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Claims  |
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What is claimed and desired to be secured by United States Letters Patent
is:
1. A control circuit for an array of memory cells which are arranged in
rows and columns and which are in communication with a microprocessor
through the use of a memory control module that provides control signals
to the control circuit and that sends a row-address and a column-address
of a selected memory cell to be communicated with by the microprocessor to
the control circuit, the control circuit comprising:
(a) internal logic means for responding to the memory control module in
order to generate an internal command that enables the array of memory
cells to produce and receive data;
(b) synchronizing means for coordinating the beginning of one of a
column-address decoding operation and a row-address decoding operation
through timing of the internal command, such that said one of a
column-address decoding operation and a row-address decoding operation
begins only between predetermined repeating intervals; and
(c) means for beginning the other of said one of a column-address decoding
operation and a row-address decoding operation independently of the
predetermined repeating intervals of the synchronizing means.
2. A control circuit for a memory array as recited in claim 1, wherein the
synchronizing means coordinates the row-address decoding operation, and
wherein the column-address decoding operation begins independently of the
predetermined repeating intervals of the synchronizing means, and further
comprising:
(a) an address bus communicating between the memory control module and the
means for beginning the other of said one of a column-address decoding
operation and a row-address decoding operation independently of the
predetermined repeating intervals of the synchronizing means, the address
bus alternately being provided with the column-address and the row-address
by the memory control module; and
(b) means for decoding the column-address, and wherein the means for
beginning the other of said one of a column-address decoding operation and
a row-address decoding operation independently of the predetermined
repeating intervals of the synchronizing means comprises means for sensing
the occurrence of a new column-address on the address bus and passing the
new column-address to the column-address decoding means in order that the
column-address decoding means independently of the predetermined repeating
intervals of the synchronizing means may begin decoding the new
column-address as soon as the new column-address appears on the address
bus.
3. A control circuit for a memory array as recited in claim 2, wherein the
means for sensing a new column-address comprises an automatic transition
detection circuit.
4. A control circuit for a memory array as recited in claim 3, wherein the
means for sensing the occurrence of a new column-address further comprises
a column-address latch in communication with the automatic transition
detection circuit.
5. A control circuit for a memory array as recited in claim 3, wherein the
means for beginning the other of said one of a column-address decoding
operation and a row-address decoding operation independently of the
predetermined repeating intervals of the synchronizing means further
comprises:
(a) a column-address latch separating the address bus from the
column-address decoding circuitry; and
(b) means for setting the column-address latch to a transparent state in
response to a control signal indicating that the row-address is being
placed on the address bus, the column-address latch being responsive to
the means for sensing the occurrence of a new column-address to latch in
the new column-address when in the transparent state.
6. A control circuit for a memory array column as recited in claim 1,
further comprising:
(a) means for decoding the column address;
(b) a post-decode latch located between the means for decoding the
column-address and the array of memory cells, the post-decode latch
prohibiting a decoded column-address from transferring from the means for
decoding the column address to the array of memory cells until the
post-decode latch is enabled; and
(c) a communication line extending between the internal logic means and the
post-decode latch for enabling the post-decode latch upon receipt from the
memory control module of a column-address strobe signal verifying the
presence of the column-address on the address bus.
7. A control circuit for a memory array as recited in claim 5, further
comprising a communication line between the internal logic means and the
column-address latch for resetting the column-address latch to be
non-transparent once a desired column-address has been received by the
column-address latch.
8. A control circuit for an array of memory cells which are arranged in
rows and columns and which are in communication with a microprocessor
through the use of a memory control module that provides control signals
to the control circuit and that sends a row-address and a column-address
of a selected memory cell to be communicated with by the microprocessor to
the control circuit, the control circuit comprising:
(a) means for decoding the column-address;
(b) an address bus communicating between the memory control module and the
means for decoding the column-address, the address bus alternately being
provided by the memory control module with the column-address and the
row-address;
(c) internal logic means for responding to the memory control module in
order to generate an internal command that enable the array of memory
cells to produce and receive data;
(d) synchronizing means for coordinating a row-address decoding operation,
the beginning of which is dictated by the internal command from the
internal logic means, such that the row-address decoding operation begins
only between predetermined repeating intervals; and
(e) means for sensing the occurrence of a new column-address on the address
bus in order that the means for decoding the column-address, independently
of the predetermined repeating intervals of the synchronizing means, may
begin decoding a new column-address as soon as the new column-address
appears on the address bus.
