Title: Apparatus and method of error detection and correction in a radiation-hardened static random access memory field-programmable gate array
Abstract: The present system comprises a radiation tolerant programmable logic device having logic modules and routing resources coupling together the logic modules. Configuration data lines providing configuration data control the programming of the logic modules and the routing resources. Error correction circuitry coupled to the configuration data lines analyzes and corrects any errors in the configuration data that may occur due to a single event upset (SEU). The present system also comprises a method for correcting errors in a programmable logic device having configuration data to program the programmable logic device. The method comprises a background reading of the configuration data. Next, the configuration data is analyzed for errors. Finally, the configuration data is then corrected and the configuration data is rewritten if errors are located.
Patent Number: 6,838,899 Issued on 01/04/2005 to Plants
| Inventors:
|
Plants; William C. (Sunnyvale, CA)
|
| Assignee:
|
Actel Corporation (Mountain View, CA)
|
| Appl. No.:
|
335234 |
| Filed:
|
December 30, 2002 |
| Current U.S. Class: |
326/9; 326/10; 326/11; 326/38; 326/39 |
| Intern'l Class: |
H03K 019/003 |
| Field of Search: |
326/9-12,16,38,39,14
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Tran; Anh Q.
Attorney, Agent or Firm: Sierra Patent Group, Ltd.
Claims
What is claimed is:
1. A radiation tolerant programmable logic device comprising:
logic modules;
routing resources coupling said logic modules;
configuration data lines having configuration data controlling said logic
modules and said routing resources; and
error correction circuitry coupled to said configuration data lines,
wherein said error correction circuitry analyzes and corrects errors in
said configuration data.
2. A radiation tolerant programmable logic device according to claim 1
wherein said error correction circuitry further comprises:
a radiation tolerant read/write amplifier; and
a radiation tolerant shift register coupled to said radiation tolerant
read/write amplifier.
3. A radiation tolerant programmable logic device according to claim 2
wherein said wherein said error correction circuitry further comprises:
a voting block coupled between said radiation tolerant read/write amplifier
and said configuration data line.
4. A radiation tolerant programmable logic device according to claim 3
wherein said wherein said error correction circuitry further comprises a
plurality of latches having at least one transistor large enough to
withstand a Qcnt or larger particle strike, said latches coupled to said
voting block.
5. A radiation tolerant programmable logic device according to claim 1
wherein said wherein said error correction circuitry further comprises:
a plurality of electronic code correction (ECC) latches each having a
plurality of inputs and an output;
a first multiplexer having an input and a plurality of outputs;
a voting logic circuit having inputs coupled to at least one output of said
plurality of ECC latches and coupled to at least one of said outputs of
said multiplexer and said voting logic circuit having an output coupled to
first input of a two-input multiplexer;
a radiation tolerant shift register coupled to one of said plurality of
outputs of said first multiplexer and an output coupled to a second input
of said two-input multiplexer;
a write latch having an input coupled to an output of said two-input
multiplexer; and
a read and write amplifier having an input coupled to an output of said
write latch and at lease one of said plurality of outputs of said first
multiplexer.
6. A radiation tolerant programmable logic device according to claim 5,
wherein said voting logic circuit further comprises:
three two-input AND gates; and
a three-input AND gate having inputs coupled to an output of each of said
three two-input AND gates.
7. A radiation tolerant programmable logic device according to claim 5,
wherein said read and write amplifier further comprises:
a write amplifier circuit having a plurality of inputs and a plurality of
outputs;
a precharge circuit coupled to said input of said write amplifier circuit
and having a plurality of outputs; and
a sense circuit having inputs coupled to said outputs of said write
amplifier circuit and said outputs of said precharge circuit and having an
output.
8. A radiation tolerant programmable logic device according to claim 5,
wherein said ECC latches further comprise:
a first transistor of a first nominal size having an source coupled to a
first node forming an input to said ECC latch, a drain coupled to a second
node and a gate coupled to a logic module signal line;
a second transistor of a nominal size having an source coupled to said
first node, a drain coupled to said second node and a gate coupled to an
inverted logic module signal line;
a first two-input NAND gate having a first input coupled to said second
node and a second input coupled to a reset signal line;
a second two-input NAND gate having a first input coupled to an output of
first two-input NAND gate and a second input coupled to a set signal line
and having an output to said output of said ECC latch;
a third transistor of second larger size having a source coupled to a third
node, a drain coupled to a fourth node and a gate coupled to a second
logic module signal line; and
a fourth transistor of second larger size having a source coupled to said
third node, said third coupled to said output of said second two-input
NAND gate, a drain coupled to said forth node, said fourth node coupled to
said first input of said first two-input NAND gate, and a gate coupled to
a second inverted logic module signal line.
