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Adaptation of the FPGA to Logic Failures
Tyurin S.F., Grekov A.V., Gromov O.A.
Abstract - The paper proposes the restoration of logic programmable logic integrated circuits such as FPGA (field-programmable gate array) for critical applications by adapting to failures of logic elements. The principle of adaptation FPGA is to switch to the remaining functionality of the LUT (Look Up Table), with the possibility of hardware and software they use in the event of hardware failure after massive failures. Asked to ensure the preservation of the basis in the sense of Post logic functions that allow you to calculate the input for a longer time at a given failure model.
I. Introduction
Modern FPGA, containing several billion of transistors [1], provide wide opportunities for logic reconfiguration, but do not use them to adapt to failures. Thus, one of the leading experts in FPGA area Yervant Zorian said: "Now the main problem of system on a chip repair is development of embedded technologies and methods of the logic repair that occupies no more than 10% of chip area" [2].
To solve this problem we may provide retaining of basic in the terms of Post theorem [3] logic functions that allow to calculate the input for a longer time at a given pattern of failures, that is - the reservation bases elements, the use of elements with an excess basis [4, 5]. In case of failures it is possible to calculate the initial logic functions - all or only critical parts of the residual bases of all or a subset of items with the possible use of software and hardware implementation. [6] With that the scheme is adapted to the conditions of a fault with the appropriate reconfiguration.
Contemporary programmable logic - FPGA (field-programmable gate array) provide wide opportunities of logic reconfiguration, but do not use them to adapt to the failures and logic recovery [13].
Let us consider the proposed principle and characteristics of recovery logic FPGA for critical applications by adapting to failures of logic elements.
Manuscript received September 10, 2013.
Tyurin S.F. is with the Perm National Research Politechnical
University, Russia, e-mail: tvurinsergfeo@vandex.ru
Grekov A.V. is with the Perm Military Institute of Internal Troops of the Ministry of Internal Affairs of the Russian Federation, e-mail: grekartemvl@mail.ru
Gromov O.A. is with the Perm National Research Politechnical
II. The principle of adaptation to failure of 8-1
MULTIPLEXER
Let us consider the gate FPGA - multiplexer with three address inputs xb x2, x3 - 8 channels a, b, c, d, e, f g, h, (81), consisting of seven elementary multiplexers 2-1 (Fig. 1).
Fig. 1. The 8-1 multiplexer (eight channels), consisting of seven elementary multiplexers
on the assumption that there is not a single failure data inputs a, b, c, d, e, f, g, h, or not more than one failure in seven elementary multiplexers 2-1 propose the switch to the "half" of the scheme.
Let there be a failure in the element, which is connected to the input channels c, d. Then it is necessary to do after finding out the transition to the second half of the scheme -channels e, f, g, h. And the failures may occur on the input -but not of the last element.
When a fault is detected, for example, by external means, it is necessary to perform two tests - on the one and the other half. But this is allowed only in case of failure of elements and data inputs (one failure).
If there is half the items (allow refusal on the inputs of all the elements and even by choice but out on the exit of the last element), for example, the older variable is equal to zero:
University, Russia, e-mail: ogromov@inbox.ru
z = 0 x2(cxi v dxi) v x2(axi vbxi) v
V 0 x2 (gxi v hxi ) v x2 (exi v fxi)
Then we get:
z = 0 x2(cxi v dxi) v x2(axi v bxi) (2)
or
Z\ = x2(cxi v dxi) v x2(axi v bxi) (3).
The second half of channels will be implemented similarly:
z2 = x2 (gxi v hxi) v x 2 (exi v fxi) (4).
That is, to restore one eight-channel multiplexer of the three "half" of the four channels is necessary, so that the third multiplexer plug on the leading variable either one or the other half, that is operated in a two-channel multiplexer. Therefore, to set up the third component is necessary to:
z3 = x3 (zi0 v 10) v x3( z 2O v 10), (5)
which corresponds to the two-channel multiplexer functions z3 = Z\ x3 v z2 x3 . (6)
If there is a failure (one-time constant) to the address inputs - everything is much more complicated.
Table I shows how to rewire channels to counter such denial. Accordingly, the process of failure detecting is getting slow.
TableI
Required reconnect channels with constant denials eight-
channel MULTIPLEXER ADDRESS INPUTS
x2 x1 x0 No fault x20 x2 Failure x\0 x\ * О 0 x1 0
0 0 0 0 0 4 0 2 0 1
0 0 1 1 1 5 1 3 0 1
0 1 0 2 2 6 0 2 2 3
0 1 1 3 3 7 1 3 2 3
1 0 0 4 0 4 4 6 4 5
1 0 1 5 1 5 5 7 4 5
1 1 0 6 2 6 4 6 6 6
1 1 1 7 3 7 5 7 6 6
"Half" of the logical elements can be used alone, but to restore a full multiplexer requires three "half" of the multiplexer.
III. Features of FPGA logic elements
Currently, FPGA contain configurable logic blocks (CLB) [i, 7], consisting of the logic elements,
programmable local and global matrix connections Mc -Fig. 2.
