Design and Implemen a ion o
a SNTP Clien on FPGA
J. Viejo, J. Juan, M. J. Bellido, E. Os ua, A. Millan, P. Ruiz-de-Cla ijo, A. Mu˜
noz, and D. Gue e o
G upo ID2 (In es igacion y Desa ollo Digi al)
Depa amen o de Tecnologia Elec onica-Uni e sidad de Se illa
E. T. S. Ing. In o ma ica, Campus Uni e si a io Reina Me cedes
41012 Se illa (SPAIN)
Email: [email p o ec ed], [email p o ec ed], [email p o ec ed], [email p o ec ed],
[email p o ec ed], [email p o ec ed], am i[email p o ec ed], [email p o ec ed]
Abs ac — This con ibu ion p esen s he design and imple-
men a ion o a SNTP clien module sui able o IEC 61850
en i onmen s ully done in ha dwa e. The module is able o
p o ide synch oniza ion and accu a e ime e e ence wi hin a mi-
c osecond wi h espec o a SNTP se e , in a ex emely compac ,
cos -e ec i e and low powe de ice comple ely implemen ed in a
low g ade FPGA chip. The e o e i can be an ideal eplacemen o
expensi e compu e -based solu ions o dedica ed GPS ecei e s in
a wide ange o indus ial applica ions. This SNTP clien is pa
o a common echnological pla o m o implemen ing Remo e
Te minal Uni s (RTUs) unde IEC 61850.
I. INTRODUCTION
Time s amping is a c i ical ask in many indus ial con ol
sys ems. Da a acquisi ion by Remo e Te minal Uni s (RTUs)
being a ypical example. In his sense, he indus y no m
IEC 61850 [1] de ines he Simple Ne wo k Time P o ocol
(SNTP) [2] o e E he ne as a s anda d way o synch onize
a se o subs a ions wi h a ime se e . SNTP is a simpli ied
e sion o he mo e gene al Ne wo k Time P o ocol (NTP)
[3] ha is commonly used in In e ne se e s and ou e s.
Bo h SNTP and NTP sha e he same communica ion p o ocol
and da a o ma , he main di e ence being ha NTP uses
sophis ica ed algo i hms ha ensu es a co ec synch oniza ion
wi h mul iple se e s unde highly a iable la ency da a links,
which is common in a wo ld wide ne wo k like he In e ne . On
he con a y, SNTP co e s he synch oniza ion wi h a single
se e and uses a simpli ied s a eless algo i hm, which makes
i sui able o embedded sys ems in a con olled indus ial
en i onmen . Ne e heless, a SNTP clien may communica e
wi h ei he a SNTP se e o a ull NTP se e .
In his scena io, a ime se e will ypically ga he ac-
cu a e ime in o ma ion om an absolu e e e ence like an
accu a e clock o a GPS ecei e . SNTP clien s loca ed a
he subs a ions will synch onize wi h he se e h ough he
Local A ea Ne wo k (LAN) making i unnecessa y o ins all
absolu e e e ences a he subs a ions. SNTP clien s will hen
p o ide he nea by elec onic equipmen wi h he necessa y
ime in o ma ion.
This con ibu ion is pa o a p ojec suppo ed by he
Minis y o Indus y o Spain and leaded by he Tel en
company [4] which inali y is o de elop a Common Tech-
nological Pla o m (PTC) o acili a e he implemen a ion o
he unc ionali y ypically ound in Remo e Te minal Uni s
(RTU) used o con ol he public powe g id. A key poin in he
p ojec is o assu e he synch oniza ion o elec onic equipmen
wi hin he ange o a ew mic oseconds. This synch oniza ion
is achie ed by he use o SNTP clien s and se e s ully
implemen ed in ha dwa e. SNTP clien and se e design is
di ided in h ee main phases:
1) Basic p o ocol s ack implemen a ion and in eg a ion
wi h E he ne con olle . A leas he ollowing p o ocols
a e needed: IP, ARP, UDP, and NTP.
2) NTP clien implemen a ion: issuing o eques s, answe
p ocessing and local clock synch oniza ion.