9. A method for correcting errors in a programmable logic device having
configuration data to program the programmable logic device, said method
comprising:
background reading of the configuration data;
analyzing said configuration data for errors;
correcting said configuration data if errors are located; and
re-writing said configuration data if errors are located.
10. The method of claim 9 where said analyzing of said configuration data
for errors is accomplished with voting logic.
Description
BACKGROUND
1. Field of the System
The present invention relates to integrated circuits. In particular, the
present invention relates to a method for error detection and correction
in a radiation tolerant static random access memory (SRAM) for a field
programmable gate array (FPGA).
2. Background
A major concern in building a radiation-hardened SRAM-based FPGA for a
space application is the reliability of the configuration memory. Memory
devices used in satellites and in other computer equipment, can be placed
in environments that are highly susceptible to radiation. A satellite
memory cell in a space environment can be exposed to a radiation-induced
soft error, commonly called a single event upset (SEU), when a cell is
struck by high-energy particles. Electron-hole pairs created by, and along
the path of, a single energetic particle as it passes through an
integrated circuit such as a memory typically cause a soft error or SEU.
An SEU typically results from alpha particles (helium nuclei), beta
particles or other ionized nuclei rays impacting a low-capacitance node of
a semiconductor circuit. Should the energetic particle generate the
critical charge in the critical volume of the memory cell, the logic state
of the memory is upset. This critical charge, by definition, is the
minimum amount of electrical charge required to change the logic state of
the memory cell. It is commonly called Q Critical (Q.sub.crit).
An SEU can change the contents of any volatile memory cell. If that bit of
memory is doing something besides merely storing data, such as controlling
the logic functionality of an FPGA, the results can be catastrophic. While
other technologies may be better suited for the most sensitive control
functions of a spacecraft, there is a significant advantage to be had by
being able to change a portion of the spacecraft's functionality remotely,
either during prototyping on the ground or later during the mission.
Spacecraft designers accept the idea that SEUs will inevitably occur.
Based on the inevitable, they are willing to use SRAM-based FPGAs in
non-critical portions of the vehicle provided the error rate is
reasonable, sufficient error trapping is available and the recovery time
is reasonable.
When a heavy ion traverses a node within a memory storage cell, the ion can
force the node from its original state to an opposite state for a period
of time. This change of state is due to the charge that the heavy ion
deposits as it passes through the silicon of the Metal Oxide Semiconductor
(MOS) transistor of the memory cell. If this node is held in the opposite
state for a period of time longer than the delay around the feed back loop
of the memory cell, the cell can switch states and the stored data can be
lost. The period of time the node is held in the opposite state can depend
on several factors. The most critical being the charge deposited.
FIG. 1 a is a simplified schematic diagram of a logic gate 104. Logic gate
102 comprises a p-channel transistor 102 and an n-channel transistor 100.
P-channel transistor has a source coupled to Vcc, a drain coupled to node
Q 105 and a gate coupled to node QB 106. N-channel transistor 100 has a
source coupled to ground, a drain coupled to Q node 105 and a gate coupled
to QB node 106.
FIG. 1b is an illustration of a charged particle strike on transistor 100.
Transistor 100 comprises a drain 160, a source 165 and a gate 162. Gate
oxide 163 separates gate 162, drain 160 and source 161. As shown in FIG.
1b the drain 160 is being struck by the charged particle (ion) 110 along
the strike path 180. When the charged particle 110 tears though a
semiconductor transistor 100 (potentially at relative velocities of 10,000
miles per hour or more), it leaves a wake of hole and electron pairs 120
behind. If it strikes the output diffusion of a Complementary Metal Oxide
Semiconductor (CMOS) logic gate 104, as illustrated in FIG. 1a, all of
those carriers are available as drift current 130 if an electric field is
present. If no electric field is present then the drift current 130
ultimately diffuses. If the output of the CMOS gate is not at the voltage
of the surrounding material of the diffusion that is struck (for example,
N+ diffusion 160 at Vcc in a P-substrate 190 at ground), then such an
electric field exists and the current will pull that diffusion towards the
voltage of the P-substrate 190. Problems occur from a strike to the N+
diffusion 140 of a gate 162 is driven to Logic-1 or the P+ diffusion of a
gate is driven to Logic-0.
