U.S. patent application number 09/745691 was filed with the patent office on 2001-05-17 for vpx bank architecture.
Invention is credited to Guliani, Sandeep K., Sundaram, Rajesh, Taub, Mase J..
Application Number | 20010001263 09/745691 |
Document ID | / |
Family ID | 23624969 |
Filed Date | 2001-05-17 |
United States Patent
Application |
20010001263 |
Kind Code |
A1 |
Guliani, Sandeep K. ; et
al. |
May 17, 2001 |
VPX bank architecture
Abstract
A method for a VPX banked architecture. The method of one
embodiment first segments a memory array into at least two banks.
Each bank comprises of memory cells. The banks are provided with a
supply voltage.
Inventors: |
Guliani, Sandeep K.;
(Folsom, CA) ; Sundaram, Rajesh; (Fair Oaks,
CA) ; Taub, Mase J.; (Elk Grove, CA) |
Correspondence
Address: |
Peter Lam
BLAKELY, SOKOLOFF, TAYLOR & ZAFMAN LLP
12400 Wilshire Boulevard, Seventh Floor
Los Angeles
CA
90025-1026
US
|
Family ID: |
23624969 |
Appl. No.: |
09/745691 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09745691 |
Dec 22, 2000 |
|
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09410493 |
Sep 30, 1999 |
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Current U.S.
Class: |
365/230.03 ;
365/185.11; 365/189.11 |
Current CPC
Class: |
G11C 8/12 20130101; G11C
16/30 20130101; G11C 5/14 20130101 |
Class at
Publication: |
365/230.03 ;
365/189.11; 365/185.11 |
International
Class: |
G11C 016/04 |
Claims
What is claimed is:
1. A method of arranging a memory array comprising: segmenting said
memory array into at least two banks, said banks comprising of
memory cells; and providing said banks with a supply voltage.
2. The method of claim 1 wherein each of said banks comprises a set
of X-decoder cells.
3. The method of claim 1 wherein each of said sets of X-decoder
cells has a separate N-well.
4. The method of claim 1 wherein said providing comprises coupling
said supply voltage from a global signal to a local signal.
5. The method of claim 1 wherein said each of said banks comprises
bank switch logic, said switch logic coupling a global signal to a
local signal.
6. A method for reducing program current comprising: segmenting a
capacitance that has to be charged; and inserting a dummy row in a
memory array.
7. The method of claim 6 wherein said dummy row is an unused
wordline to keep said memory array contiguous.
8. The method of claim 6 wherein said two dummy rows are inserted
into said memory array.
9. The method of claim 6 wherein said segmenting comprises dividing
a memory array into a plurality of banks, each of said banks
comprising memory cells.
10. The method of claim 9 wherein each of said banks comprises a
set of X-decoder cells.
11. The method of claim 9 wherein N-wells of said banks are
separate.
12. The method of claim 9 further comprising generating local
signals for each of said banks.
13. The method of claim 12 wherein said generating comprises using
switching logic to couple global signals to said local signals.
14. The method of claim 9 wherein each of said banks comprises of
switching logic.
15. The method of claim 9 wherein a bank is activated when an
address in said bank is accessed.
16. An apparatus for accessing memory comprising: a global signal;
a memory array coupled to said global signal, said memory array
segmented into at least two banks, said banks comprising memory
cells.
17. The apparatus of claim 16 further comprising a charge pump,
said charge pump providing a pumped supply voltage.
18. The apparatus of claim 17 wherein said charge pump is coupled
to high voltage switch logic, said high voltage logic for switching
said pumped supply voltage to a global signal.
19. The apparatus of claim 16 wherein each of said banks comprises
bank switching logic, said bank switching logic for coupling said
global signal to a local signal within in each of said banks.
20. The apparatus of claim 16 wherein each of said banks comprises
a set of X-decoder cells, each set of X-decoder cells coupled to
bank switching logic.
21. The apparatus of claim 20 wherein each of said X-decoder cells
couples said local signal to a global wordline.
22. The apparatus of claim 16 wherein N-wells of said banks are
separate.