9. A control circuit for a memory array as recited in claim 8, further
comprising:
(a) a column-address latch separating the address bus from the means for
decoding the column-address; and
(b) means for setting the column-address latch to a transparent state in
response to a control signal indicating that the row-address is being
placed on the address bus, the decode latch being responsive to the means
for sensing the occurrence of a new column-address to latch in the new
column-address when in the transparent state.
10. A control circuit for a memory array as recited in claim 8, further
comprising:
(a) a post-decode latch located between the means for decoding the
column-address and the array of memory cells, the post-decode latch
prohibiting a decoded column-address from transferring from the means for
decoding the column-address to the array of memory cells until the
post-decode latch is enabled; and
(b) a communication line extending between the internal logic means and the
post-decode latch for enabling the post-decode latch when the internal
logic means has verified that the column-address is present on the address
bus and that data can be written to or received by the array of memory
cells.
11. A control circuit for a memory array as recited in claim 8, wherein the
means for sensing a new column-address comprises an automatic transition
detection circuit.
12. A control circuit for a memory array as recited in claim 11, wherein
the means for sensing the occurrence of a new column-address further
comprises a column-address latch in communication with the automatic
transition detection circuit.
13. A control circuit for a memory array as recited in claim 8, wherein the
means for decoding the column-address begins decoding the new
column-address before the synchronization of a column-address strobe
signal with the synchronizing means.
14. A control circuit for an array of memory cells which are arranged in
rows and columns and which are in communication with a microprocessor
through the use of a memory control module that provides control signals
to the control circuit and that sends a row-address and a column-address
of a selected memory cell to be communicated with by the microprocessor to
the control circuit, the control circuit comprising:
(a) a decoder circuit receiving address data from an address bus
communicating between the memory control module and the decoder circuit
and decoding the address data to provide an address to the array of memory
cells;
(b) an internal logic control circuit in communication with the memory
control module for generating internal control signals;
(c) a clock signal to which at least a substantial portion of said internal
control signals by the internal logic control circuitry are synchronized;
(d) a column-address latch between the address bus and the decoder circuit;
(e) a communication line between the internal logic control circuit and a
decode latch that provides a control signal to the decode latch for making
the decode latch transparent;
(f) an address transition detection circuit for detecting when a new
column-address is present on the address bus and for enabling the
column-address latch to receive the new column-address, such that the
decoder circuit can begin decoding the new column-address independently of
the clock signal;
(g) a post-decode latch located between the decoder circuit and the array
of memory cells, the post-decode latch prohibiting a decoded
column-address from transferring from the decoder circuit until the
decoder circuit is enabled; and
(h) a communication line extending between the internal logic control
circuit and the post-decode latch for enabling the post-decode latch when
the memory control module has verified that the new column-address is
present on the address bus.
15. A method of addressing a memory cell array, the method comprising:
(a) providing address bus lines;
(b) providing a memory circuit including a memory array;
(c) providing an external logic control for placing a separate column and
row-address in the address bus lines and for generating a RAS signal when
the row-address is stable and a CAS signal when the column-address is
stable;
(d) providing a synchronizing clock signal for synchronizing at least a
significant portion of all operations conducted in response to the
external logic control;
(e) providing a row-address decoding circuit for decoding the row-address
in response to the RAS signal;
(f) providing a column-address decoding circuitry and a column-address
latch previous to the column-address decoding circuitry for separating the
column-address decoding circuitry from the address bus lines, the
column-address latch being set to a transparent state in response to the
reception of the RAS signal;
(g) providing an address transition detection circuit for detecting when a
new column-address appears on the address bus line, wherein the address
transition detection circuit is in communication with the column-address
latch for enabling the column-address latch when a new column-address is
present on the bus lines;
(h) providing the row-address on the address bus lines and providing the
RAS signal to notify that the column-address is stable;
(i) decoding the row-address;
(j) providing a column-address on the address bus lines;
(k) sensing the presence of the column-address on the address bus lines
with the address transition detection circuit and passing the
column-address to the column-address decoding circuitry;
(l) decoding the column-address with the column-address decoding circuitry;
(m) providing a CAS signal to the column-address decoding circuitry that
the column-address is the desired column-address and is stable and should
be passed into the memory array;
(n) latching the column-address and allowing no new column-address to be
decoded by the column-address decoding circuitry until the occurrence of a
subsequent RAS signal; and
(o) conducting a data communication operation by the memory cell array.