In such an occurrence, there are two sources of current vying for control
of the node Q: the CMOS p-channel device 102 (shown in FIG. 1a) that
originally drove the node to the correct logic level and the pool of
charge in the so-called "field funnel" 150 in FIG. 1b. The larger current
controls the node. If the strength of p-channel device 102 is large
relative to the available drift current 130, then the node will barely
move. If the strength of p-channel device 102 is small relative to the
energy strike, then the drift current 130 in FIG. 1b controls. Drift
current 130 controls until all its charge dissipates, at which time the
CMOS device can restore the node to the correct value.
Unfortunately, it takes time for a small CMOS device to regain control
against a high-energy strike. In the case, for example, of a victimized
gate being part of the feedback path in a sequential (i.e. memory) element
with the incorrect logic level propagating around the loop, the CMOS
device gets shut off and is never able to make the needed correction and
the memory element loses state. If the memory element controls something
important, system or subsystem failure can result.
FIG. 2a is a simplified schematic diagram illustrating a particle strike on
cross-coupled transistors. Transistors 102a, 102b, 100a and 100b are
identical to two logic gates as shown as one logic gate 104 in FIG. 1a. In
FIG. 2a, particle strike 210 is shown hitting the N+ region of n-channel
transistor 100a. FIG. 2b illustrates the waveform associated with this
strike.
FIG. 2b is a diagram depicting the waveforms 200 associated with a particle
strike 210. The particular case shown is for a particle not quite capable
of producing the critical charge required to flip the latch. At time T1,
the particle hits and then node Q drops from its equilibrium value of Vcc
very quickly due to the drift in the field funnel. Meanwhile, transistor
102 feeding node Q pumps current into node Q at T2, when all the charge in
the field funnel 150 in FIG. 1b is exhausted, node Q quickly returns to
its original equilibrium value of Vcc. Since the case depicted is close to
the maximum amount of charge that the cell can withstand, the voltage on
node Q approaches the trip point 230 at V.sub.trip. If the charged
particle had created substantially more charge carriers than the
transistor could have overcome, then node Q would have dropped to ground
potential and the latch would have flipped into the opposite state
permanently.
SRAM in an FPGA may also be specified as CSRAM or USRAM. CSRAM is
Configuration SRAM. This CSRAM is used to hold the configuration bits for
the FPGA. It is physically spread out over the entire die and is
interspersed with the rest of the FPGA circuitry. At least one of the two
nodes in the static latch that make up the SRAM cell can be connected to
the FPGA circuitry that controls it. When the contents of the CSRAM
change, the logic function implemented by the FPGA changes. What is needed
is a solution to insure the data integrity is maintained.
USRAM is the abbreviation for user SRAM. This is memory that is part of a
user logic design and is concentrated inside a functional block dedicated
to the purpose. What is needed is a solution to insure the data integrity
of an USRAM is maintained.
In an SRAM based FPGA, there are a variety of separate elements that go
into the making of a useful product. There are configuration memory bits
in the CSRAM, which allow the user to impose his/her design on the
uncommitted resources available. There are the combinational and
sequential modules that do the user's logic. There are the configurable
switches, signal lines, and buffers that allow the modules to be connected
together. There are support circuits like clocks and other global signals
like enables and resets, which allow the building of one or more
subsystems in different time domains. There are blocks like the SRAM and
DLL that allow the user access to more highly integrated functions than
can be built out of an array of logic modules and interconnect.
Hence, there is a need for an apparatus and method of providing error
detection and correction in a radiation-hardened SRAM based FPGA, which
can easily be implemented using conventional CMOS processes, and which has
performance speed comparable to an SRAM based FPGA that has not been
radiation-hardened.
SUMMARY OF THE INVENTION
The present system comprises a radiation tolerant programmable logic device
having logic modules and routing resources coupling together the logic
modules. Configuration data lines providing configuration data control the
programming of the logic modules and the routing resources. Error
correction circuitry coupled to the configuration data lines analyzes and
corrects any errors in the configuration data that may occur due to a
single event upset (SEU).
The present system also comprises a method for correcting errors in a
programmable logic device having configuration data to program the
programmable logic device. The method comprises a background reading of
the configuration data. Next, the configuration data is analyzed for
errors. Finally, the configuration data is then corrected and the
configuration data is rewritten if errors are located.
