IEEE Transactions on Nuclear Science, Vol. NS-33, No. 6, December 1986

ON-ORBIT OBSERVATIONS OF SINGLE EVENTUPSET IN HARRIS HM-6508 1K RAMS

J. B. BlakeSpace Sciences LaboratoryThe Aerospace Corporation

R. MandelLockheed Missiles and Space Company

Abstract: The SEU and latchup rate of a satellitesubsystem containing 384 Harris HM-6508 RAMs wasobserved over a 2 year period. The satellite was in alow polar orbit. A total of 47 single SEUs, 9 double,1 triple and 1 quadruple events were observed; themultiple events were 19% of the total. No latchup wasobserved. The error rate was 7.6 x 10 2 SEUs/chip-year and an upper limit was found of the ratio oflatchups/SEUs of 1.7 x 10 2. The observed SEU ratewas consistent with predictions based upon laboratorytest, using the Lawrence Berkeley Laboratory 88"cyclotron.

Introduction

Single event upset (SEU) due to single chargedparticles in spacecraft microelectronics has grown asa subject of interest in the last several years from acuriosity, often eliciting disbelief, to a subject ofvital interest. Binder et al.1 published the firstanalysis. The importance of understanding the vulner-ability of various microelectronic components to SEUhas resulted in a substantial effort in ground test-ing. Many components have been ground tested usingvarious particle accelerators, and the in-flight errorrate estimated from the test results. The IEEE Trans-actions on Nuclear Science regularly contain detaileddescriptions of laboratory test and analysis. Howeverfew correlations of extensive flight data with esti-mates from ground testing have been published, inparticular where detailed testing of the parts withthe same pedigreed as those used in the spacecraftpreceeded launch. Correlative data are a vital partof obtaining confidence in the accuracy of SEU rateestimates based upon ground test only.

In this paper we give the results of two years offlight observation of a subsystem consisting of HarrisHM-6508 1K RAMs, and compare the observations of SEUsand absence of latchup with estimates based uponground testing with the Lawrence Berkeley Laboratory88" cyclotron.

Once the exact location of the error is found,the correct value is reloaded into memory. After thememory is reloaded, the contents of the memory loca-tion in question is verified in order to determine ifthe error was a soft error generated by an SEU or ahard error generated by a part failure or cosmic-rayinduced latchup.

The data presented in this paper were acquired inthe two year period from 1 January 1983 through 31December 1984.

Results

A total of 72 SEUs were observed during 731 days(2 years) of flight. Several were multiple events;the multiplicity distribution is given in Table 1.

Table 1SEU MJLTIPLICITY DISTRIBUTION

Number of Events

4791

Errors per Event

1234

It can be seen that there have been a substantialnumber of multiple events, 19%.

The reader may recall from the subsystem descrip-tion that the memory contents verified only once every90 minutes, and wonder if the multiple SEUs are notjust two independent events that occurred betweenverifications. In the following we show that theprobability of two independent events between verifi-cations is very small.

The total number of events, assuming multipleevents are due to a single cosmic ray, is 58. There-fore the error rate is

Description of Observations

The Harris HM-6508 1K x I RAMs are part of asubsystem of a satellite in a low, polar orbit. Thememory module, used in the subsystem containing theRAMs, consists of three printed circuit cards, witheach card containing eight 2K byte memory hybrids, fora total of 48K bytes. Each memory hybrid contains 16HM-6508 RAM chips. On a regular basis all but 256bytes of the 48K bytes are examined for bit errors.

Two different techniques were used for detectingbit errors. The first technique, a memory check sum,was capable of automatically detecting all single bitand some double bit errors which occurred within a

page of memory. A memory page consists of 256bytes. Memory check sum tests are performed approxi-mately every 90 minutes. To detect a multiple erroror to determine the exact location of the bit errorwithin the page the entire contents of the memory isdumped and compared to the load file. Memory dumpsare normally performed once a month, or immediatelyafter the check sum routine detects an error.

58 = 7.9 x 10-2 SEUs/day,731SEs/a (1)

or

12.6 days/SEU. (2)

Assuming Poisson statistics we compute the probabili-ty that two independent errors occur during the sameday, and during the period between verifications

P(same day) = 2.9 x 10 3

P(between verifications) = 1.4 x 10-5

(3)

(4)

The result given in (3) indicates we expect twoevents in a single day about once a year. Our twoyear data set has one such occurrence. The resultgiven in (4) indicates two independent events insideof a single verification period is expected about onceper 13 years. We observed 11 multiple events in thetwo years of observations and thus they cannot, ingeneral, be independent events occurring in a singleverification period.