16. A control circuit for a memory array as recited in claim 1, wherein the
synchronizing means coordinates the row-address decoding operation, and
wherein the column-address decoding operation begins independently of the
predetermined repeating intervals of the synchronizing means.
17. A control circuit for a memory array as recited in claim 2, wherein the
memory array is capable of operating in burst mode in which contiguous
column-addresses of the array of memory cells are accessed successively
with additional column-address decoding operations.
18. A control circuit for a memory array as recited in claim 18, wherein
the beginning of each additional column-address decoding operation is
coordinated by the synchronizing means when the memory array is operating
in burst mode.
19. A control circuit for a memory array as recited in claim 18, wherein
the memory array comprises an array of synchronously dynamically
refreshable random access memory cells. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to a control circuit and a related method for
improving the access time of synchronous DRAM memory. More particularly,
the present invention is directed toward decreasing the access time of
synchronous DRAM memory through the use of partially asynchronous
circuitry.
2. The Relevant Technology
Dynamically refreshable random access memory (DRAM) is currently highly
utilized for providing rapid data storage and retrieval in computerized
equipment at a reasonable cost. DRAM technology is evolving rapidly. One
recent emergence in the DRAM field is the use of synchronous operation of
the DRAM control circuitry.
FIGS. 1 through 3 illustrate the use of synchronous circuitry to control a
DRAM memory cell array. FIG. 1 is a functional block diagram depiction of
a synchronous DRAM circuit. Shown therein is an internal control logic
module 12, which receives control commands on pins numbered 14 through 26,
and which generates the internal controls for either reading data located
on pins denoted DQM through DQ8 into the memory bank or producing data
from the memory bank onto the pins DQM through DQ8. Typical DRAM addresses
are broken into two portions by an external logic control module (not
depicted). These two portions comprise a row address and a column-address
in order that a narrower bus width can be used. Also shown in FIG. 1 are
two paths for the row and column-addresses which are typically provided on
an internal address denoted by pins A0 through A10 by the memory control
module. In response to signals from internal control logic module 12, the
row address is routed through the row address decoding circuitry including
a row address latch 28, a row multiplexer 30, row address buffers 32, and
a row address decoder 34. Row multiplexer 30 is used only where more than
1 bank of memory cells is included. This allows for bank switching, which
is an improvement gained by the advance to synchronous DRAM. Typically,
two memory array banks, bank 0 and bank 1, are multiplexed by row
multiplexer 30 in response to the state of address line A10.
The column-address is routed through the column decoding circuitry,
including column-address latch 36, burst counter 38, column-address buffer
40, column decoder 42, and sense amplifiers I/O gating module 44. Data is
transferred to or from the memory array bank 10 from the data bus,
comprising pins DQM through DQ8, through either a data in buffer 46 or a
data out buffer 48, as well as a latch 50 and sense amplifiers I/O gating
44. The signals are synchronized with the number 1 clock generator 52 and
the number 2 clock generator 54. Mode register 56 is for setting up the
memory array bank and control module in one of a predetermined number of
functional modes. The refresh circuitry including refresh controller 58,
refresh counter 60, and self refresh oscillator and timer 62 provide the
dynamic refresh functions necessary at regular intervals to maintain the
data voltage level in memory array bank 10.
FIG. 2 is a flow chart describing the operation of the synchronous DRAM and
control circuitry of FIG. 1 during a read operation. The process described
is a read operation and is discussed for illustration purposes only, as
synchronous DRAM read and write operations are commonly known in the art.
The first step, denoted at block 60, is the issuance of a read command by
the microprocessor or other circuitry which is utilizing memory array bank
10 for storage of data. Throughout this document, generalized names will
be given to signals which may also be known by other names. For instance,
the read command may also be known as the "memory access command." These
commands will be recognizable by those skilled in the art.
The read command is typically received by the memory control module which
typically comprises a PC decoding chip set. In the next step, denoted at
block 62, the address is issued by the microprocessor onto the address bus
lines which communicate between the microprocessor and the memory control
module. This address is received by the decoder chip set, which divides
the address into two portions. In block 64, the row address portion is
transmitted to the memory module of FIG. 1 on the internal address bus
denoted by pins A0 through A10 on FIG. 1. Thereafter, the decoder chip set
issues a RAS signal to the internal control logic 12 of the memory module.