A better understanding of the features and advantages of the present
invention will be obtained by reference to the following detailed
description of the invention and accompanying drawings which set forth an
illustrative embodiment in which the principles of the invention are
utilized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a simplified schematic diagram of a logic gate.
FIG. 1b is an illustration of a charged particle strike though a
semiconductor and creating a wake of hole and electron pairs.
FIG. 2a is a simplified schematic diagram illustrating a particle strike on
cross-coupled transistors.
FIG. 2b is the waveform associated with a particle not quite capable of
producing the critical charge required to flip a latch.
FIG. 3 is a simplified schematic diagram illustrating SRAM memory
architecture having radiation tolerant reading and writing circuits as
disclosed in the present invention.
FIG. 4 shows a simplified block diagram of CSRAM interface circuit.
FIG. 5 is a simplified schematic diagram of a radiation hardened latch
shown as ECC latches and write latch in FIG. 4.
FIG. 6 illustrates the logic diagram and truth table for the majority of
three voting circuit of FIG. 4.
FIG. 7 is a simplified block diagram of a field programmable gate array
(FPGA) core within an integrated circuit having multiple core tiles.
FIG. 8 is a simplified schematic diagram illustrating an FPGA core having
only one core tile.
FIG. 9 is a simplified block diagram illustrating one embodiment of the
placement of EEC check bits in a FPGA core having two FPGA tiles.
FIG. 10 is a simplified schematic diagram illustrating in greater detail
the radiation tolerant read and write amplifiers for configuration static
random access as shown in FIG. 4.
FIG. 11 is a simplified schematic diagram illustrating in greater detail
the radiation tolerant read and write amplifiers as shown in FIG. 3 for
user static random access (USRAM).
FIG. 12 is a simplified block diagram illustrating the USRAM circuit with
the electronic correction code circuitry of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiment of the invention is discussed in detail below.
While specific implementations are discussed, it should be understood that
this is done for illustration purposes only. A person skilled in the
relevant art will recognize that other components and configurations may
be used without parting from the spirit and scope of the invention.
In this disclosure, various circuits and logical functions are described.
It is to be understood that designations such as "1" and "0" in these
descriptions are arbitrary logical designations. In a first implementation
of the invention, "1" may correspond to a voltage high, while "0"
corresponds to a voltage low or ground, while in a second implementation,
"0" may correspond to a voltage high, while "1" corresponds to a voltage
low or ground. Likewise, where signals are described, a "signal" as used
in this disclosure may represent the application, or pulling "high" of a
voltage to a node in a circuit where there was low or no voltage before,
or it may represent the termination, or the bringing "low" of a voltage to
the node, depending on the particular implementation of the invention.
The disclosed invention relates to a method for designing a
radiation-hardened FPGA and the required circuit designs for conversion
from a commercial Static Random Access Memory (SRAM) based Field
Programmable Gate Array (FPGA) to a radiation-hardened version. The
radiation-hardened FPGA described herein greatly reduces the (Single Event
Upset) SEU issues associated with prior-art devices.
Since radiation-hardened circuits tend to be rather large relative to their
non-radiation hardened counterparts, making all parts of the circuit hard
is not practical due to area considerations. A method is needed to
prioritize the need for radiation hardness of the various items and only
implement the essential items radiation hardened circuits.
Radiation-hardened design is very much a statistical approach. Described
below is the approach used to create a radiation-hardened FPGA.
The CSRAM must be hardened since the product may not be commercially viable
unless the part can reliably store the logic design. Also, it should be
done in an extremely area efficient way since there are millions of
configuration bits which comprise about 25% of the core area. A background
READ.backslash.READ.backslash.READ.backslash.WRITE on error scheme with
the Error Correction Code (ECC) bits to ensure the data is kept accurate
is employed.
The configuration data input circuitry, the read or write systems, and the
CSRAM wordline driver/decoder and associated control logic will be
required to be hardened, since they control the memory loading and
background checking.