0018-9499/86/1200-1616$01.00 © 1986 IEEE

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The location of the multiple errors gives addi-tional and compelling evidence that the multiple hitsare due to a single cosmic ray. Knowledge as to thelocation of multiple hits also is of great interest inunderstanding how to minimize deleterious effects frommultiple hits. Recall that the HM-6508 RAMs arepackaged on 3 circuit cards, 8 hybrids to a circuitand 16 RAMs to a hybrid. Table 2 is a listing of themultiple events giving some of the location details.

Table 2MULTIPLE HIT NUMBERS

BitsFlipped

RAMs Hybrids BoardAffected Affected Affected

22222242232

?2112211132

I2I111I1112

1111111I1I1

In the first case in the Table the identity ofthe RAMs affected was lost.

Figure 1 shows bit maps of HM-6508 RAMs for thesix cases in Table 2 where only 1 RAM was affected,The location of the affected bits is marked. Notethat in the 2-error cases the bits were adjacent, andin the 4-error case lay essentially on a straight lineas would be most likely if caused by a single cosmicray. Needless to say, the probability of this order-ing being statistical in nature is truly nil.

Unfortunately we do not have information on thehybrid layout and therefore cannot comment on thephysical configuration of the error locations whenmore than one RAM on the same hybrid were involved.We do know that in the 2 cases where 2 hybrids wereinvolved the hybrids were in adjacent locations on thesame board.

In the two years of flight no HM-6508 RAM latchedup. This observation gives an upper limit for thelatchup rate of

R < 1latchup - 384 RAMs x 2 years

1.3 x 10 3 latchups/chip-year. (5)

Figure 1. Harris HM-6058 bit maps are shown for five cases of a double error and one case of a quadruple error.The location of the errors are shown as filled circles.

This may be compared with the SEU rate of

R- 58 -2RE 384 7.6 x 102 SEUs/chip-year., (6)

or if each error of a multiple event is counted separ-ately

U 472 9.4 x 10 SEUs/chip-year. (7)SEU 384x 2

Thus the ratio of latchup to SEUs is given by

< 1.3 x 10 3 -.2Latchup/SEU - - x = 1.7 x 10 (8)

7.6 x 10It is well known that the galactic cosmic-ray

intensity varies over the solar cycle. Therefore itis of interest to look for a time dependence in theSEU rate. These results are given in Table 3.

Table 3TIME DEPENDENCE OF SEU RATE

Time Period

1 Jan - 30 June 832 July - 31 Dec 831 Jan - 30 June 841 July - 31 Dec 84

SEU Number SEU Rate (upsets/day)

14 (7.7 ± 2.1) x 10-212 (6.5 ± 1.9) x 10-210 (5.5 ± 1.7) x 10-222 (12.0 ± 2.6) x 10-2

These results are plotted in Figure 2. It can beseen that the data suggest that the cosmic flux mayhave increased in the second half of 1984. Independ-ent observationsbelow.

0.15F

0.10I=/)

0.05

of the cosmic ray flux are discussed

1 JAN -30 JUN 1 JUL- 31 DEC 1 JAN -30 JUN 1 JUL - 31 DEC1983 1984

Figure 2. The average upset rate per day is plottedas a function of time. The data is aver-

aged over

Discussion

It is of interest to compare the observationswith predictions based upon laboratory testing.Figures 3 and 4 show data for SEU and latchup, respec-tively, acquired by the Space Sciences Laboratory ofThe Aerospace Corporation using the Lawrence BerkeleyLaboratory 88" cyclotron. These data indicate thatthe critical charge for both latchup and SEU are

approximately the same, - 0.29 picocoulombs. Howeverthe cross sections differ by a factor of the order of100. The in-flight results, discussed above (8), givea ratio of greater than 50. The observations andtests are consistent but clearly substantially more

data are required to arrive at a finite value for thelatchup rate.

Figure 3. Experimental data is shown of bit-errorcross section as a function of the LET ofthe incident particle for three differentHM-6508 RAMs. The three parts are labeledSN2, SN3, and SN4.

