Cm bin mt dy (Phn 1) Ng Din Tp i Hc Quc Gia H Ni (Phn 1) Do nhng tin b ca cng ngh, cc cm bin khng ch c nng cao v mt cht lng m nhiu loi cm bin mi ln lt ra i; cm bin mt dy l mt th d. Trc ht xin ni v tn gi: cm bin mt dy khng c ngha l cm bin ny ch c mt dy ra m thut ng mt dy (1 wire) c dng ch nhn mnh mt c im ca loi cm bin ny l ng dn tn hiu li ra v ng dn in p ngun nui c th dng chung trn mt dy dn v khng ch chung cho mt cm bin m nhiu cm bin c th s dng chung mt ng dn. Trn hnh 1 l mt cm bin nhit mt dy nhng ta vn thy r l c 4 dy dn ra.

Hnh 1: Mt loi cm bin mt dy ang lu hnh trn th trng Nh li hi cui nhng nm 60 ca th k trc, cc bc n anh a tn hiu o lng t di l khoan (k thut carota) ln trn mt t bng chnh ng dn in t ngun acquy nui cho thit b o t trong l khoan c xem nh mt k tch th by gi vi s ra i ca cm bin mt dy, thnh tu trn c thc hin ch bng mt hai mi hn; l do l mi th c tch hp sn trn cm bin! Do s dng chung ng dn s liu o lng vi ng cp in p ngun nn cc cm bin mt dy c bit thch hp vi cc ng dng o lng a im, khi m s lng cc ng dn s liu o lng v ng cp in p ngun tr thnh mt con s rt ln. Chng hn, vi mt kho bo qun c 40 phng, trong mi phng cn o nhit ti 3 im (3 cm bin nhit ) v m ti mt im (1 cm bin m); nu tnh trung bnh mi im o cn 4 ng dn th ti im tp trung s dy dn ln n 640 dy, ngha l cn n mt b cp cc k ln ! Trong nhng trng hp ny, cc cm bin mt dy s lm thay i hon ton gii php k thut. Nhng thng gn y c mt s cm bin mt dy c gii thiu vi th trng, nhng v mt cu trc v nguyn l hot ng u c nhng im chung, cho nn n gin ta tm hiu cm bin mt dy thng qua vic tm hiu chi tit loi cm bin nhit mt dy DS1820.

DS1820 l mt sn phm ca cng ty Dallas (Hoa K), y cng l cng ty ng gp nhiu vo vic cho ra i bus mt dy v cc cm bin mt dy. Hnh dng bn ngoi ca cm bin mt dy DS1820 c m t trn hnh 2, trong dng v TO-92 vi 3 chn l dng thng gp v c dng trong nhiu ng dng, cn dng v SOIC vi 8 chn c dng o nhit b mt, k c da ngi!

Hnh 2: Dng ng v v b ngoi ca cm bin DS1820 Cc c im k thut ca cm bin DS1820 c th k ra mt cch tm tt nh sau:

S dng giao din mt dy nn ch cn c mt chn ra truyn thng. phn gii khi o nhit l 9 bit. Di o nhit -55oC n 125oC, tng bc 0,5oC, c th t chnh xc n 0,1oC bng vic hiu chnh qua phn mm. Rt thch hp vi cc ng dng o lng a im v nhiu u o c th c ni trn mt bus, bus ny c gi l bus mt dy (1-wire bus) v s c trnh by chi tit trong s tp ch ti y. Khng cn thm linh kin bn ngoi. in p ngun nui c th thay i trong khong rng, t 3,0 V n 5,5 V mt chiu v c th c cp thng qua ng dn d liu. Dng tiu th ti ch ngh cc nh. Thi gian ly mu v bin i thnh s tng i nhanh, khng qu 200 ms. Mi cm bin c mt m nh danh duy nht 64 bit cha trong b nh ROM trn chip (on chip), gi tr nh phn c khc bng tia laze.