23. The apparatus of claim 16 wherein said memory array further
comprises a dummy row, said dummy row located between said
banks.
24. The apparatus of claim 23 wherein said dummy row is an unused
wordline to keep said memory array contiguous.
25. The apparatus of claim 21 wherein a local block select device
is coupled to said global wordline, said local block select device
to connect said global wordline to a local wordline depending on
address of memory address being accessed.
26. A memory comprising: a charge pump, said charge pump to provide
a pumped supply voltage; an X-path switch coupled to said charge
pump, said X-path switch for coupling said pumped supply voltage to
a global signal; and a memory array, said memory array segmented
into at least two banks, each bank comprising bank switch logic for
coupling said global signal to a local signal; a set of X-decoders
coupled to said local signal, said X-decoders coupled to global
wordlines; and local block selects to connect said global wordlines
to local wordlines.
27. The memory of claim 26 wherein N-wells of each set of
X-decoders are separate.
28. The memory of claim 26 wherein said memory array further
comprises a dummy row, said dummy row located between said banks to
keep said memory array contiguous.
29. The memory of claim 26 wherein said memory is a flash
memory.
30. A digital processing system comprising: a processor; a memory
coupled to said memory comprising a memory array, said memory array
segmented into at least two banks, each bank comprising bank switch
logic for coupling a global signal to a local signal; a set of
X-decoders coupled to said local signal, said X-decoders coupled to
global wordlines; and local block selects to connect said global
wordlines to local wordlines.
Description
FIELD OF THE INVENTION
1. The present invention relates generally to the field of
computers and computer systems. More particularly, the present
invention relates to a method and apparatus for a VPX bank
architecture.
BACKGROUND OF THE INVENTION
2. Many of today's computing applications such as cellular phones,
digital cameras, and personal computers, use nonvolatile memories
to store data or code. Non-volatility is advantageous because it
allows the computing system to retain its data and code even when
power is removed from the computing system. Thus if the system is
turned off or if there is a power failure, there is no loss of code
or data. Such nonvolatile memories include Read-Only Memory (ROMs),
Electrically Programmable Read-Only Memory (EPROMs), Electrically
Erasable Programmable Read-Only Memory (EEPROMs), and flash
Electrically Erasable Programmable Read-Only Memory (flash EEPROMs
or flash memory).
3. Nonvolatile semiconductor memory devices are fundamental
building blocks in computer system designs. One such nonvolatile
memory device is flash memory. Flash memory can be programmed by
the user, and once programmed, the flash memory retains its data
until the memory is erased. Electrical erasure of the flash memory
erases the contents of the memory of the device in one relatively
rapid operation. The flash memory may then be programmed with new
code or data. The primary mechanism by which data is stored in
flash memory is a flash memory cell. Accordingly, outputs of a
flash memory device are typically associated with an array of flash
cells that is arranged into rows and columns such that each flash
cell in the array is uniquely addressable.
4. A flash EEPROM memory device (cell) is a floating gate MOS field
effect transistor having a drain region, a source region, a
floating gate, and a control gate. Conductors are connected to each
drain, source, and control gate for applying signals to the
transistor. A flash EEPROM cell is capable of functioning in the
manner of a normal EPROM cell and will retain a programmed value
when power is removed from the circuitry. A flash EEPROM cell may
typically be used to store a one or zero condition. If multilevel
cell (MLC) technology is used, multiple bits of data may be stored
in each flash EEPROM cell. Unlike a typical EPROM cell, a flash
EEPROM cell is electrically erasable in place and does not need to
be removed and diffused with ultraviolet to accomplish erasure of
the memory cells.
5. Arrays of such flash EEPROM memory cells have been used in
computers and similar circuitry as both read only memory and as
long term storage which may be both read and written. These cells
require accurate values of voltage be furnished in order to
accomplish programming and reading of the devices. Arrays of flash
EEPROM memory devices are typically used for long term storage in
portable computers where their lightweight and rapid programming
ability offer distinct advantages offer electromechanical hard disk
drives. However, the tendency has been to reduce the power
requirements of such portable computers to make the computers
lighter and to increase the length of use between recharging. This
has required that the voltage potentials available to program the
flash memory arrays be reduced.