3) NTP se e implemen a ion: synch oniza ion wi h an
ex e nal e e ence and eques p ocessing.
This pape desc ibes he cu en s a us o he clien imple-
men a ion ha includes phase 1 and pa o phase 2.
The es o his con ibu ion is o ganized as ollows: a
b ie in oduc ion o NTP and SNTP is included in sec ion II,
sec ion III enume a es he equi emen s and gene al speci ica-
ions o he sys em, sec ion IV gi es he de ails o he design
and implemen a ion o he ha dwa e SNTP clien , sec ion V
includes some implemen a ion esul s and sec ion VI discusses
some conclusions.
II. NTP/SNTP PROTOCOL BASIC OPERATION
The ope a ion o he NTP/SNTP p o ocol is e y simple
(Fig. 1). The clien sends a eques o he se e by issuing
an UDP da a packe whe e he ime o i s local clock (T1) is
included. When he eques is ecei ed a he se e a new ime
s amp T2 is gene a ed wi h he ecep ion ime as gi en by he
se e ’s local clock. A e p ocessing he eques , he se e
issues a eply including he ime a which he eply lea es
he se e (T3). When he clien ecei es he eply, he a i al
ime (T4) is also anno a ed. Wi h his se o imes amps he
clien can calcula e he ound ip ime ( d) and he ime o se
be ween he clien ’s and se e ’s clocks ( o se ). Assuming a
symme ic connec ion i gi es:
d = (T4−T1)−(T3−T2)
o se =(T2−T1)+(T3−T4)
2(1)
Fig. 1. Ope a ion o he NTP/SNTP p o ocol.
Using he calcula ed o se , he clien can co ec i s local
clock o ma ch he se e ime. So wa e implemen a ions
o NTP ipically achie e ime synch oniza ion wi hin a mil-
lisecond wi h espec o he se e [3]. The e a e wo main
sou ces o e o . The i s one is he asymme y in he ne wo k
communica ion when he ime spen by he clien ’s eques o
each he se e is di e en o he ime spen by he answe
o each he clien . This is due o unp edic able la ency in
ne wo k equipmen , specially when collisions ake place and
he numbe o he de ices in ol ed inc eases. The second main
sou ce o e o is due o he a iable ime gap be ween he
ins an he ime s amp is egis e ed in he da ag am and he
eal ins an he da ag am lea es o eaches he hos . In ypical
so wa e implemen a ion, hese ime s amps a e egis e ed
by clien /se e so wa e unning as a use le el applica ion
(Fig. 2) so he ime s amp e o will depend on he ime spen
p ocessing he da ag am as i goes h ough he p o ocol s ack
and so wa e laye s. This e o will la gely depend on sys em
load, de ailed so wa e implemen a ion, e c. The p ecision o
he NTP synch oniza ion can be la gely imp o ed by doing he
ime-s amping ope a ion in lowe laye s [5], he e o e some
ope a ing sys em ke nels like Linux o F eeBSD suppo NTP
p ocessing in he ke nel [6]. This way, p ecision may each
some ens o mic oseconds.
The highes p ecision in he ime-s amping ope a ion is only
achie able i done by he E he ne de ice ha dwa e as soon as
he packe s a i e o lea e he in e ace.
III. SYSTEM SPECIFICATION
The main objec i e o his con ibu ion is o build cos -
e ec i e, au onomous, compac and highly accu a e SNTP
clien and se e modules sui able o , bu no limi ed o, IEC
61850 en i onmen s. The SNTP se e will use a s anda d
GPS ecei e as a ime e e ence. I will use he PPS (Pulse Pe
Second) signal and NMEA da a om he GPS o synch onize
i s in e nal clock. The SNTP clien will synch onize wi h he
se e h ough he local a ea ne wo k using he NTP p o o-
col, and will p o ide a PPS signal and NMEA in o ma ion
Fig. 2. Laye s whe e NTP can be implemen ed.
h ough a se ial in e ace hus emula ing a GPS ecei e . A
ypical scena io is depic ed in Fig. 3 whe e he se e (H-
SNTPD) ga he s he ime om he GPS ecei e and clien s
(H-SNTPC) p o ide ime and synch oniza ion in o ma ion o
emo e e minal uni s (RTU).