u o nhit s DS1820 a ra s liu biu th nhit o c di dng m nh phn 9 bit. Cc thng tin c gi n v nhn v t DS1820 trn giao din 1-wire, do ch cn hai ng dn gm mt ng cho tn hiu v mt ng lm dy t l kt ni vi iu khin n im o. Ngun nui cho cc thao tc ghi/c/chuyn i c th c trch t ng tn hiu, khng cn c thm ng dy ring cp in p ngun. Mi vi mch o nhit DS1820 c mt m s nh danh duy nht, c khc bng laser trong qu trnh ch to vi mch nn nhiu vi mch DS1820 c th cng kt ni vo mt bus 1-wire m khng c s nhm ln. c im ny lm cho vic lp t nhiu cm bin nhit ti nhiu v tr khc nhau tr nn d dng v vi chi ph thp. Theo chun 1-wire

di ti a cho php ca bus l 300 m. S lng cc cm bin ni vo bus khng hn ch. nng cao phn gi ln trn 9 bit ta phi tnh ton thm bng phn mm da trn cc s liu lu tr trn cc thanh ghi nhit , COUNT REMAIN v COUNT PER C trong nhm cc thanh ghi nhp (scratchpad). vic tnh ton da theo phng trnh sau:

Khi s dng phng trnh ny cm bin mt dy DS1820 c nng cp tr thnh mt cm bin nhit c phn gii cao. B nh ROM 64BIT Mi cm bin nhit DS1820 c mt dy m 64 bit duy nht c lu tr trong b nh ROM t khi sn xut bng k thut laze. ngha ca 64 bit m c gii thch trn hnh 3:

Hnh 3: Ni dung dy m 64-bit trn b nh ROM Nh vy dy m c chia ra thnh 3 nhm, trong :

Tm bit u tin l m nh danh h mt dy, m ca DS1820 l 10h. 48 bit tip theo l m s xut xng duy nht, ngha l mi cm bin DS1820 ch c mt s m. Tm bit c ngha nht l byte m kim tra CRC (cyclic redundancy check), byte ny c tnh ton t 56 bit u tin ca dy m trn ROM (xem hnh 3).

truy cp ln cm bin mt dy DS1820 ta phi s dng hai nhm lnh: cc lnh ROM v cc lnh chc nng (function commands) b nh, cc lnh ny c th c m t ngn gn nh sau: Sau khi thit b ch (thng l mt vi iu khin) pht hin ra mt xung presence pulse, n c th xut ra mt lnh ROM. C 5 loi lnh ROM, mi lnh di 8 bit. Thit b ch phi a ra lnh ROM thch hp trc khi a ra mt lnh chc nng giao tip vi cm bin DS18S20. Lnh ROM - READ ROM (33h)

Cho php c ra 8 byte m khc bng laser trn ROM, bao gm: 8 bit m nh tn linh kin (10h), 48 bit s xut xng, 8 bit kim tra CRC. Lnh ny ch dng khi trn bus c 1 cm bin DS1820, nu khng s xy ra xung t trn bus do tt c cc thit b t cng p ng. - MATCH ROM (55h) Lnh ny c gi i cng vi 64 bit ROM tip theo, cho php b iu khin bus chn ra ch mt cm bin DS1820 c th khi trn bus c nhiu cm bin DS1820 cng ni vo. Ch c DS1820 no c 64 bit trn ROM trung khp vi chui 64 bit va c gi ti mi p ng li cc lnh v b nh tip theo. Cn cc cm bin DS1820 c 64 bit ROM khng trng khp s tip tc ch mt xung reset. Lnh ny c s dng c trong trng hp c mt cm bin mt dy, c trong trng hp c nhiu cm bin mt dy. - SKIP ROM (CCh) Lnh ny cho php thit b iu khin truy nhp thng n cc lnh b nh ca DS1820 m khng cn gi chui m 64 bit ROM. Nh vy s tit kim c thi gian ch i nhng ch mang hiu qu khi trn bus ch c mt cm bin. - SEARCH ROM (F0h) Lnh ny cho php b iu khin bus c th d tm c s lng thnh vin t ang c u vo bus v cc gi tr c th trong 64 bit ROM ca chng bng mt chu trnh d tm. - ALARM SEARCH (ECh) Tin trnh ca lnh ny ging ht nh lnh Search ROM, nhng cm bin DS1820 ch p ng lnh ny khi xut hin iu kin cnh bo trong php o nhit cui cng. iu kin cnh bo y c nh ngha l gi tr nhit o c ln hn gi tr TH v nh hn gi tr TL l hai gi tr nhit cao nht v nhit thp nht c t trn thanh ghi trong b nh ca cm bin. Lnh chc nng b nh Sau khi thit b ch (thng l mt vi iu khin) s dng cc lnh ROM nh a ch cho cc cm bin mt dy ang c u vo bus, thit b ch s a ra cc lnh chc nng DS1820. Bng cc lnh chc nng thit b ch c th c ra v ghi vo b nh nhp (scratchpath) ca cm bin DS1820. khi to qu trnh chuyn i gi tr nhit o c v xc nh ch cung cp in p ngun. Cc lnh chc nng c th c m t ngn gn nh sau: - WRITE SCRATCHPAD (4Eh)