Logic gate FPGA - is a super redundant basis, and it is constructed as a read-only memory ROM (LUT - Look Up Table), which is a variable for the four multiplexer i6-i (tree multiplexers), input data is set up so-called configurable memory cells [i] - Fig. 3.
In Fig. 3 inputs - S0, S\, S2, S3, the element is set to implement the sum modulo two S0®S\®S2®S3. On a specific set of variables is realized the only way from input to output, for example, from input i4: S3S2Si (not S0) [i].
Elementary multiplexers 2-і is implemented as a switch (this is also a multiplexer) for example, on the basis of two chains of two transmit MOS transistors [1] - Fig. 4.
Memory configuration (configuration data logic elements and matrices compounds) - this is the configurational cells, each of them contains six transistors [1, 8].
IV. Features of adaptation to failures of transistors and inputs of LUT FPGA
Given the great redundancy logic elements on the basis of the conversion tables LUT, it is possible to restore the faulty conversion table. It is obvious that in this case there is a loss of functionality, but even LUT with limited functionality can be used for the synthesis of a large number of Boolean functions.
Let us consider a simple model of single constant failures. And we shall consider themselves as failures based transistors, which are built LUT, and the failure of address inputs.
Fig. 2. Configurable logic block of Altera's FPGA
Interconnect Matrix
Fig. 4. The switch signals from the local interconnect to the input CLB: multiplexers’ tree (a), implementation of the multiplexer transmitting MOSFETs (b)
ABC D
Input failure provides that the address input LUT has fixed logic level "0" and "1." A constant refusal to "1" in the CMOS transistor circuit includes sample source-drain or latching gate. A constant refusal to "0" CMOS transistor -is an open circuit source-drain or open the shutter.
Consider the possible cases of failure of the transistor.
Suppose there was a single constant denial of transistor VT29 (Fig. 5).
If you set a single constant refusal to "0", in this case, the upper part of the network goes down, because the information from the SRAM cells that are connected to transistors VT1-VT8, can not be transferred to the output. But setting D=0, we can always connect the bottom of the exit and realize the function of three variables A, B and C. If you set up once the constant refusal of "1", the top part of the circuit is always connected to the output. In this case, setting D=1, turn off the lower part of the scheme and prevent the occurrence of faults. At the top of the chart can also be synthesized function of three variables A, B, C.
Consider the failures in transistors connected to the line C. Suppose there was a single constant refusal to "0" in the transistor VT27. This means that the information from the SRAM cells that are connected to the transistors VT9-VT11, will never be passed on to the input. However, you can set C=0 and for all open transistors VT28 and VT26. Then we can implement the function of three variables A, B and D. In the event of a "1" on the transistor VT27, can not turn this thread from the transistor VT30. But setting C=1, we will close the transistors VT26 and VT28. Here again we can implement arbitrary functions of three variables A, B and D.
Consider the failures in transistors, which are connected to the line B. Let there was a failure to "0" in the transistor VT19. This means that the transistors and VT5 VT6, can never be switched to the input of the transistor VT26. Filed B=0 we open transistor VT18, VT20, VT22 and VT24. In this case the cells 1,2,5,6,9,10,13,14 will not be used when using LUT. However, in the remaining 8 SRAM can build the function of 3 variables A, C, and D. In the event of a "1" in the transistor VT19, is served B=1 and also implement a function of three variables A, C, and D from the remaining cells and SRAM transistors.
Suppose there was a failure in the transistor of the first stage, for example, not "0" in the transistor VT7. In this case, we can work with even the SRAM cell, it needs to set A=0. In this case, we can construct a function of the variables B, C and D. In case of refusal to "1" in the transistor VT7, similarly set A=1 and work with odd SRAM cells. Here, again, the remaining elements can be synthesized function of three variables B, C and D.
It is obvious that a similar situation will occur during the failure of the other transistors. Thus, a single failure in any transistor reduces the functionality of the item, however,
due to the large redundancy is still possible to build a function of three input variables.
At single constant failures of the inputs LUT actually enforce the same situation arises that in case of failure of transistors. That is forcibly turned off one half of the LUT and a conversion table is converted from the multiplexer 16 to 1 in 8 to 1 multiplexer. But in this case it is also possible to synthesize a function of three variables.
V. Conclusion
Thus, this article presents an approach for FPGA logic partial recovery for critical applications by adapting to failures of logic elements based on the Look Up Table. The principle and the example of recovery of the LUT in single constant failures of transistors and inputs - go to the "half" or, more precisely, a "partial" functional. Use classical LUT for 4 variables, however, the same procedure can be applied to a larger number of LUT inputs. This gives rise to new opportunities parry multiple failures in a more complex tree transistors.
In the case of failure of hardware (logic elements) after massive failures, for example, in catastrophic situations, it is also possible software-hardware utilization failed elements.
In the future, should spread this approach, which could be called "partial firmware functionality" in other areas -for the implementation of energy-efficient FPGA.
In addition, it is useful to explore the possibility of using partial functionality for diagnosing FPGA.
References
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