Mo e speci ically, he SNTP clien should mee he ollow-
ing c i e ia:
•The clien will ope a e in a s anda d 10/100/1000 MHz
E he ne LAN.
•The clien will be con igu ed au oma ically using he
BOOTP [7] p o ocol so ha he con igu a ion o all he
clien s can be cen alized in a single BOOTP se e .
•The p ecision o he local clock a he clien should
be wi hin 10 µs wi h espec o he se e ’s clock in
op imal condi ions: ha dwa e ime-s amping in he se e
and di ec LAN connec ion wi hou swi ches. In ypical
condi ions (so wa e se e and s anda d swi ch connec-
ion) p ecision should be always wi hin 1 ms.
•The whole clien design should i in a single, low densi y
FPGA chip and should need no addi ional ha dwa e, so
ha cos o sys em pa s will be unde $20 be o e mass
p oduc ion.
•Low powe . Implemen ed in a low densi y, low equency
FPGA, he clien will consume unde 1 W o a e age
powe which is much lowe han a compu e -based im-
plemen a ion ha would consume abou 100 W.
IV. DESIGN AND IMPLEMENTATION
In his sec ion, he mos impo an aspec s o design and
implemen a ion a e commen ed. A diag am o he modules
ha o m he SNTP clien is shown in Fig. 4. We can
dis inguish he ollowing pa s: con ol uni , E he ne MAC
con olle , SNTP clien module, and PPS gene a ion and RMC
ame ansmission module. Nex , we will b ie ly explain he
unc ionali y o each one o hese subsys ems.
The con ol uni is in cha ge o a bi a ing he ope a ion o
he es o he modules in o de o pe o m he adequa e ask
in each momen . The module has been modeled as a Fini e
S a e Machine using Ve ilog coding acco ding o he s uc u e
Fig. 3. Typical scena io o deploying ha dwa e SNTP clien and se e s.
Fig. 4. Block diag am o he SNTP clien .
desc ibed in [8]. The con ol uni de ines wo main ope a ing
modes:
1) Con igu a ion. When he SNTP clien s a s o ope a e
o a e a sys em ese , an au oma ic con igu a ion p o-
cess is pe o med acco ding o he Boo s ap P o ocol
(BOOTP) [7]. This p ocess consis s o inding he SNTP
clien MAC add ess and a se ies o con igu a ion pa-
ame e s like SNTP clien and se e IP add esses and
he RS-232 se ial po baud a e. We ha e used BOOTP
because o i s simplici y compa ed o DHCP [9] which
makes i mo e sui able o be implemen ed in ha dwa e,
while he ex ended capabili ies o DHCP a e no use ul
o he in ended applica ion and would only in oduce
ex a de elopmen and esou ce cos s.
2) No mal ope a ion. Once he con igu a ion p ocess has
inished, he SNTP clien begins o wo k in o he no mal
ope a ion mode. In his mode, he de ice ca ies ou
di e en asks which we a e going o summa ize nex .
Fi s ly, he SNTP clien needs o know he SNTP se e
MAC add ess, so he clien includes a simple implemen-
a ion o he Add ess Resolu ion P o ocol (ARP) [10]; as
well, he clien mus be able o send an ARP eply packe
whene e ano he de ice eques s i s MAC add ess.
Secondly, he clien mus ansmi a ime eques packe
(SNTP message) a secondly in e als. Finally, when
he SNTP clien ecei es he ime eply packe , he
imes amps ob ained a e egis e ed so ha he SNTP
clien module can synch onize he local clock.
The E he ne MAC con olle is in cha ge o con olling a
s anda d Fas E he ne PHY de ice, allowing us o ansmi and
ecei e E he ne ames acco ding o IEEE 802.3 speci ica ion.