Lnh ny cho php ghi 2 byte d liu vo b nh nhp ca DS1820. Byte u tin c ghi vo thanh ghi TH (byte 2 ca b nh nhp) cn byte th hai c ghi vo thanh ghi TL (byte 3 ca b nh nhp). D liu truyn theo trnh t u tin l bit c ngha nht v k tip l nhng bit c ngha gim dn. C hai byte ny phi c ghi trc khi thit b ch xut ra mt xung reset hoc khi c d liu khc xut hin. - READ SCRATCHPAD (BEh) Lnh ny cho php thit b ch c ni dung b nh nhp. Qu trnh c bt u t bit c ngha nhy ca byte 0 v tip tc cho n byte rh 9 (byte 8 - CRC). Thit b ch c th xut ra mt xung reset lm dng qu trnh c bt k lc no nu nh ch c mt phn ca d liu trn b nh nhp cn c c. - COPYSCRATCHPAD (48h) Lnh ny copy ni dung ca hai thanh ghi TH v TL (byte 2 v byte 3) vo b nh EEPROM. Nu cm bin c s dng trong ch d cp ngun l bt u vic o. - CONVERT T (44h) Lnh ny khi ng mt qu trnh o v chuyn i gi tr nhit thnh s (nh phn). Sau khi chuyn i gi tr kt qu o nhit c lu tr trn thanh ghi nhit 2 byte trong b nh nhp Thi gian chuyn i khng qu 200 ms, trong thi gian ang chuyn i nu thc hin lnh c th cc gi tr c ra u bng 0. - READ POWER SUPPLY (B4h) Mt lnh c tip sau lnh ny s cho bit DS1820 ang s dng ch cp ngun nh th no, gi tr c c bng 0 nu cp ngun bng chnh ng dn d liu v bng 1 nu cp ngun qua mt ng dn ring. n y ta c th thy l vic s dng cc lnh va trnh by l mt cng vic khng n gin, nht l vi nhng bn c cha tng tm hiu v k thut vi x l. Cng v vy m trong khun kh mt bi vit ta cha th hiu h cch lm ch hot ng ca mt cm bin mt dy. Trong phn sau s trnh by thm v cch ghp ni cm bin mt dy vi vi iu khin v t chc ca bus mt dy.

Phn 2C l cm bin nhit mt dy DS1820 l mt trong s t cc cm bin mt dy kh giao tip nht (tt nhin ta c th t an i l nu lm ch c DS1820 th cc cm bin mt dy khc cng tr nn d dng b khng ch !). Trng hp cm bin m mt dy

DS2438 li hon ton khc, s phc tp v mt cu to c nh sn xut m nhn nn ngi dng c hng nhiu thun li. V cc cm bin m ta u bit: xc nh chnh xc m tng i th c ba thng s l nhit ca cm bin, in p li ra ca cm bin v in p ngun nui phi c xc nh (o) chnh xc. V vy nu c mt cch no y thc hin hin c ba php o k trn mt cch ng thi hoc c thc hin ch bng mt linh kin th php o m tng i s tr nn n gin i rt nhiu m vn bo m c chnh xc mong mun. Xut pht t tng , cng ty Dallas (Hoa K) thit k v gii thiu vi th trng mt b gim st in p c dng tch hp thnh cm bin mt dy vi k hiu l DS2438, trong cc b bin i A/D dng cho vic o in p v nhit suy ra m u c tch hp trn chip. Ngoi ra trn chip cn c b bin i dng in (current converter) 10 bit, mt b tch dng (current accumulator) mt b m thi gian. Vi mch DS2438 thng c gi l b gim st pin thng minh (Smart Battery Monitor). Cha phi l tt c, trn chip DS2438 cn c b nh khng t mt d liu (nonvolatile) vi dung lng 40 byte lu tr cc thng tin nh chun cho cm bin.