This alerts the control logic circuitry that a stable address is present
on pins A0 through A10. Once again, the term "RAS signal" is a generic
term taken from standard DRAM terminology.
In actual operation, a certain combination of signals on pins 18-24 of FIG.
1 are given, and may otherwise be known as the "bank active command."
Control logic circuit 12 then issues the appropriate commands to row
address latch 28 and row multiplexer 30, such that the row address can be
entered into row address buffers 32 and row decoder 34, and decoded as
denoted in block 70.
Thereafter, the proper row address passes into memory array bank 10, and
the row is selected as denoted in block 72. Next, the memory control
module issues the column-address to the memory module of FIG. 1 where it
will be present on pins A0 through A10. This is denoted in block 74. In
block 76 it is further denoted that the decoder chip set then issues a CAS
signal to control logic 12 to alert it that a stable column-address is
present on pins A0 through A10.
The CAS signal is also a generalized term denoting a specific combination
of signals on pins 18 through 24 of FIG. 1, and may otherwise be termed
the "read/write signal." The CAS signal must wait a certain amount of time
for setup and hold the row-address to the column-address, which is
typically about 20 ns and is denoted t.sub.RCD. t.sub.RCD is further
lengthened by the necessity of waiting for the occurrence of a
synchronizing clock signal, often increasing the delay up to 30 ns.
Thereafter, control logic circuit 12 generates the internal signals to
column-address latch 36 such that the column-address passes through burst
counter 38 and into column-address buffer 40 where it is then decoded by
column decoder 42. This is depicted by block 80. Once decoded, the
column-address passes into memory array bank 10, and the column is
selected as depicted in block 82. Once the column has been selected,
memory array bank 10 places the requested data on data bus lines DQM
through DQ8 through data-out buffer 48, as denoted in block 84. This
completes the first read operation. In burst mode, the circuitry will
automatically thereafter load a series of adjacently addressed data onto
the data bus.
FIG. 3 is a timing diagram depicting the timing of the above-discussed
first read operation depicted in FIG. 2. The timing diagram shows the
procedure for reading a double burst of information, Dout.sub.m and
Dout.sub.M+1 stored in memory array bank 10. Thus, the memory array module
is operating in burst mode with a burst of 2. In burst mode, a specified
number of addresses will be written in sequence, wherein the addresses are
located in memory locations having the same row-address, and having
column-addresses varying as M and M+n, wherein n is the specific number of
addresses set up in mode register 56 seen in FIG. 1 by a command code at
the power-up stage to burst at every memory access.
FIG. 3 shows that the memory module of FIG. 1 completes the first read
operation denoted by Dout on line DQ, which is the data line, in four
clock cycles. The timing diagram of FIG. 3 shows the sequence of the read
command from the time the system is enabled, denoted by a high signal
level on signal CKE. The sequence comprises the command line entering an
active state, while at the same time the row-address is placed on pins A0
through A9. Thereafter, there is a delay while the command line is in the
no operation mode, and while the row-address decoder is decoding the
row-address. Next, during clock cycle T1, and after delay t.sub.RCD,
discussed above, the column-address is placed on pins A0 through A9. A
read command is issued, which corresponds to the issuance of the CAS
signal. Following the read command, there is another delay, denoted by no
operation on the command line, and denoted with the delay time t.sub.AA,
while the column-address is being decoded. Next, the requested data is
presented on line DQ and the first read operation is completed.
Afterwards, the further burst mode read operations denoted for the first
burst Dout.sub.m+1, are conducted.
Synchronous DRAM is a new and emerging technology that is still being
improved upon rapidly. Advantages of synchronous DRAM technology are that
it is more accurate, with a reduced tendency to misfire from noise on the
control lines. Furthermore, synchronous DRAMs are capable of burst
addressing and bank switching, as discussed above, to achieve very high
speeds. High speed is the key desired trait in the movement to develop
improved memory devices. Nevertheless, synchronous DRAM achieves this
higher speed at the sacrifice of certain desirable functions of
traditional DRAM technology. For instance, synchronous DRAM is presently
incapable of fast page mode addressing. Using fast page mode, current
asynchronous DRAM can begin column-addressing as soon as a new
column-address is present on the column-address bus lines without waiting
for a CAS signal and a concurrent synchronizing clock signal. This allows
for a faster t.sub.AA time, the time from when a stable column-address is
present on the internal address bus lines until the read or write
operation is completed.