FIG. 3 is a simplified schematic diagram illustrating CSRAM architecture
300 having radiation tolerant reading and writing circuits as disclosed in
the present invention. SRAM memory architecture comprises an EPROM control
block 312 that, as is well known to those of ordinary skill in the art,
outputs a serial data stream (SD) from an external source (i.e., EPROM,
CPU, etc.) and a corresponding clock signal (SCK) to synchronize the
operations of the various blocks. The data stream includes a preamble that
tells the various blocks how to process the rest of the data. The preamble
may contain information such as partial load versus full load or partial
load address, among other possibilities. The serial data stream signal is
coupled to row counter 314 through signal line 313, control logic 332,
column counter 330 and READ/WRITE amplifiers 328 (used during testing
only). Row counter 314 is coupled to FPGA core 316. As is well known to
those of ordinary skill in the art, an FPGA core may be employed as a
stand-alone FPGA core, repeated in a rectangular array of core tiles, or
included with other devices in a system-on-a-chip (SOC). The core FPGA
tile may include an array of logic modules surrounded by input/output
modules. An FPGA core tile may also include other components such as read
only memory (ROM) modules. Horizontal and vertical routing channels
provide interconnections between the various components within an FPGA
core tile. Programmable connections are provided by programmable elements
between the routing resources.
Column counter 330 is coupled to FPGA core 316 through READ/WRITE
amplifiers 328. FPGA core 316 is coupled to radiation tolerant READ/WRITE
amplifier and error correction code circuit 318 which is then coupled to
the cyclic redundancy check circuit (CRC) 326 through radiation tolerant
shift register 320 and two-input multiplexers 322 and 324. Radiation
tolerant READ/WRITE amplifier and error correction code circuit 318 and
radiation tolerant shift register along with row counter 314 perform
background error correction code refresh operations.
Row counter 314 raises the word of any row only if every cell in the row is
to be accessed for reading or writing and leaves the word low (and the
memory cells protected) the rest of the time. To accomplish this, the
radiation tolerant shift register 320 and radiation tolerant READ/WRITE
amplifier and error correction code circuit 318 of the present invention
have been added to load the data into the circuit in a radiation
environment.
FIG. 4 shows a simplified block diagram of one bit 400 of radiation
tolerant amplifier and error correction code circuit 318 and radiation
tolerant shift register 320. CSRAM interface circuit 400 interfaces
between each memory column in the CSRAM core and the ECC circuitry. CSRAM
interface circuit 400 comprises a plurality of ECC latches, in this
illustrative example there are three 402, 404 and 406. ECC latches are
coupled to a majority of three voting block 408. Voting block also has
inputs coupled to 7:1 multiplexer 410. Multiplexer 410 sources the shift
register of flip-flop 414 discussed below. The output of voting block 408
is coupled to one input of two-input multiplexer 412 which has a second
input coupled to 7:1 multiplexer 410 through flip-flop 414. The shift
register flip-flop 414 is one bit of radiation tolerant shift register 320
as shown in FIG. 3. Flip-flop 414 may be used to load data into the CSRAM,
but may also be used in conjunction with 7:1 multiplexer 410 to observe
the rest of the circuits in CSRAM interface circuit 400 for test purposes.
The output of two-input multiplexer 412 is coupled to READ/WRITE amplifier
418 through a write latch 416. Two-input multiplexer 412 allows for
sourcing of the write data from either the ECC or shift register flip-flop
414.
ECC latches 402, 404 and 406 implement a triple-mode redundancy (TMR) for
the results of the ECC circuitry. In the present invention, as will be
discussed in greater detail below, the background ECC checking employs
four cycles: three consecutive read cycles followed by a write cycle (even
though a write operation will only occur during the write cycle when a
correction is necessary). After each read cycle the results of the CRC
check are stored in one of the radiation tolerant ECC latches 402, 404 or
406. Radiation tolerant ECC latches 402, 404 or 406 are identical to
radiation tolerant write latch 416. Thus, the check is run through three
times and the results are voted on a bit-by-bit basis in majority of three
voting block 408. The voting logic is illustrated in FIG. 6.
FIG. 5 is a simplified schematic diagram of a radiation hardened latch 500
shown as ECC latch 402, 404, and 406 and write latch 416 in FIG. 4.
Radiation hardened latch 500 is used to reliably hold the data being
written into the CSRAM. Latch 500 operates normally as a CMOS, level
sensitive, asynchronous set/reset latch except that transistors in the
feedback loop from 508 to 504 to 506 are larger than the minimum the
process allows to make it radiation tolerant. The size of the transistor
is determined by the size needed to resist a Qcrit or larger particle
strike without the use of poly resistors. During operation, when L is high
and LB is low, the left transmission gate 502 is closed and the right
transmission gate 504 is open so the latch ignores input IN and the
feedback loop from OUT to LG to LQB stores the data. When L is low and LB
is high, the top transmission gate is off (breaking the feedback loop) and
the left transmission gate is open making latch 500 responsive to the data
on IN. The circuit for the transmission gate is illustrated in inset 510.