Hnh 1: B tr chn ca vi mch DS2348. B gim st DS2438 c ng gi khi xut xng di dng vi mch 8 chn hai hng (DIP) vi cch b tr chn c m t trn hnh 1. Nh vy l v thc cht th b gim st DS2438 khng phi l mt cm bin c thit k tch hp vi mt cm bin m thng thng hnh thnh cm bin 1 dy, l cm bin m HIH-3605 ca cng ty Honeywell. V vy trc khi tm hiu bn cht mt dy ca DS2438 ta nn tm hiu chi tit v cm bin HIH-3605 Nhng c tnh chnh ca HIH-3605 c th k ra l:

in p li ra thay i tuyn tnh theo %RH Kh nng thay th c tinh chnh bng k thut laze

Tiu tn nng lng t chnh xc cao (2% RH, trong khong 0-100% RH khng b ng nc, 25oC v in p ngun nui 5 VDC) Thi gian p ng nhanh Hot ng n nh, tri thp Chu tc ng ca ho cht.

Cm bin m HIH-3605 l mt vi mch tch hp n khi (monolithic) c thit k cho mc ch OEM (Original Equipment Manufac-turer) vi s lng ln. Cho n nay cm bin m HIH-3605 l mt trong s t cc cm bin tch hp c nhiu c tnh u vit nh: chnh xc, daei o rng, khng b ng nc v khng t. Do c quan h tuyn tnh gia m tng i v in p li ra nn c th u cm bin trc tip vo mt li vo ca vi iu khin. Vi dng tiu th in hnh ch c 200 mA, cm bin HIH3605 t ra l mt linh kin l tng s dng trong cc thit b o cm tay dng ngun nui bng pin. Cm bin m HIH-3605 thng c ng gi khi xut xng di dng vi mch hai hng chn SIP (Single In-line Package), c th hn c. Cu to ca cm bin HIH-3605 c m t trn hnh 2. Mt c im c th d dng nhn thy l hai lp platin xp c xen k bng mt lp polyme dn nhit (thermoset). Pha trn li c thm mt lp polyme loi b nh hng ca bi bn v chng nc ng li khi c m 100%. Vi cch cu ny, cm bin m tng i HIH-3605 v thc cht l mt phn t in dung c lp cch in c ch to bng vt liu polyme cch nhit thermoset c vi chnh bng cng ngh laze trong qu trnh ch to (laser trimmed thermoset), vi cc phn t nh dng tn hiu c tch hp ngay trn chip. Cm bin HIH-3605 c nhng bin tng khc nhau, ch yu lin quan n dng ng v v c tnh ca tn hiu li ra, c phn bit bng cc ch ci b sung nh A, A-CP, B v B-CP.

Hnh 2: Cu trc ca cm bin m HIH-3605. Do c im thit k, cc cm bin HIH 3605 rt nhy cm vi s phng in tnh in v vt nn cn thn khi nh dng tay cm cc cm bin loi ny.

Mt s cc thng s cn quan tm n khi thet k cc thit b o m dng cm bin HIH-3605 c th k ra l: midity tuyn tnh RH: 0.5% RH (in hnh) tr RH: 1.2% RH cc i lp li RH: 0.5% RH Thi gian p ng RH: 15 sec n nh ca gi 1% RH, khi m c gi tr o: c nh 50% trong 5 nm (!) in p ngun nui: 4-5,8 V, gi tr nh chun ghi 5 VDC Dng tiu th: 200 mA 5 VDC, 2 mA 9 VDC

Hnh 3: Mt cm bin mt dy c kch thc nh c th pht hin c s thay i m do nc bc hi t tay ngi, trn th ch ra s trhay i m tng i t 38% ln 81%. Mt im ng lu khi s dng cc cm bin m loi ny l phn ng ca cm bin tng i nhy vi nh sng; khi chiu nh sng c cng ln vo bn ngoi cm bin c th gy nn s tri gi tr n 3% 90% RH. V vy cc cm bin m thng c ng gi trong mt hp bng nha (xem hnh 5). Cm bin HIH 3085 c ng gi thnh cm bin mt dy c kch thc nh hn u ngn tay (xem hnh 3), Trn v cm bin ta co th nhn r dng ch 1-wire H (H: Honeywell). Bng loi cm bin ny ngi ta th o s thay i m do nc bc ra t lng bn tay khi c t thm vo mt cm bin m. T th trn hnh 4, m thay i t 38% RH ln n gi tr 81% RH.