Asynchronous DRAM technology typically uses automatic transition detection
(ATD) to detect when the new column is present so that column-address
decoding can begin immediately thereafter. Using ATD in fast page mode in
this manner, multiple reads and writes can be achieved one after the other
in a pseudo-burst mode. Synchronous DRAM, on the other hand, is tied to
the clock and is incapable of performing such a function. Thus, often a
whole clock cycle is lost waiting for the column-address strobe (CAS) to
signal the presence of a desired stable column-address after delay
t.sub.RCD and for CAS to synchronize with the clock so that the decoding
of the column-address can begin.
Thus, it becomes apparent that there is a need for a method of improving
access times of synchronous DRAM memory to overcome delays, such as the
delay between the stable presence of a column-address on the address lines
and the generation of a column access strobe signal from the decoder
circuitry. Such a step has not been taken in the art, presumably because
it would appear to be a step back in the advancement of DRAM technology,
which has recently migrated from asynchronous DRAM to sychronous DRAM
control, to go back to partially asynchronous DRAM. This is especially the
case, as the more efficient burst mode of synchronous DRAM has made the
pseudo-burst mode of fast page mode obsolete. From the above discussion,
however, it can be seen that it would be a great improvement in
synchronous DRAM technology to take an apparent step back and incorporate
the asynchronous column-addressing capability of traditional DRAM to the
newer synchronous DRAM technology.
SUMMARY AND OBJECTS OF THE INVENTION
The present invention seeks to resolve the above and other problems which
have been experienced in the art. More particularly, the present invention
constitutes an advancement in the art by providing a synchronous DRAM
memory module and control circuitry with partially asynchronous address
decoding, which achieves each of the objects listed below.
It is an object of the present invention to provide a memory module which
retains the advantages gained by the advancement into synchronized DRAM
technology, while also benefitting from the beneficial aspects of fast
page mode of asynchronous DRAM technology, which was previously lost by
the move to synchronous technology.
It is also an object of the present invention to provide a synchronous DRAM
memory module with partially asynchronous operation, whereby the
column-address decoding can begin immediately upon the presence of a
stable column-address on the address bus lines and without waiting for the
column-address strobe to synchronize with the rising or failing edge of
the synchronizing clock signal.
It is another object of the present invention to provide such a synchronous
DRAM memory module with partially asynchronous column decoding
capabilities, whereby the presence of the stable column-address is
detected by an address transition detection circuit.
It is a farther object of the present invention to provide such a
synchronous DRAM memory module with partially asynchronous decoding
circuitry, whereby the column-address is fully finished with decoding upon
the occurrence of the column-address strobe signal, such that the
column-address can be immediately input into the memory module at that
time, and thus speeding up the read or write process by up to a full clock
cycle.
It is likewise an object of the present invention to provide such a
synchronous DRAM with partially asynchronous decoding circuitry with
arbitrating signals from the internal control logic circuit, whereby the
column-address is latched into the column-address decoding circuitry upon
the notification of a stable column-address, and whereby the decoded
column-address is allowed into the memory module upon receipt of that
signal.
To achieve the foregoing objects, and in accordance with the invention as
embodied and described herein, the present invention comprises a
synchronous DRAM memory module with decoding circuitry having asynchronous
column decoding capability. The present invention comprises means for
beginning the decoding of the column-address prior to the synchronization
of the column-address strobe and the synchronizing clock signal. This is
accomplished with means for sensing the occurrence of a new column-address
on the address lines in order that the column-address decoding circuitry
may begin decoding a new column-address as soon as a new column-address
appears on the address bus lines, independently of the synchronizing
means.
In one embodiment, the means for beginning the decoding of the
column-address independent of the synchronizing of the column-address
strobe and the synchronizing clock signal comprises an address transition
detection circuit. The address transition detection circuit allows the
column-address to be sent to the column-address decoder immediately upon
the arrival of a stable column-address on the address bus lines of the
memory module. At the arrival thereafter of the column-address strobe
signifying that the column-address is final and is desired to be entered
into the memory array bank, the column-address will be already decoded and
can be immediately transferred into the memory array bank.
In a typical read process using the present invention, the following steps
will occur. First, the read command is issued by the microprocessor. The
read command is typically received by a memory control module, which in
current microcomputers, presently the largest user of DRAM memory,
comprises a PC decoding chip set. Thereafter, the address is issued on the
external address bus lines and is transferred to the memory control
module. The memory control module then breaks the address into two
portion, a row-address portion and a column-address portion, and issues
the row-address portion to the DRAM memory module.