ECC latches 402, 404 and 406, as shown in FIG. 4, implement a triple-mode
redundancy (TMR) for the ECC circuitry. In the present invention, as will
be discussed in greater detail below, the background ECC checking employs
four cycles: three consecutive read cycles followed by a write cycle (even
though a write operation will only occur during the write cycle when a
correction is necessary). After each read cycle the results of the CRC
check are stored in one of the radiation tolerant ECC latches 402, 404 or
406. The radiation tolerant ECC latches 402, 404 or 406 are identical to
radiation tolerant write latch 416. The check is run through, for example,
three times and the results are voted on a bit-by-bit basis in majority of
three voting block 408. The voting logic is illustrated in FIG. 6.
In one embodiment of the present invention, only a single error correction
scheme may be used, thus it is important that the error correction code
circuit is designed such that there will be one error to correct in any
ECC word line. However in some cases, errors referred to as "double
strike" errors occur. These errors occur when a particle hits a circuit at
a relatively shallow angle, upsetting two or more programming bits in a
single word line simultaneously. In the ECC circuit of the present
invention, the "double strike" problem is solved by physically separating
the bits in any ECC word line by a distance larger than the "double
strike" distance. Though the "double strike" distance is an estimate, it
is believed to be approximately 20 um in a 0.25 mm CMOS process. In the
present embodiment, the memory cell size in a first dimension (parallel to
the word lines) may be approximately 7.66 um. Thus, three memory cells
span a greater distance than the 20 um in a first dimension and that
memory cells four or more places apart on a word line are insulated from
"double strikes." In addition, there is almost always FPGA circuitry
distributed among the memory columns such that the distance is usually
greater than the above distances. Thus, in one embodiment of the present
invention, four ECC word lines are interdigitated at one so that all the
bits on any single word line are guaranteed to be more than the "double
strike" distance apart. Because space on any integrated circuit is
crucial, it is desirable to implement the FCC scheme of the present
invention using data word lines as wide as possible. An example of an
embodiment of this scheme is shown in FIG. 6 below.
FIG. 7 is a simplified block diagram of a field programmable gate array
(FPGA) core 700 within an integrated circuit having multiple core tiles
702. As set forth above, an FPGA core 700 may be employed as a stand-alone
FPGA core, repeated in a rectangular array of core tiles, or included with
other devices in a system-on-a-chip (SOC). The core FPGA tile may include
an array of logic modules surrounded by input/output modules. An FPGA core
tile may also include other components such as read only memory (ROM)
modules. Horizontal and vertical routing channels provide interconnections
between the various components within an FPGA core tile. Programmable
connections are provided by programmable elements between the routing
resources. In this illustrative example FPGA core 700 comprises six FPGA
tiles 702. It will be clear to those of ordinary skill in the art having
the benefit of this disclosure that other configurations are possible and
the present configuration is set forth as an example only. FPGA core 600
also comprises horizontal half channel FPGA tiles 706, vertical half
channel FPGA tiles 704 and turn blocks 708.
FIG. 8 is a simplified schematic diagram illustrating an FPGA core 800
having only one core tile 804. FPGA core 800 comprises two vertical half
channels 802 each having 19 memory columns 810, one core tile 804 having
442 memory columns 812 and an additional 8 memory columns 814 for the
internal ECC user SRAM (USRAM) circuit. The internal ECC user SRAM (USRAM)
circuit will be discussed in greater detail below. CSRAM block 800 has
total number of 488 memory columns.
As stated above, in one embodiment of the present invention, four ECC words
are interdigitated on each word line so that all the bits in any single
word line are guaranteed to be more than the "double strike" distance
apart. In one illustrative embodiment, the total number of memory columns
is 488, thus, in this embodiment, four ECC decoder/encoders that can
accept a 122-bit data word line (488/4=122). As is well known to those of
ordinary skill in the art, EGG uses hamming encode/decode with parity. In
one example, to implement a single error correctionldouble error detection
(SECDED) scheme (as shown in FIG. 4), 9 extra bits are required per data
word line, or 9.times.4=36 additional bits per memory block.
To spread out the delays for the FPGA routing resources, it is desirable to
distribute the 36 check bits in groups of four (one bit for each ECC word
line) over the width of FPGA core 902 as illustrated in FIG. 9. Each FPGA
core 902 is associated with 4 check bits and the left vertical half
channel 906 has 4 check bits. One of ordinary skill in the art having the
benefit of this disclosure would realize that the above number of data
bits, word lines and check bits may change according to a variety of
factors including, but not limited to, FPGA core size. Thus, the above
number of data bits, word lines and check bits are set forth for
illustrative purposes only and are in no way meant to limited the present
invention.