6. FIG. 1 is a typical prior art memory architecture 100. A charge
pump 102 provides a pumped voltage potential 104. Pump voltage 104
is supplied to X-path switches 106. Logic circuits of the X-path
switches 106 control the voltage potentials coupled to the X-path
during read, write, and erase modes in the memory. The outputs of
the X-path switches 106 are coupled to X-decoders 112, 122. Each
supply voltage from the switched outputs 108 from the X-path
switches 106 have to supply all the X-decoder devices 112, 122 in
both planes 110, 120.
7. The embodiment in FIG. 1 has a memory array divided into two
planes 110, 120. The first plane 110 and second plane 120 are
similar in construction. Global wordlines 114, 124 from the
X-decoders 112, 122 are coupled to local block selects 116, 126 in
each block of the memory block in the corresponding planes 110,
120. The local block selects 116, 126 determine whether the global
wordlines 114, 124 are coupled to the local wordlines 118, 128 in a
block.
8. The X-path switches of prior art designs provided a single set
of high voltages signals that are coupled to circuits for the
entire memory array. A high voltage signal can be coupled to
devices on both planes of memory. In other words, whenever each
high voltage signal transitioned from one voltage to a higher
voltage potential, that high voltage signal needed to supply
current to all the circuit devices coupled to its signal. Hence,
each high voltage signal has to charge up a large amount of
capacitance, which increases the current and power consumption.
9. A number of the electronic systems that use flash memories are
small portable devices that rely on batteries for power. As new
applications emerge, system designers are open to alternative
methods of increasing the battery life of these devices by reducing
power consumption.
SUMMARY OF THE INVENTION
10. A method for a VPX banked architecture is described. The method
comprises of one embodiment first segments a memory array into at
least two banks. Each bank comprises of memory cells. The banks are
provided with a supply voltage.
11. Other features and advantages of the present invention will be
apparent from the accompanying drawings and from the detailed
description that follow below.
BRIEF DESCRIPTION OF THE DRAWINGS
12. The present invention is illustrated by way of example and not
limitations in the figures of the accompanying drawings, in which
like references indicate similar elements, and in which:
13. FIG. 1 is a typical prior art memory architecture;
14. FIG. 2 is a computer system with a memory using a VPX bank
architecture in one embodiment;
15. FIG. 3 is a block diagram of the high voltage and banking
architecture of one embodiment;
16. FIG. 4 is a circuit diagram of an X-decoder cell; and
17. FIG. 5 is a block diagram of a banked memory architecture.
DETAILED DESCRIPTION
18. A method and apparatus for a VPX bank architecture is
disclosed. The described architecture enables banking a memory
array in nonvolatile writable memory. The embodiments described
herein are described in the context of a nonvolatile writable
memory or flash memory, but is not so limited. Although the
following embodiments are described with reference to nonvolatile
writable memories and flash memory, other embodiments are
applicable to other circuits that have memory arrays or voltage
supplies. The same techniques and teachings of the present
invention can easily be applied to other types of memory devices
that use charge pumps.
19. Designers of portable devices have been concerned with reducing
power and current consumption in order to increase system
performance. However, another feature important for improving
system performance is program time. Hence, memory parts having fast
reads and fast programs are also desired. For instance, cell phone
manufacturers have found that products having a longer battery life
are more competitive in the marketplace. Hence, low power
components are greatly in demand. This is really important at low
voltages since the savings are very significant. Methods for
reducing power consumption have included utilizing standby modes,
deep power-down, and lower voltages.
20. But at lower voltages, programming flash memory cells becomes
more difficult. First, certain circuits such as the X-decoders need
to be larger in size. The X-decoders were enlarged because the read
path and sensing slowed down at lower voltages. The larger size
helped compensate for the performance difference. However, the
amount of capacitance due to the X-decoders increased. Second, the
pump efficiency of the charge pumps decrease. Third, the size of
the charge pump area increases because more pump stages are
required to meet the current demands.