Next the memory control module issues a row-address strobe signal (RAS) to
denote the presence of a stable row-address on the address bus lines of
the DRAM memory module. Upon the issuance of RAS, the column-address latch
is set to be transparent. That is, it is set to allow new addresses to
flow through it upon notification by an address transition circuit of the
presence of a new address. At the same time, the row-address is latched
into the row-address latch. Then, the row-address is decoded and the row
is selected. The memory control module then issues the column-address to
the DRAM memory module.
Immediately upon the presence of a new column-address on the address bus
lines of the DRAM memory module, the address transition circuit detects
the presence and alerts the column-address latch. The column-address latch
allows the address in, and the column-address decoder then begins decoding
the column-address. Thereafter, when the memory control module finally
issues a column-address strobe signal (CAS), the column-address is already
fully decoded and can be sent immediately to the memory array bank. The
occurrence of CAS also latches the post decode latch, which allows the
decoded column-address to pass into the memory array bank. The column is
then selected, and the desired data is transferred from the memory array
bank to the data-out buffer and the data bus lines. This completes the
read operation. Write operations are similar, with the exception that data
is transferred into the memory array bank after the column-address passes
into the memory array bank through a data-in buffer.
With the use of the present invention, the data will be present on the data
out lines up to a full clock cycle earlier. This is because the delay
between the presence of a stable column-address and the appearance of data
on the data bus is shortened because the decoder now does not need to wait
until the occurrence of CAS to begin decoding.
The present invention can be used with multiple memory array bank switching
and can be operated in burst mode, wherein the memory is set to decode a
series of column-addresses, column.sub.m to column.sub.m+n, where n is the
preselected number of addresses to be decoded. Thus, the present invention
recoups previously lost advantages of asynchronous DRAM and retains other
benefits gained by the advancement to synchronous DRAM technology.
Thus, it can be seen that the present invention, while retaining the
benefits of the high speed of synchronous DRAM technology, also
incorporates the benefits of asynchronous DRAM technology and in
particular, the fast page mode. As a consequence, in many cases, a full
clock cycle can be omitted from the read and write operation, since
decoding can begin earlier than the occurrence of the column-address
strobe and its synchronization with the internal synchronous clock. In 66
MHz machines for instance, this will result in an increase for every read
and write operation of approximately 15 nanoseconds. This is a significant
improvement in the access time of these devices, which typically require
about 60 nanoseconds for the entire first data out operation.
These and other objects and features of the present invention will become
more fully apparent from the following description and appended claims, or
may be learned by the practice of the invention as set forth hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the manner in which the above-recited and other advantages
and objects of the invention are obtained, a more particular description
of the invention briefly described above will be rendered by reference to
a specific embodiment thereof which is illustrated in the appended
drawings. Understanding that these drawings depict only a typical
embodiment of the invention and are not therefore to be considered to be
limiting of its scope, the invention will be described and explained with
additional specificity and detail through the use of the accompanying
drawings in which:
FIG. 1 is a functional block diagram depicting a synchronous DRAM memory
array and control logic module of the prior art.
FIG. 2 is a flow chart depicting a typical read operation of the memory
array and control logic module of FIG. 1.
FIG. 3 is a timing diagram showing the delays inherent in the read
operation of the flow chart of FIG. 2 and the functional block diagram of
FIG. 1.
FIG. 4 is a functional block diagram of the synchronous DRAM memory with
asynchronous column decoding of the present invention.
FIG. 5 is a block diagram delineating the steps of a read operation of
synchronous DRAM memory with asynchronous column decoding of the present
invention which is depicted by the functional block diagram of FIG. 4.
FIG. 6 is a timing diagram showing the timing of the read function outlined
in FIG. 5 for the circuitry of the present invention illustrated by the
functional block diagram of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention comprises a synchronous DRAM memory array and method
of addressing and controlling the memory array in a partially asynchronous
manner. The results of this invention are decreased access times of the
DRAM memory module for read and write operations. Thus, in accordance with
the present invention, it has been found that beneficial characteristics
of synchronous DRAM can be retained while recouping certain favorable
characteristics of asynchronous DRAMs in the same memory array that were
formerly lost to the synchronous DRAM technology.