FIG. 10 is a simplified schematic diagram illustrating in greater detail
the read and write amplifiers as shown in FIG. 4. Write amplifier 1010
comprises an enable input 1015 coupled to an inverter 1016. Inverter 1016
has an output coupled to the gate of P-channel transistors 1020 and 1034
and to two-input NOR gate 1022. Two-input NOR gate 1022 has a second input
coupled to a first input 1041 of precharge circuit 1014 through inverter
1042 and an output coupled to the gate of N-channel transistors 1026 and
1038. Write amplifier 1010 has a data input 1027 coupled to the gates of
P-channel transistors 1018, 1032 and N-channel transistors 1028 to 1032
and 1040 through inverter 1030. Data input 1027 of write amplifier 1010 is
coupled to write latch 416 of FIG. 4.
Referring still to FIG. 10, P-channel transistor 1018 has a source coupled
to Vcc and a drain coupled to the source of P-channel transistor 1020.
P-channel transistor has a drain coupled to node comprising an output
1024. N-channel transistor 1026 has a drain coupled to node comprising an
output 1024 and a source coupled to the drain of N-channel transistor 1028
that has a source coupled to ground. P-channel transistor 1032 has a
source coupled to Vcc and a drain coupled to the source of P-channel
transistor 1034. P-channel transistor has a drain coupled to node
comprising a logic column bar (LCB) output 1036. N-channel transistor 1038
has a drain coupled to node comprising an output 1036 and a source coupled
to the drain of N-channel transistor 1040 that has a source coupled to
ground.
Precharge circuit 1014 comprises a precharge input 1041 coupled to inverter
1042. Inverter 1042 has an output coupled to an input of inverter 1044
which has an output coupled to the gate of P-channel transistors 1046 and
1048. P-channel transistor 1046 has a source coupled to Vcc and a drain
coupled to LCB output 1024. P-channel transistor 1048 has a source coupled
to Vcc and a drain coupled to logic column (LC) output 1036.
Precharge periods occur between all read and write operations. For example,
precharge input 1041 may be at logic 0 during precharging. When precharge
input 1041 is at logic 0, the input of inverter 1044 is at logic 1 which
forces node 1047 to logic 0 and disables write circuit 1010 pull-down
transistors 1026 and 1038. When precharge input 1041 is at logic 0, it
also forces node 1047 to logic 0 turning on the precharge P-channel
transistors 1046 and 1048 and drive LC output 1036 and LCB output 1024
lines to Vcc.
Precharge circuit 1014 also comprises a current source (VCS) input 1051 to
VCS generator comprising P-channel transistors 1052 and 1054. VCS
generator provides a bleed current into LC line 1036 and LCB line 1024.
Sense amplifier 1012 comprises a reset/set (RS) latch. In one illustrative
embodiment, RS latch is formed by a first and second cross-coupled
two-input AND gates 1064 and 1066. First two-input NAND gate 1064 has a
first input coupled to LC line 1036 from write amplifier circuit 1010 and
precharge circuit 1014, a second input coupled to the output of second
two-input NAND gate 1066 and an output coupled to a first input of second
two-input NAND gate 1066 and inverter 1062. Second two-input NAND gate
1066 has a first input coupled to the output of first two-input NAND gate
1064, a second-input coupled to LCB line 1024 from either write amplifier
circuit 1010 or precharge circuit 1014 and an output coupled to a second
input of first two-input AND gate 1064. Inverter 1062 has and output
coupled to data out line 1068.
FIG. 11 is a simplified schematic diagram illustrating in greater detail
the radiation tolerant read and write amplifiers as shown in FIG. 3 for
user static random access (USRAM). Write amplifier 1110 comprises an
enable input 1115 coupled to an inverter 1116. Inverter 1116 has an output
coupled to the gate of P-channel transistors 1120 and 1134 and to
two-input NOR gate 1122. Two-input NOR gate 1122 has a second input
coupled to a first input 1141 of precharge circuit 1114 through two-input
NAND gate 1142 and an output coupled to the gate of N-channel transistors
1126 and 1138. Write amplifier 1110 has a data input 1127 coupled to the
gates of P-channel transistors 1118, 1132 and N-channel transistors 1128
and 1140.
