P.H.G. l-leseab.ci-l LABORATORIES - LABORATORY 1WTE. Di vision a r.e ed arose for decade pulse counters possessing certain

Transcription

1 .E. Y. 3/ P.H.G. -ESEAB.- LABORATORES - LABORATORY 1WTE,... ;;' ~ Decadc Puse ounter Empoyng a ove Seque9ce of States 1, ntrouctc~: n t r,e ccu r se of deveopment of eq_upment n the Frequency Standards D vson a r.e ed arose for decade puse counters possessng certan characterstcs. Thasa are: (a) H~ fan-cut carry capabty. (o) Decoabty, such that no decma output ne from a decoder can possby carry fase puses caused by non-zero swtchng deays o:r other non-dea operatons. (c) Encodabty, such that any combnaton of states may be set nto the counter. n partcuar, the states correspondng to count 1 : must be settabe by operatng a snge contro ne - even, f uece aeaz-y, whe the nput puse tran contnues. ''he counter to be descrbed empoys four fp-fops and a number of two-nput gates to mpement the desred sequence of count states. The sequence s known as Decade Sequence 23 o , Desgn hoces: The desgn s based upon HGH true NAND ogc. The prnted crcut board, RP 281, has been ad out to accept Texas nstruments SN7~N seres TTL ntegrated crcuts. To mnmse puse deay from nput to output of the counter, the sequence s cesgned for synchronous edge-trggerng. To ad decodabty the sequence of states nvoves no more than one fp-fop chancng state'"when advancng the count by one. Tbs s, of course, ony possbe when the cosed sequence conssts of an even number of steps. t aso mpes that the code produced by the sequence must be an unweghted code.

2 To ad encodabty, the set of fp-fop states correspondng to count 1 1 shoud consst of or For reasons whch w become apparent the set 1111 was chosen. The Type'])' fp-fop was chosen snce ts LEAR a~d PRESET nputs take precedence over cock sgnas, aowng encodng to take pace whe the nput puse tran contnues. 3. Practca Desp;n: The fna desgn s shown on the ogc dagram. The requred gatng Y1as decded by the use of Karnaugh Maps. To ad understandng of the waveforms obtaned, a truth tabe s aso gven. As we, a cyc L c state-sequence ndcates the paths of entry nto the de s r-eu sequence from unde s r-ed commencement states. The Karnaugh M:ap s arranged n such a way that f, n a sequence of states, movement from one set of states to the next requres ony vertca or horzonta movement from one square to a neghbourng square, then ony one varabe changes at each step. The two vertca edges of the map can be regarded as beng adjacent n the above sense, as can the t,vo horzonta edges. Thus the map can be vsuased as consstng of the unbroken surface of a doughnut. The frst Karnaugh Map beow shows the path foowed by the count sequence adopted. 6fs4-4± ~ / 1 7 f 8 1'1 9 ; X f X == don't care. J 11 X X t + 1 1,x x x 1 \:3 v /-ADJAENT...

3 3. The foowng features are apparent: () The ower haf of the map s argey avoded. Ths effectvey prevents the A output beng oaded by sequence gatng. () Feature () eads to the choce of A to provde the maxmum fan-out carry output. () The ony Oto 1 transton for the A fp-fop occurs at the step from natura BD number 7 to number 15. Ths decdes the choce of representaton of count ' 1 by the combnaton of states 1J11. (v) The set of states correspondng to natura BD number 6 s avoded. Ths avodance s based. upon a practca consderaton. By so dong the use of three-nput gates s avoded and the number of two-nput gates requred s kept to a mnmum wthout compromsng the other desred features of the counter. The mnmum gatng necessary to ensure that the sequence s foo~ed s obtaned n the conventona way be settng up a map for escn ;D; nput and groupng to obtan the approprate mnterms. We make use o: tne fact that the Type 1 D 1 fp-fop has ony one data nput and ts output equas the 'D' nput one bt-tme earer. The maps and resutng equac ons are shown beow. The terms n the equatons are aso s hcvrn on the ogc dagram. The ogca mpementaton of the equatons s shown beow. t can be observed here that when usng a counter empoyng a natura BD countng code, s generay desgnated D. the fp-fop generatng the carry output The natura BD or 248 code s a weghted code, wth A beng the east sgt1fcant dgt and D the most sgnfcant. The sequence descrbed here generates an unweghted code and thus the fp-fop generatng the car-r-y output coud equay we have been caed A, B, or D. The choce of A s purey a functon of the abeng of the map showng the sequence of states.