Hnh 4: th m t s thay i m do hi nc bc ra t tay ngi cm cm bin. Thng thng, cm bin mt dy trn c s HIH-3605 c ng v trong mt hp nha c kch thc khng nh. Trn hnh 5 l mt cm bin m tng i mt dy ang c gii thiu trn th trng, khi ch cn hai dy dn li ra. Loi cm bin ny khi ng v chung vi mt cm bin nhit c gi khong 199 la M.

Hnh 5: Mt cm bin m mt dy c gi thiu trn th trng. hnh thnh mt cm bin m mt dy, cm bin Honeywell HIH-3605 c ghp ni vi b gim st in p DS2438 theo mch in trn hnh 6. Phin bn thu nh ca loi cm bin ny tch hp hai chip trong hnh v ngay cnh nhau trn mt bn mch. Khi

t cm bin s ch c hai ng dn ra: mt ng ni t (GND) v mt ng dng chung cho d liu (DATA) v in p ngun nui.

Hnh 6: Kt ni cm bin m vi b gim st DS2438 to ra cm bin m mt dy. o m tng i bng cm bin 1 dy Hot ng ca mch in c th m t ngn gn nh sau: Ngun nui cho cm bin c cp t nhm cc linh kin CR1 v C1. tnh ra m tng i b gim st DS2438 c gi tr in p st trn cm bin Vdd, v in p li ra ca cm bin Vad. ng thi DS2438 cng to ra tn hiu b tr nhit t mt cm bin nhit 13 bit tch hp sn trn chip. Cp linh kin R1 v C2 ng vai tr mt b lc thng thp. Trong mch in trn hnh 5 t in C1 v mt trong cc it Schottky trong nhm CR1 to thnh mt b chnh lu na chu k cung cp in p nui c cm bin v vi mch gim st DS2438 bng cch nhp (stealing) in p ny t bus khi in p t gi tr 5 V trong cc khong thi gian khng din ra qu trnh truyn thng tin. y l cch cp ngun nui k sinh c s dng ph bin trong cc linh kin 1 dy nhm to ra in p ngun nui cm bin t hot ng. it Schottky cn li ni gia ng DATA v t (GND) m nhn nhim v bo v mch bng cch ghim khng cho mc tn hiu xung thp hn in p t qu 0,4 V. Di s iu khin ca vi mch ch bus U1, DS2438 gim st in p ngun nui cp cho U2, cm bin m v in p Vdd (pin 5) dng nui chnh b gim st. Tip b gim st o in p li ra ca U2 v trn chn vo analog (chn 4). Cui cng, b gim st DS2438 thng bo nhit tnh theo C t li ra ca b bin i 13 bit c tch hp trn chip. Tip phn mm np sn trn vi iu khin hoc my tnh s s dng cc d liu ny tnh ton v hin th gi tr ng ca m tng i. im I/O DATAchnh l mt li vo ca vi iu khin. Khi vic tnh ton gi tr m tng i a ra hin th c th c m t tm tt nh sau: in p li ra ca cm bin m Honeywell HIH-3605 l in p analog c bin t l vi in p ngun nui. Do bin i trc tip bin in p thnh m tng