Referring still to FIG. 11, P-channel transistor 1118 has a source coupled
to Vcc and a drain coupled to the source of P-channel transistor 1120.
P-channel transistor has a drain coupled to node comprising an output
1124. N-channel transistor 1126 has a drain coupled to node comprising an
output 1124 and a source coupled to the drain of N-channel transistor 1128
that has a source coupled to ground. P-channel transistor 1132 has a
source coupled to Vcc and a drain coupled to the source of P-channel
transistor 1134. P-channel transistor has a drain coupled to node
comprising an LCB output 1136. N-channel transistor 1138 has a drain
coupled to node comprising an output 1136 and a source coupled to the
drain of N-channel transistor 1140 that has a source coupled to ground.
Precharge circuit 1114 comprises a precharge input 1141 and a MASKB input
1143 coupled to two-input NAND gate 1142. Two-input NAND gate 1142 has an
output coupled to an input of inverter 1144 which has an output coupled to
the gate of P-channel transistors 1146 and 1148. P-channel transistor has
a source coupled to Vcc and a drain coupled to LCB output 1124. P-channel
transistor 1548 has a source coupled to Vcc and a drain coupled to LC
output 1136.
Precharge periods occur between all read and write operations. For example,
precharge input 1141 may be at logic 0 during precharging. When precharge
input 1141 is at logic 0, the input of inverter 1144 is at logic 1 which
forces node 1127 to logic 0 and disables write circuit 1110 pull-down
transistors 1126 and 1138. When precharge input 1141 is at logic 0, it
also forces node 1147 to logic 0 turning on the precharge P-channel
transistors 1146 and 1148 and drive LC output 1136 and LCB output 1124
lines to logic 1.
MASKB input 1143, when asserted low, forces a value of logic 0 into sense
amplifier 1112 and forces the precharge circuit into the precharge state
as if precharge input 1141 had been asserted low. This masks the data
sensed on LC/LCB because USRAM bits can change value after initial loading
so they have to be masked during background ECC as is well known in the
art. The USRAM bits in dynamic applications contain a logic 0 for refresh
purposes regardless of the value initially loaded into a particular bit.
Precharge circuit 1514 also comprises VSC input 1151 to VCS generator
comprising P-channel transistors 1152 and 1154. VCS generator provides a
bleed current into LC line 1136 and LCB line 1124.
Sense amplifier 1112 comprises an RS latch. In one illustrative embodiment,
RS latch is formed by a cross-coupled three-input NAND gate 1164 and
two-input NAND gate 1166. Three-input NAND gate 1164 has a first input
coupled to LC line 1136 from write amplifier circuit 1110 and precharge
circuit 1114, a second input coupled to MASKB input line 1143 and a third
input coupled to the output of two-input NAND gate 1166 and an output
coupled to a first input of two-input NAND gate 1166 and inverter 1162.
Two-input NAND gate 1166 has a first input coupled to the third output of
three-input NAND gate 1164, a second input coupled to LCB line 1124 from
either write amplifier circuit 1110 or precharge circuit 1114 and an
output coupled to a third input of three-input NAND gate 1164. Inverter
1162 has and output coupled to data out line 1168.
FIG. 12 is a simplified block diagram illustrating the USRAM circuit 1200
with the electronic correction code circuitry (ECC) of the present
invention. USRAM circuit 1200 comprises write port 1602 having an ECC
encoder 1210 coupled to write data line 1208. Write data line 1208 is
coupled to USRAM core 1204 via signal line 1209 and coupled via signal
line 1211 to USRAM ECC through ECC encoder. ECC encoder generates check
bits from the write data input line 1208 before a write operation takes
place. Read port 1206 contains ECC decoder coupled to USRAM core via
signal line 1218 and coupled via signal line 1220 to USRAM ECC. ECC
decoder 1214 has an output coupled to read data line 1216. ECC decoder
1214 uses the stored data and check bits to make single corrections when
necessary after a read operation. The read port ECC corrects the data as
it leaves the USRAM block.
In static applications (AROM, etc.) masking is unnecessary. In dynamic
applications (RAM. FIFO, etc.) masking is necessary.
From this disclosure, it will be apparent to persons of ordinary skill in
the art that various alternatives to the embodiments of the disclosed
system described herein may be employed in practicing the disclosed
system. It is intended that the following claims define the scope of the
disclosed system and that structures and methods within the scope of these
claims and their equivalents be covered thereby.
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