4 4. ~D AB,, oo / 6 4 r X - 11 X X o 1 ' 1 X X 2 ount Sequence x \ D AB \ j ( X, J X X X X X Fp-Fop D - - DD = A.B + A. \ AB D AB r 1 /, O ? X r x x 1 o?j X X ' Fp-Fop Fp-Fop B D ~ B.D +.D BD = G.D. + B.D. D D AB ~ 11 X 1 ~ X X --====.r-:---~ ~ 1 X -~ w L)_J X O?p-Fop" A Paths From Undesred States to Desred States n the Sequence.

5 5. B> /Jo A.B A.B. + A. A > > ) A. : : ) e. 1 - B.D +.D D.D >---- B.D +.D D.D B> \ )]3:c > jl- - LOGAL tpl.sj.jentaton O GAT\JG EQUATONS

6 A truth tabe, ncudng the undesred commencement sets of states, s shown beow: OUNT A B D NATUJAL BD NUtJ3ER o o o [_1 _~J 3 oofo,t._1_ J 5 t 6 O O O O r r..., o / u:~ JStLD S.8'1S OF STJ~TBS -T A H L H L.1 " r, L ~ o 1 o o 1 o 5 '7 ' 1 o 6 o o o o o \ 4 _ D H L S,67

7 7. The vaues of A, B, and D for each of the undesred sets of states can be substtuted nto the equatons and the vaues of AD, BD' D and DD obtaned. These vaues w be cock'ed nto A, B, s.nd D respectvey by the next cock puse. f the new ABD s a esred set of states, the desred sequence w then progress ro rma Ly v th each cock puse advanc1,3 the count by one. f the n ew A.BD s another undesred set of states a further step s needed to ~each a desred set of states n the sequence. The dagram beow shows the processon of cycc sets of states together wth entry paths from the undesred sets of states. :.ach set of states s represented by ts na tu ra. BD equvaent, wth t:o.e a;pr:;prate count number shown n brackets. 8~ \(7) 12~ \(6). \.,..,.r [:,.. / 11 \.. 6/' \ /' 9 13

8 4. Summary: The crcut descrbed performs the foowng functons: (a) Startng n random condton, t w automatcay be drven nto the desred sequence of states by the nput cock puses. (b) By hodng the GATE ARRY /P ne HGH, two separate outputs each capabe of drvng 1 oads are made avaabe. Aternatvey the A ne can be used as a 1 oad drect output and the GATED ARRY /P can be gated on or off accordng to the state of a contro sgna on the GATE ARRY /P ne. (c) By drvng the ENABLE ENODE ne LOW, the states exstng on the ENODE A, B, and D nes are set nto ther respectve fp-fops. f the ENODE A, B, and D nes are permanenty hed HGH, resettng the counter to ' 1 s smpy accompshed by momentary drvng the ENABLE ENODE ne LOW. These functons can be carred out regardess of the state of the cock ne. (d) Snce n proceedng from one set of states to the next ony one fp-fop changes state, unambguous decodng wth no possbty of fase puses can be performed usng prnted crcut card RP 282A. (e) Snce ony two eves of gatng are used n the sequence gatng and the fp-fops are cocked synchronousy, the counter w ope rat e up to a cock rate very neary as hgh as that of a Type 'D' fp-fop aone. 5. Acmowedr,ement: enjoyed frendy argument wth G.M. Ws durng the course of ths work. ~ ' y OM. J.Lo.,u\ ~ '. P. HacFarane Eng~eer ass 2 N-. ~ ~ R.1., 1ra1nor Dvsona Engneer Frequency Standards Dvson '),., J >.,:.-"n t "'. \..;".,, :'.-.._; f 1~.J\.JV

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