circuits have employed row decode circuits with level translators as disclosed in U.S. Pat. Nos. 5,668,485, 5,808,482 and 5,696,721. The row decode circuit of U.S. Pat. No. 5,696,721 disclosed in
FIG. 7
, for example, uses a level translator to increase the word line voltage and avoid a threshold voltage loss at the storage capacitor due to the access transistor. The row decode circuit is activated when block select signal BS
—
is driven low and address signals RFJ and RFK are driven high. The resulting low signal at the control gate of transistor
706
couples the control gate of transistor
712
to high voltage supply Vpp, thereby activating word line drive circuit
722
. The decode circuit is reset when block select signal BS
—
goes high. This high level turns off transistor
704
and turns on transistor
710
. In this state, transistors
706
and
710
are both conducting. Transistor
706
must have a width-to-length ratio designed for rapid activation of drive circuit
722
. Transistor
710
must have a width-to-length ratio, however, that is sufficient to overcome transistor
706
and turn off the decode circuit. This conflict between transistor
706
and
710
is illustrated by the simulated waveforms of FIG.
8
. Therein, a width of transistor
710
is held at 2.0 &mgr;m while the width of transistor
706
assumes widths of 0.8 &mgr;m, 3.0 &mgr;m and 6.0 &mgr;m. Although a width of 6.0 &mgr;m for transistor 706 might improve rise time of a word line in response to a low level of row address strobe signal RAS, the decode circuit fails to reset in response to the high level of row address strobe signal RAS. This failure to reset results in an intermediate state of the output signal and lost data along a word line that fails to turn off. A corresponding increase in the width of transistor
710
would have a disadvantage of requiring grater area for the decode circuit.
SUMMARY OF THE INVENTION
These problems are resolved by a decode circuit having a first output terminal and coupled to receive an address signal having a first voltage range. The decode circuit produces a first output signal having one of a first and second logic levels. An output circuit is coupled to receive the first output signal and a power supply signal. The output circuit produces a second output signal having a second voltage range. A first latch transistor is coupled to receive the second output signal. The first latch transistor is arranged to couple the first output terminal to a voltage terminal in response to one of a first and second logic state of the second output signal. A second latch transistor is coupled to receive the second output signal. The second latch transistor is arranged to couple the first output terminal to a reference terminal in response to another of the first and second logic state of the second output signal.
The power supply signal of the present invention eliminates the significance of relative widths of drive and reset transistors when the decode circuit is reset without an increase in required area.
REFERENCES:
patent: 4798977 (1989-01-01), Sakui et al.
patent: 5278802 (1994-01-01), Kersh, III et al.
patent: 5327026 (1994-07-01), Hardee et al.
patent: 5696721 (1997-12-01), McAdams et al.
patent: 5825205 (1998-10-01), Ohtsuka
Horenstein. Microelectronic Circuits and Devices. Prentice Hall: New Jersey. pp. 752-753, 1990.
DeSoto Stewart M.
Scott David B.
Brady III Wade James
Holmbo Dwight N.
Paik Steven S.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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