i RH cn phi bit (bng cch o !) c in p ngun nui v in p li ra ca cm bin p dng cng thc: RHcb = (VOUT/ VSUPPLY) 0,16/ 0,0062 y RHcb l m tng i cm bin. Cng thc ny c p dng cho nhit 25oC. hiu chnh ht qu do sai lch nhit tin hnh o v do vy s nhn c gi tr ng ca m tng i, ta s dng cng thc: RHtrue = RHcb/ (1,0546 0,00216 T) y T l nhit c tnh ra C. Do DS2438 thng bo nhit theo n v o l C nn vic p dng cong thc trn rt thch hp i vi cc cm bim mt dy. Trong trng hp nhit c tnh theo F th gi tr ng ca m tng i RH c tnh theo cng thc: RHtrue = RHcb/ (1,093 0,0012 T) vi T l nhit c tnh ra F. y l nhng cng thc cn c ch n khi lp trnh x l cc kt qu c c trong vi iu khin v a ra hin th. Nh vy, bng vic tch hp mt cm bin o m thng thng vi mt vi mch gim st in p ta nhn c mt cm bin m mt dy theo ng ngha: d liu v in p ngun nui c truyn ch trn mt ng dn (1 dy). M nhn dng (ID) ca linh kin c to ra t DS2438 cn d liu c ra kt qu c truyn t chn Vdd. Trong phn sau s trnh by tip v cch ghp ni cm bin mt dy vi vi iu khin thnh mt h thng o v t chc ca bus mt dy.

(Phn cui)Khi mun a cc cm bin 1 dy vo s dng ta cn hiu r cch u ni cc cm bin vi nhau v vi cc b phn ph tr nh th no. Cc cm bin mt dy c ni vo bus 1 dy. Trc ht gi l bus v c nhiu thnh vin c u ni vo hay nh thng ni l c nhiu thnh vin tham gia; cn l bus 1 dy v cc thnh vin lin h vi nhau ch thng qua mt dy (khng k ng dy dn ni t). Khi s dng nhiu cm bin th bus mt dy hnh thnh mt mng (network) v cng c nhng c tnh ring. V vy trc ht ta tm hiu v tp ca mng.

Tp mng 1 dy: Mc d mng 1 dy vn thng c xem l "hon ton t do" v mt cu trc, nhng da trn s phn b ca cc cm bin "t" v cch t chc ca cc ng dy ni ta vn c th phn loi ra mt s kiu: 1) Tp ng thng: Bus ng thng l mt ng dn n l, bt u t mun ch v ko di ti thnh vin "t" c t xa nht.

2) Tp phn nhnh: Bus 1 dy l mt ng trc chnh n l bt u mun ch v kt thc thnh vin "t" nm xa nht. Cc thnh vin t khc c gn vo ng trc chnh trn khong cch t 3 m tr ln.

3) Tp hnh sao: Bus 1 dy c phn chia hoc gn mun ch v to ra thnh nhiu nhnh vi nhng di khc nhau, u cui ca mi nhnh l mt thit b "t" nh m t trn hnh v di y.

Mc ch t ra khi thit k cc bus 1 dy l lm sao bus c cu hnh n gin nh c th. Trn thc t, khi thit k cc mng 1 dy ngi ta thng a vo cc in tr m (xem hnh di). in tr ny c t v tr m nhnh c ni vi ng trc chnh. Gi tr ca in tr m cho mi nhnh thng c chn l 150 W.

Mt s thut ng trong mng 1 dy: Khi bn lun v mng mt dy c hai thut ng thng c s dng m t v nh gi tnh nng ca mng. l:

Bn knh: ca mng l khong cch (cng l di ca ng dn) o bng mt v tnh t mun ch n thnh vin xa nht Trng s (weight): ca mng l di tng cng ca cc ng dn kt ni trong mng, tnh bng mt. Chng hn trong mt mng hnh sao c cc nhnh l: 10 m, 20 m v 30 m th bn knh ca mng l 30 m cn trng s ca mng l 60 m. V mt ngha, trng s ca mng s hn ch thi gian tng ca xung trn cp, trong khi bn knh li quyt nh thi gian phn x chm nht ca tn hiu. Thng thng, khng c mt mng 1 dy no c c bn knh ln hn 750 m, v khong cch ny thi gian tr s gy nh hng qu ng n giao thc truyn. C iu l trn thc t li c nhiu h s khc tc ng lm cho bn knh nh hn gi tr ny.Mng chuyn mch

Khi c nhiu thnh vin tham gia bus th mc phc tp ca mng, bn kinh v trng s ca mng cng tng theo. trnh hu qu ny c th s dng mt phng php, theo bus c phn chia ra thnh nhng nhnh ri s dng bin php chuyn mch in t ln lt kt ni tng nhnh vi thnh vin ch (xem hnh v). Nh vy bng vic s dng mt linh kin chuyn mch in t, ti mt thi im mt nhm cc thnh vin c u ni ch theo tp ng thng. Cc nh sn xut cng thit k ring cc linh kin ch dng vo mc ch chuyn mch, chng hn loi DS2409 ca cng ty Dallas (Hoa K). Cng cn lu l trong trng hp mng hnh sao, nu b sung mt linh kin chuyn mch gia cc nhnh ca hnh sao th ti mt thi im ta cng c tp ng thng.