21. Two different aspects relating to the supply current are
important during memory programming. One is the average programming
current. The higher the current requirements, the more charge that
the charge pumps have to supply. The other is the time necessary to
slew the supply voltage. The larger the load or capacitance coupled
to a power supply node, the more time that is necessary for the
node to slew up to the desired voltage potential.
22. One embodiment of the invention introduces a bank architecture
that segments a memory array into multiple banks of memory cells
and X-decoder cells. Each bank is supplied with its own set of high
voltage signals. When a word is programmed in memory, the high
voltage signals for the bank in which the word to be programmed
resides is charged up and the high voltage signals of the other
banks are left floating. Thus, the amount of capacitance to be
charged during programming in one embodiment is reduced by a factor
equal to the number of banks. For example, if a memory array is
divided into four banks, the total capacitance to be charged is
reduced by a factor of four. Furthermore, the charging current and
supply slew time are reduced by a similar factor. This enhancement
can be especially useful at low voltages such as 2 volts and lower.
The charging current and slew time reductions are directly related
to the total capacitance. The larger the capacitance, the more
current that is needed from the voltage supply to charge up the
capacitance, resulting in longer slew times on the supply node.
23. Referring now to FIG. 2, there is a computer system 200 that
includes the present embodiment. Sample system 200 may have a
memory incorporating a VPX banked memory architecture, in
accordance with the present invention, such as in the embodiment
described herein. Sample system 200 is representative of processing
systems based on the PENTIUM.RTM., PENTIUM.RTM. Pro, PENTIUM.RTM.
II, PENTIUM.RTM. III microprocessors available from Intel
Corporation of Santa Clara, Calif., although other systems
(including PCs having other microprocessors, engineering
workstations, set-top boxes and the like) may also be used. In one
embodiment, sample system 200 may be executing a version of the
WINDOWS.TM. operating system available from Microsoft Corporation
of Redmond, Wash., although other operating systems and graphical
user interfaces, for example, may also be used. Thus, the present
invention is not limited to any specific combination of hardware
circuitry and software.
24. FIG. 2 is a block diagram of a system 200 of one embodiment.
System 200 is an example of a hub architecture. The computer system
200 includes a processor 202 that processes data signals. The
processor 202 may be a complex instruction set computer (CISC)
microprocessor, a reduced instruction set computing (RISC)
microprocessor, a very long instruction word (VLIW) microprocessor,
a processor implementing a combination of instruction sets, or
other processor device, such as a digital signal processor, for
example. FIG. 2 shows an example of an embodiment of the present
invention implemented in a single processor system 200. However, it
is understood that other embodiments may alternatively be
implemented as systems having multiple processors. Processor 202 is
coupled to a processor bus 210 that transmits data signals between
processor 202 and other components in the system 200. The elements
of system 200 perform their conventional functions well known in
the art.
25. System 200 includes a memory 220. Memory 220 may be a dynamic
random access memory (DRAM) device, a static random access memory
(SRAM) device, flash memory device, or other memory device. Memory
220 may store instructions and/or data represented by data signals
that may be executed by processor 202. A cache memory 204 can
reside inside processor 202 that stores data signals stored in
memory 220. Alternatively, in another embodiment, the cache memory
may reside external to the processor.
26. A system logic chip 216 is coupled to the processor bus 210 and
memory 220. The system logic chip 216 in the illustrated embodiment
is a memory controller hub (MCH). The processor 202 communicates to
a memory controller hub (MCH) 216 via a processor bus 210. The MCH
216 provides a high bandwidth memory path 218 to memory 220 for
instruction and data storage and for storage of graphics commands,
data and textures. The MCH 216 directs data signals between
processor 202, memory 220, and other components in the system 200
and bridges the data signals between processor bus 210, memory 220,
and system I/O 222. In some embodiments, the system logic chip 216
provides a graphics port for coupling to a graphics controller 212.
The MCH 216 is coupled to memory 220 through a memory interface
218. The graphics card 212 is coupled to the MCH 216 through an
Accelerated Graphics Port (AGP) interconnect 214.