In order to control the memory array in a partially asychronous manner, a
means for beginning the decoding of the column-address independently of
the synchronizing clock signal is used. This further comprises, in a
preferred embodiment, means for sensing the occurrence of a new
column-address on the memory address lines in order that column-address
decoding circuitry may begin decoding a new column-address as soon as the
new column-address appears on the address bus lines, independently of the
synchronizing means.
In one embodiment of the present invention, illustrated in FIG. 6, the
means for beginning the deeming of the column-address comprises circuitry
that begins the decoding of the column-address as soon as the column is
stable on the address bus lines. This is done asynchronously of the clock,
and in most instances will improve the access time for read and write
commands by a full clock cycle. Shown in FIG. 4 is memory array bank 10 in
communication with data bus DQM-DQ8 and with the column and row decoding
circuitry, together with control logic module 12 for controlling the
decoding circuitry, as discussed above with respect to FIG. 1. Also shown
are address bus lines A0 through A10. As also discussed, these are
connected to row-address latch 28, then to row multiplexer 30, row-address
buffer 32, row decoder 34, and then to memory array bank 10. They are also
connected to column-address latch 36, then to burst counter 38,
column-address buffer 40, column decoder 42, sense amplifiers and I/O
gating 44, and finally to the memory array bank 10.
New additions, in accordance with the present invention, comprise a post
decode latch 86, with a control line 89 for communication with the
column-address strobe command generated in control logic circuit 12. Also
added to accomplish the present invention is an address transition
detection circuit 88 for notifying column-address latch 36 of the presence
of a new address on address bus lines A0-A10. Using the circuitry of FIG.
4, the circuit designer can be given the choice of operating in standard
synchronous DRAM mode, as illustrated in the flow chart of FIG. 2, or of
operating in partially asynchronous mode, illustrated in the flow chart of
FIG. 5.
Shown in FIG. 5 is a flow chart of the partially asynchronous operation of
the circuitry of FIG. 4. Therein, block 90 denotes the generation by the
microprocessor of a read or write command. For illustration purposes, only
the read command is being shown and discussed, as it will be clear from
the discussion to one skilled in the art how to thereby accomplish a write
command under the present invention. After the read command is issued, a
valid address is placed on the external address bus line, as denoted in
block 92. The read command and the address to be read to are both received
by the decoding chip set. The decoding chip set then breaks the address in
two portions, one being the row-address and the second portion being the
column-address. This is typically done in order to fit a large number of
addresses on a limited number of bus lines, as also discussed above. The
row-address is then sent out over address bus lines A0-A10, as denoted in
block 94.
After the foregoing, as denoted in block 96, a row-address strobe (RAS)
signal is sent to row-address latch 28. The row-address is then latched
into row-address latch 28, as denoted in block 100, and then passes into
row-address decoder 34, where it is decoded, as denoted in block 102. It
is then passed onto memory array 10 and the row is selected as denoted at
block 104.
Concurrently, under the present invention, column-address latch 36 also
receives the RAS signal, and in response becomes transparent. That is,
after the row-address is latched, it allows any new addresses to pass
through into the decoder circuitry upon notification by address transition
detection circuit 88. Address detection transition circuit 88 thereafter
detects the presence of a new address subsequent to the row-address latch
and every new address that thereafter appears on address bus lines A0-A10,
as denoted in block 108, and notifies column-address latch 36, which then
latches in the column-address. This occurs upon the issuance of a
column-address, as denoted at block 106.
Thus, the column decoding circuit begins decoding the new address every
time a new address appears, which is done asynchronously of the clock
signal. When the column-address strobe (CAS) does arrive, as denoted in
block 110, the column-address will have anticipated it and will be already
completely decoded. Each new column-address will be decoded immediately
after it is present on the address lines and undesired column-addresses
will be discarded, while desired column-addresses are input into the
memory array bank immediately upon the presence of the column-address
strobe which denotes that the column-address is final.
In one case, where the frequency of the synchronizing clock is 66 MHz, a
clock cycle can be eliminated for every read or write command, saving
approximately 15 nanoseconds. Upon the arrival of CAS, the decoded address
is latched into the memory array as denoted in block 114, after which the
memory array will produce the dam from the designated memory location and
transfer it onto the data bus through dam-out buffer 48, as denoted at
block 118. Alternatively, if a write command is being performed, the
memory array will receive the data on the data bus and transfer it into
data-in buffer 46, and thence into the designated memory location of
memory array bank 10. Multiple sequential addresses | | |