Trong th d trn hnh v, nu s dng mng hnh sao th bn knh ca mng l 150 m, cn trng s l 450 m; trong khi nu s dng chuyn mch in t th bn knh vn gi nguyn nhng trng s gim xung ch cn 150 m. Nhng kt qu, nhng li bn lun v mng khng chuyn mch vn c th p dng cho tng nhnh ca mng chuyn mch. Trn thc t, chng ti thy s dng mng chuyn mch s tn thm mt linh kin chuyn mch (DS2409), nhng vic vit chng trnh cng nh vic a h thng vo ng dng s n gin hn rt nhiu.

Giao tip ca bus 1 dy v mt phn cng vi mun ch tng i n gin. Trong trng hp n gin nht bus 1 dy c ni trc tip vo mt cng vo/ ra ca vi iu khin (xem hnh v trn), ng thi c thm mt in tr ni ln ngun dng (in tr pull-up) ng vai tr ti ca mch cc gp li ra v to mc lgic HIGH mc nh.

Trong trng hp mun ci thin c tnh ca bus ta c th b sung thm mt tranzito MOS nh hnh v trn. im cui cng cn nm vng lm ch bus 1 dy l hiu c qu trnh trao i d liu trn bus. Qu trnh ny din ra trong nhng khong thi gian ngn, c gi l nhng khe thi gian (time slot). Cc khe thi gian c mun ch to ra bng phn cng, nhng thi im to ra v thng s va khe thi gian li do phn mm quy nh, nn trn thc t vic iu khin qu trnh trao i d liu trn bus 1 dy cng khng phc tp. Th d trn hnh v di y m t mt khe thi gian "Read One" hoc "Write One".

c khe thi gian ny, mun ch (thng l vi iu khin hoc PC) ko ng dn bus xung mc lgic thp trong khong thi gian chng 10 micr giy ri tr li mc cao mc nh. Nn ch l sn xung ca xung c iu khin bng mch tc tng/ gim xung (slew rate). Khe thi gian ko di khong 70 micr giy sau li chuyn sang mt khe thi gian khc. Mt th d khc l to xung reset h thng nh m t trng hnh di y.

c c dng xung ny, trong trng hp cn reset h thng, mun ch ko ng dn bus xung mc lgic thp trong 480 micr giy. y l khong thi gian di hn bt k mt khe thi gian no c th gi bus 1 dy mc thp. Tt c cc thnh vin trn bus nhn dng hin tng ny nh l thao tc bo hiu reset bus, ng thi p ng li xung

reset bus bng cch pht ra xung bo hiu c mt (presence pulse). Tt c cc thnh vin "t" trn bus u pht xung trnh din trong cng mt chu k v cc xung s ph ln nhau to thnh mt xung n nh trn hnh v. Mt th d cui cng l thao tc ghi gi tr "0" (Write Zero), gin thi gian c m t nh trn hnh di y:

to khe thi gian ghi gi tr "Zero" mun ch ko ng dn bus xung mc lgic thp trong khong thi gian 60 micr giy, sau tr li mc HIGH mc nh trong 10 micr giy trc khi bt u mt khe thi gian khc. Mt im chung trong ba th d trn l di ca khong thi gian m mun ch ko bus xung mc lgic LOW ng mt vai tr quyt nh i vi loi khe thi gian. Thay li kt lun: Vic tm hiu v lm ch mt h thng bus 1 dy trong mt khong thi gian khng di l mt vic lm khng d dng. Nhng nu hiu c bn cht ca qu trnh th ta c th tip cn dn dn, t cc h thng vi mt-hai cm bin, thc hin mt vi cu lnh n cc h thng c s lng ln. Vn quan trng nht l c k ti liu hng dn ca tng loi cm bin mt dy nh dng. (Theo: Tp ch THNN)