10-

10-

ECNF=u

cL

10-5

10- 0.2LET (pC/micrometer)

0.4

Figure 4. Experimental data is shown of the latchupcross-section as a function of LET of theincident particle for three different HM-6508 RAMs. The three parts are labeledSN2, SN3, and SN4.

Figures 5 and 6 present observations of the fluxof iron nuclei in two energy channels (98.4 - 418MeV/nucleon and > 418 MeV/nucleon) for the time periodfrom 1973 through 1984. These data are from theUniversity of Chicago experiment aboard the IMP 8

1618

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10

E

CD

C-1

olO

cr-CD)

m

10

100.2LET (pC/micrometer)

TI..

iI I

li i i~~~~~~~~~~~~~~~~~~~~~~~~~~

HM 6508 RAMS

0-------S

S0N2 -* SN 3

TC%fI. LIMIT ASN 4|UPPER lIMITFOR ALL HM 6508SAMPLES TESTED(NO LATCHUP OBSERVED)

.1r

U

9-1

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23satellite (Chenette; Chenette and Dietrich3. It canbe seen that the iron flux reaches a minimum valueduring 1981 and 1982 and then started to increase.Note that these data suggest that the SEU rate willincrease by a factor of 3 or more over the next fewyears.

Iron 98.4 - 418. MeV/nucleonThe University of Chicago IMP 8

0.12 - 11 I0.11

~0.1:> 0.09 - X2 0.08 _ _

0.07 - X, 0.06 - _C 0.05 _ _, 0.04 -

0.03-0.02

73 75 77 79 81 83 85YEAR

Figure 5. Observations of the galactic cosmic-ray ionflux as a function of time for the energyrange from 98.4 to 418 MeV/nucleon.

Iron >418. MeV/nucleonThe University of Chicago - IMP 8

the vulnerable cell in the RAM. The program containsa model of the galactic cosmic-ray flux and its solarcycle modulation, geomagnetic cutoff effects and thefinite size of the Earth. This program has been usedto compute the expected SEU rate in errors/bit-day.

The HM-6508 parameters used in the calculationwere a critical charge of 0.29 picocoulombs and a cellsize of 20 x 20 x 1 micrometers, taken from the dataillustrated in Figure 2. The proper orbital parame-ters were used and the time in the solar cycle takenas 1984.0, the middle of the period of observation.

Figure 7 gives the calculated SEU rate as afunction of aluminum shielding thickness. The averageSEU rate over the two year period, (2.07 ± .27) x10 7, is plotted also as a stippled band. It can beseen that the observed error rate is consonant with aspacecraft shielding of a few gram/cm2, the correctvalue.

10-6

m

-7

X 10H-

1 -2 w 1 - 1 1 IR I 'l

102 10 1 1 10 102 103SHIELDING THICKNESS, gm/cm2 aluminum

=w 24 -C/$ 22 -

E

~20

rang abv 4 1 e/nce

LU

~1614

73 75 77 79 81 83 85

YEAR

Figure 6. Observations of the galactic cosmic-ray ion

flux as a function of time for the energy

range above 418 M4ev/nucleon.

A compu er program, called CREME, has been writ-

ten by Adams which calculates the SEU rate given the

measured value of the critical charge and the size of

Figure 7. A plot of the calculated upset rate as afunction of shielding using the laboratorymeasured parameter for the H14-6508. Thestriped bar denotes the measured, on-orbitrate and its statistical uncertainty.

Acknowledgments: The authors would like to thank Drs.W. A. Kolasinski and R. Koga for many discussionsconcerning SEU test results, Drs. D. L. Chenette andW. F. Dietrick for their galactic cosmic-ray data, andDr. J. H. Adams, Jr. for a copy of his computerprogram for generating upset rates from test data.

This work was supported at The Aerospace Corpora-tion by the United States Air Force Space Divisionunder contract F04701-84-C-0085.

References

1. D. Binder, E. C. Smith and A. B. Holman, IEEETrans. Nucl. Sci., NS-22, p. 2675 (1985).

2. D. L. Chenette, private communication (1985)3. D. L. Chenette and W. F. Dietrich, IEEE Trans.

Nucl. Sci., NS-31, p. 1217 (1984).4. J. H. Adams, Jr., private communication (1985).

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