27. System 200 uses a proprietary hub interface bus 222 to couple
the MCH 216 to the I/O controller hub (ICH) 230. The ICH 230
provides direct connections to some I/O devices. Some examples are
the audio controller, BIOS 228, data storage 224, legacy I/O
controller containing user input and keyboard interfaces, a serial
expansion port such as Universal Serial Bus (USB), and a network
controller 234. The data storage device 224 can comprise a hard
disk drive, a floppy disk drive, a CD-ROM device, a flash memory
device, or other mass storage device. A VPX banked architecture
memory 226 resides in the flash memory BIOS 228 in this embodiment.
In an alternative embodiment, the BIOS 228 may be part of a
firmware hub.
28. The present embodiment is not limited to computer systems.
Alternative embodiments can be utilized in applications including
cellular phones, personal digital assistants (PDAs), embedded
systems, and digital cameras.
29. A number of circuit devices require N-wells. N-wells are needed
for all P type transistors created on a P type substrate. One flash
memory architecture utilizing block select and X-path decoding
schemes includes a large amount of N-well area on the die. However,
an N-well can contribute significantly to the capacitance on a
connected node. An N-well can behave like a capacitor when the
signal connected to the well transitions. Therefore, an N-well can
consume current when its corresponding signal transitions.
30. For instance, the N-wells that are tied to the positive pump
outputs or high voltage signals can draw current when the attached
signal changes from one voltage potential to a higher voltage
potential. When the flash memory device of one embodiment enters
into its program mode from a read mode, the positive nodes are
generally at the 5 volt read levels and need to be brought up to
the program value. If an N-well is coupled to VPX and VPX
transitions from 5 volts to 10 volts during a program sequence,
then VPX also needs to supply enough charge to increase the voltage
potential of the N-well. Hence, the N-wells that are tied to the
positive pump outputs during program have to be included as part of
the load on the program current. Charging the N-wells up to the
proper program voltages can require a large amount of time and
power.
31. An X decoder cell has a series of N-wells for its circuit
devices. High voltage nodes VPX and VPIX, and the N-wells are
sitting at 5 volts during read mode. Local block selects and local
wordlines also contribute to the N-well area. These N-wells also
sit at 5 volts during read mode. When the memory device goes into a
program, these voltages can increase to approximately 9 to 12
volts.
32. The total amount of capacitance of the positive voltage nodes
can be about 800 picofarads for one embodiment. There are a number
of sources contributing to the overall capacitance including:
N-well capacitance, gate capacitance, diode capacitance, junction
capacitance, and gate overlap. In some memory parts, the voltage
increases from 5 volts to 12 volts when the part goes from read to
program. If there is 1000 picofarads of capacitance that needs to
be charged from 5 volts to 12 volts, then a large amount of charge
has to be supplied.
33. FIG. 3 is a block diagram of the high voltage and banking
architecture 300 of one embodiment. The banked architecture 300 in
FIG. 3 comprises a charge pump 302, X-path switches 306, and two
memory planes 310, 315. Charge pump 302 is coupled to the X-path
switches 306. A pumped supply voltage 304 is supplied from the
charge pump. For one embodiment, the pumped supply voltage 304 is a
positive voltage and the charge pump 302 is a positive charge pump.
Alternative embodiments may comprise of a negative charge pump
providing a pumped supply voltage 304 of a negative voltage
potential. Similarly, the banking architecture can also be applied
to the Y-path or W-path in alternative embodiments.
34. The X-path switches 306 couple the pumped supply voltage 304 to
a number of high voltage signals 308. The high voltage signals 308
of one embodiment comprise of VPX, VPIX, VPXNW, and block selects.
X-path switches can switch the voltage potentials of these high
voltage signals 308 across a range of voltages from a ground
potential up to 12 volts depending on the mode of operation. For
instance, VPX and VPIX can be 5 volts during read mode. During a
programming pulse, VPX and VPIX can be approximately 10 volts. VPX
and VPIX can be at a ground potential during a erase sequence.
35. The memory array is divided into two planes: PLANE 0 310 and
PLANE 1 315. Each plane 310, 315 is subdivided into two banks each.