Trong cm bin mt dy (One wire bus), d liu c truyn theo tng bit, v thu nhn theo tng Bit mt. Khc vi cc chun t dy c li gn khc nh I2C, SPI, 1Wirebus khng c xung ng b, v th s ng b c thc hin qua vic qui nh thi gian mc in p tn ti trn Bus. Trong ng b kiu ny: +ng b khung cuc truyn: c thc hin bi mt xung reset mc thp ko di hn 400us. Ton b h thng 1WB s reset v bo li mt tn hiu sn sng cng bng 1 xung thp trn Bus khong hn 400us. Gia hai khong thi gian ny s tn ti mt khang thi gian bus trng thi 1. +ng b bit: Vic ng b bit cng c thc hin da trn vic b x l ch (hoc t) pht ra nhng khe thi gian vi mc logic i nghch vi mc logic pht i. Cc khe thi gian c qui nh rt cht ch v thi gian tn ti trn bus. Ni chung hiu r hn cn c k Datasheet DS1820. +Nhng im c bit: -Giao tip ch mt chn. -C kh nng tn dng in p trn tn hiu nh b lc thng thp trong chip lm VCC -o nhit rt chnh xc v n nh vi sai s +-0.5 -Rt nh gn +Nhc im: -Qui nh v thi gian cht ch qu (ang tm cch ni vi my in my tnh nhng ko bit delay1us trong VC++). -Truy cp chm (mt khong hn 1000us) => khng ph hp vi iu khin p ng nhanh. -Hu hu hu...t tin qu i mt trong khi sinh vin th ...khi ni (mua khong 50->60k Tng Lai) Vi dng lan man... Cho cc bn mi tm hiu mt on m ngun (xi OK lun) vit bng KeilC. //*********** #include unsigned char nhietdo; void delay(unsigned char time) { while(time) { time--; } } //********************8 void write_time_slot(bit data_bit) { DQ = 0; _nop_(); _nop_(); DQ = data_bit; delay(3); DQ = 1;

_nop_(); _nop_(); _nop_(); } //******************** bit read_time_slot() { bit dummy; DQ = 0; _nop_(); _nop_(); DQ = 1; _nop_(); _nop_(); _nop_(); _nop_(); _nop_(); _nop_(); _nop_(); _nop_(); _nop_(); dummy = DQ; delay(7); return dummy; } //******************** void reset() { DQ = 0; delay(300); DQ = 1; } //******************** //void device_available() { //Ham nay han che dung de phong truong hop bi treo // reset(); // delayus(70); // while(DQ) {} //} //******************** void write_byte_1820(unsigned char data_byte) { bit dummy_write; char i; for (i=0;i> 1; } DQ = 1;

delay(10); } //******************** int read_temp() { bit dummy_read; char i; int dummy_data, data_read = 0; for (i=0;i 1; } //**************** void main() { while(1) { nhan_temp(); P0 = nhietdo; //Hin th d liu ra led n } }

Bng Assempler: dq bit p2.7 gnd bit p2.6 dem equ r2 ;@@@@@@@@@@@@@@@@@@@@@ org 0000h jmp CHUONG_TRINH_CHINH write_time_slot: clr dq nop nop mov dq, c mov r5, #30 lcall delay setb dq nop nop ret read_time_slot: clr dq nop nop setb dq nop nop nop nop nop nop nop nop mov c, dq mov r5,#50 lcall delay ret reset_ds1820: clr dq mov r5,#255 lcall delay setb dq mov r5,#255 lcall delay ret write_byte: mov dem,#8

write_lai: rrc a lcall write_time_slot djnz dem, write_lai setb dq mov r5,#50 lcall delay ret read_byte: mov dem,#8 read_lai: lcall read_time_slot rrc a djnz dem, read_lai mov r5,#50 lcall delay ret delay: djnz r5,$ ret delay_long: mov r6,#0ffh delay_lai: lcall delay djnz r6, delay_lai ret CHUONG_TRINH_CHINH: setb dq clr gnd again: lcall reset_ds1820 mov a,#0cch lcall write_byte mov a,#44h lcall write_byte lcall reset_ds1820 mov a,#0cch lcall write_byte mov a,#0beh lcall write_byte lcall read_byte mov p0,a lcall reset_ds1820 lcall delay_long jmp again end