PLANE 0 310 comprises of BANK 0 320 and BANK 1 340, whereas PLANE 1
315 comprises of BANK 2 360 and BANK 3 380. Each bank 320, 340,
360, 380 comprises of a bank switch 322, 342, 362, 382, X-decoders
326, 346, 366, 386, and local block selects 330, 350, 370, 390. The
memory planes 310, 315 are constructed of continuous rows of flash
cells. Dummy rows 313, 318 are inserted between the banks in each
memory plane 310, 315 of one embodiment. The dummy rows 313, 318
are used to separate the banks such that each plane of flash memory
cells is not broken. However, the N-wells of the X-decoder devices
are broken and separated into separate N-wells for this
enhancement. The space between the X-decoder N-wells is filled with
dummy rows in the memory array to maintain continuity. The dummy
rows of one embodiment are unused wordlines for keeping the planes
of the memory array contiguous.
36. Bank selection logic separates the high voltage signals 308 for
each bank. The high voltage signals 308 are coupled from the X-path
switches 306 to the bank switches 322, 342, 362, 382. The bank
switches 322, 342, 362, 382 of the present embodiment provide a
separate set of high voltage signals for each bank 320, 340, 360,
380 of memory. For example, the bank switch 322 of BANK 0 320 can
couple the high voltage signals 308 to circuit devices in its bank
when flash memory cells in BANK 0 320 are accessed. Similarly, bank
switch 362 of BANK 2 362 can couple the high voltage signals 308 to
circuit devices in its bank when memory cells in BANK 2 360 are
accessed.
37. For one embodiment, each set of high voltage signals 324, 344,
364, 384 comprises of VPX, VPIX, VPXNW, and corresponding block
selects. Each set of signals is identical except that each set
supplies current to a different bank of memory. Hence, when a
signal such as VPX transitions from 5 volts to 10 volts in one
bank, the amount of capacitance the supply has to charge up is
significantly reduced since the individual VPX supply node is only
coupled to circuit devices in one bank, and not all four banks.
38. For simplicity, only BANK 0 320 is described in detail.
However, the description of BANK 0 320 also applies to BANK 1 340,
BANK 2 360, and BANK 3 380 since each bank of this embodiment are
identically constructed. Bank switch 322 couples high voltage
signals 308 to the X-decoders 326 of BANK 0 320. The high voltage
signals 324 dedicated to BANK 0 320 are provided from the bank
switch 322. The local signals 322 are switched versions of the
top-level high voltage signals 308. The X-decoders 326 connect
global wordlines 328 to supply voltages such as VPX based upon
selection logic. The global wordlines 328 typically extend along
the entire length of the bank 320. For this embodiment, the length
of the memory banks 320, 340, 360, 380 is the same of the length of
the planes 310, 315. The global wordlines 328 are coupled from the
X-decoders 326 to local block selects 330. The local block selects
330 of one embodiment serve as pass devices that couple the global
wordlines 328 and the local wordlines 332 together. The
architecture of one embodiment has the flash memory array further
divided into blocks. Block select signals turn on and off the block
selects of the appropriate block depending on which memory address
is being accessed.
39. Large areas of N-wells are located in the X-decoders and the
local block selects due to the number of P type transistors used in
those circuits. The embodiment of the invention can reduce the
charging current in the part. By dividing the memory array into
banks, the X-decoder N-wells are also divided into banks. Hence,
the amount of N-well capacitance that needs to be charged as the
high voltage nodes transition voltage potentials can be greatly
reduced. Thus, the input current during memory programming can also
be reduced. The voltage supply node can also slew faster since the
capacitance load has been reduced. As a result, program time may be
lower.
40. The method of one embodiment comprises segmenting capacitance
that has to charged during programming. The capacitance can be
segmented by dividing the memory array into banks, each with its
own set of X-decoders. Each bank is also supplied with its own set
of supply signals that are coupled to global signals depending on
switching logic. A dummy row can be inserted between the banks to
maintain continuity between the flash cells in the array.
41. FIG. 4 is a circuit diagram of an X-decoder cell 400. The
X-decoder cell 400 has a number of signals coupled to its circuit
devices including VPX 402, VPIX 404, and various select signals
406, 408, 410. VPX 402 and VPIX 404 are positive voltage supplies
for the X-decoder 400.
42. P type transistor T1 414 is coupled to VPIX 404 at its source
terminal. The gate of T1 414 is coupled to an "all wordlines" AWL
signal 403. In another embodiment, a ground potential can be
coupled to the gate of T1 414. The substrate of T1 414 is also
coupled to VPIX 404. N type transistors T2 416, T3 418, T4 420 are
coupled together in a series. The drain terminal of T2 416 is
coupled at node 430 to the drain terminal of T1 414, the gate
terminal of T5 432, and the gate terminal of T7 436. The source
terminal of T2 416 is coupled to the drain terminal of T3 418.
Similarly, the source terminal of T3 418 is coupled to the drain
terminal of T4 420. At one end of the transistor chain, the source
terminal of T4 420 is coupled to a ground potential. Select signals
SEL0 406, SEL1 408, and SEL2 410 are coupled to the gate terminals
of T2 416, T3 418, and T4 420, respectively. The select signals
406, 408, 410 control the discharge of node 430 by providing a path
to ground when T2 416, T3 418, and T4 420 are all turned on.
43. P type transistor T5 432 is coupled to VPX 402 at its source
terminal. The substrate terminal of T5 432 is also coupled to VPX
402. The drain terminal of T5 432 is coupled to the source terminal
of P type transistor T6 434. The node between the drain terminal of
T5 432 and the source terminal of T6 434 is also a global wordline
438. The gate terminal of T6 434 is coupled to the NDIS signal 412.
The N well of T6 434 is coupled to VPXNW. Drain terminal of N type
transistor T7 436 is coupled to the drain terminal of T6 434. The
source terminal of T7 436 is coupled to a ground potential.
44. T5 432 is the P driver to the global wordline 438. T7 436 is
the N driver to the global wordline 438. T6 434 serves as an
isolation device to prevent over-stress in the devices coupled
between VPX 402 and ground. T6 434 is used to prevent forward bias
of the drain to substrate junction of T7 436 during an erase
operation, because global wordline 438 is taken to a negative
voltage.
45. A block select signal 450 is coupled to the gate terminal of P
type transistor T8 440. T8 440 functions as a local block select
device. The source terminal of T8 440 is coupled to a global
wordline 438, while the drain terminal is coupled to a local
wordline 442. When a logic high on BLOCK SELECT 450 is applied to
the gate terminal of T8 440, T8 is turned on and the local wordline
442 is coupled to the global wordline 438. A logic low on BLOCK
SELECT 450 keeps T8 440 off. For one embodiment, the BLOCK SELECT
450 can have a negative voltage potential during read mode. The N
well of T8 440 is coupled to VPXNW.
46. Each X-decoder 400 drives a wordline of the memory array. For
one embodiment, both the VPX 402 and VPIX 404 supplies are 5 volts
during read mode and 10 volts during the program pulse. Every time
a word is programmed in the memory array, VPX 402 and VPIX 404 have
to be pumped from 5 volts to 10 volts. VPX 402 and VPIX 404
typically have a large amount of capacitance due to the number of
wordlines present in the array. For instance, the number of
X-decoders 400 for one embodiment of a flash array is 2048.
47. Each X-decoder cell 400 contributes a certain amount of
capacitance. The overall capacitance includes various components
such as N-well capacitance, gate capacitance, and diffusion
capacitance. The total VPX 402 and VPIX 404 capacitance for one
embodiment can be on the order of 500 picofarads to 1 nanofarad for
16 megabit and 32 megabit flash memory parts, respectively.
48. Raising the VPX 402 and VPIX 404 supply voltages from 5 volts
to 10 volts can comprise a significant portion of the total
programming current in some flash parts. For instance, the charge
in one embodiment is supplied from a charge pump that is powered
with a low voltage of typically 3 volts or 1.8 volts. The amount of
current necessary to charge VPX 402 and VPIX 404 from 5 volts to 10
volts during a program sequence can be determined by:
I.sub.PP=C*(V.sub.2-V.sub.1)/(T.sub.P*Pump Efficiency)
49. where C is the supply capacitance and T.sub.P is the program
time. V.sub.1 is the initial voltage potential and V.sub.2 is the
subsequent voltage. The charge required is divided by the program
time and pump efficiency. For example, if C is 800 picofarads and
T.sub.P is 20 microseconds and pump efficiency is 4% when the
supply is pumped from 5 volts to 10 volts, then I.sub.PP=(800
pF)*(10 V-5 V)/(20 .mu.s*0.04)=5 milliamps. At low voltage, the
necessary current is quite large.
50. Generally, a significant amount of time is required to charge
the VPX 402 and VPIX 404 voltage supplies. The time needed to
charge VPX 402 can be determined by:
T=C*(V.sub.2-V.sub.1)/I
51. where I is the pump output current and C is the capacitance on
VPX 402. V.sub.1 is the initial voltage potential and V.sub.2 is
the subsequent voltage. The time to slew is the charge divided by
the charge pump supply current. The charge pump current is
dependent on the pump size. If the pump output current is 1
milliamp and C is 800 picofarads when VPX 402 is pumped from 5
volts to 10 volts, then T=(800 pF)*(10 V-5 V)/1 mA=4 microseconds.
For one embodiment, 4 microseconds is approximately a quarter of
the program time.
52. In order to meet the power requirements during program, either
the charge pump has to be enlarged or the program time increased.
The tradeoff is between spreading the program current over a longer
time period versus die area. Current basically depends on the pump
size. But a charge pump has limited current capability, so the slew
time is also affected. A solution becomes more important when the
size of the X-decoders become larger and the associated capacitance
increases.
53. One embodiment of the invention divides the memory array into
four banks. Each bank comprises a set of X-decoders. However, the
X-decoder N wells are separated. Dummy rows are inserted between
the banks in the middle of each plane to separate the two banks on
each memory plane. Furthermore, the supply signals and decoding
signals are also divided from a global set into a separate set for
each bank.
54. Prior art designs routed each global signal to the circuits for
the entire array. Since the signals were global in nature, the N
wells for both planes were slewed up and down together no matter
where the chip was being programming.
55. FIG. 5 is a block diagram of a banked memory architecture 500.
Global signals 502 of the embodiment in FIG. 5 are generated from a
global X-path switch. The global signals 502 comprise of HHVPX,
HHVPIX, and HHVPXNW. The banked memory architecture 500 of FIG. 5
comprises of an array divided into four memory banks 550, 552, 554,
556. Each bank 550, 552, 554, 556 has its own X-path switch logic
510, 512, 514, 516 and set of X-decoder cells 530, 532, 534, 536.
Global signals 502 are coupled to the X-path switches 510, 512,
514, 516 of all four banks 550, 552, 554, 556.
56. The X-path switch logic controls whether the voltage potentials
from the global pumped signals 502 are coupled to the X-decoders
530, 532, 534, 536 in its corresponding bank. For one embodiment,
logic signals BK SEL0 504 and BK SEL1 506 are coupled to all the
X-path switches 510, 512, 514, 516. Logic signals BK SEL0 504 and
BK SEL1 506 control whether each bank's X-path switch 510, 512,
514, 516 is activated to couple global signals 502 to the bank's
local signals 520, 522, 524, 526. Each bank of X-path switches 510,
512, 514, 516 is coupled to its own set of local high voltage
signals 520, 522, 524, 526. For this embodiment, each local signal
has a corresponding global signal. For instance, global signal
HHVPX corresponds to local signals VPX of BANK 0 550, VPX1 of BANK
1 552, VPX2 of BANK 2 554, and VPX3 of BANK 3 556. Similarly,
global signal HHVPIX corresponds to local signals VPIX0 of BANK 0
550, VPIX1 of BANK 1 552, VPIX 2 of BANK 2 554, and VPIX 3 of BANK
3 556. Global signal HHVPXNW corresponds to local signals VPXNW0 of
BANK 0 550, VPXNW1 of BANK 1 552, VPXNW2 of BANK 2 554, and VPXNW3
of BANK 3 556.
57. The four memory banks 550, 552, 554, 556 of the present
embodiment are identically constructed. For illustrative purposes,
only BANK 0 550 is described in
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