The implemen a ion o his module has been ca ied ou
using he T i-mode E he ne MAC IP-co e a ailable om he
OpenCo es p ojec in i s web po al openco es.o g. Mo eo e ,
his IP-co e has a FIFO in e ace o use applica ions which
acili a es he SNTP clien design.
The use in e ace has been de eloped acco ding o he spec-
i ica ion documen [11]. This in e ace has been implemen ed
as a ini e s a e machine coded in Ve ilog, and is o med by
a ansmi e module and a ecei e module. So, on he one
hand, he ansmi e module is able o ansmi h ee di e en
E he ne ames: BOOTP Reques , ARP Reques /Reply and
ime eques packe s, which a e s o ed in a RAM. This module
also includes a memo y upda ing componen ha is in cha ge
o upda ing he di e en packe ields be o e ansmi ing
hem. On he o he hand, he ecei e module is able o iden i y
he ollowing ames: BOOTP Reply, ARP Reques /Reply and
ime eply packe s, uling ou he es o E he ne ames.
The SNTP clien module is in cha ge o calcula ing he
clock o se using he imes amps and synch onizing he local
ime. Addi ionally, a d i con ol is ca ied ou in o de
o imp o e he local clock accu acy. A his momen , his
componen is being de eloped using he Sys em Le el ool
Sys em Gene a o o DSP acco ding o he me hodology
p esen ed in [12].
Finally, he PPS gene a ion and RMC ame ansmission
module is in cha ge o gene a ing a synch oniza ion signal
(PPS+NMEA) which will be sen h ough he se ial po o a
Remo e Te minal Uni . To implemen his module he same
me hodology used o build he SNTP clien module will be
employed.
A he ime o w i ing, he sys em is ully speci ied and he
mos c i ical and complex pa s ha e been implemen ed: he
con ol uni , he use in e ace and pa o he SNTP clien
unc ions.
V. RESULTS
In his sec ion, simula ion and ha dwa e implemen a ion
esul s a e desc ibed in some de ail.
A. Simula ion esul s
In o de o check ha he designs wo ks co ec ly, he
ollowing simula ion p ocess has been ca ied ou . A he
i s s age, he design has been e i ied using Simulink and
ModelSim. Fo he gene a ion o he inpu s imuli, he Sou ce
Blockse o Simulink has been employed. A he second s age,
we ha e used he Xilinx ool ChipScope P o o pe o m he
on-chip e i ica ion o he clien . In his way, we ha e e i ied
TABLE I
HARDWARE IMPLEMENTATION RESULTS ON SPARTAN-3E XC3S500E.
Figu e o me i Usage (%)
Slices 1,122 (24%)
Slice Flip Flops 1,287 (13%)
4 inpu LUTs 1,303 (13%)
Bonded IOBs 35 (15%)
Block RAMs 10 (50%)
GCLKs 6 (25%)
Maximum ope a ion equency 96 MHz
he co ec ansmission and ecep ion o he di e en packe
ypes: BOOTP, ARP, and SNTP messages.
B. Ha dwa e implemen a ion esul s
In his subsec ion, ha dwa e implemen a ion esul s will be
p esen ed. Speci ically, wo igu es o me i will be analyzed:
ha dwa e esou ces and maximum ope a ion equency.
The design has been implemen ed on a Spa an-3E
XC3S500E FPGA. The Table I shows he design esul s in he
cu en de elopmen s age. I is wo h no no e ha al hough he
implemen a ion is no inished, he mos esou ce consuming
blocks a e implemen ed, which a e he use in e ace and he
con ol logic. Conside ing his and he ac ha he e is s ill
plen y o oom o op imiza ion in he al eady implemen ed
ha dwa e, i is expec ed ha he inal esou ce equi emen s
will be e y close o he da a in Table I.
VI. CONCLUSION
The design o a SNTP clien comple ely done in ha dwa e
has been p esen ed. By using a high le el me hodology and
s anda d FPGA echnology i is possible o p oduce a high
accu a e, cos -e ec i e and lexible solu ion o accu a e ime
dis ibu ion and ime s amping in indus ial en i onmen s
ha ag ees wi h in e na ional s anda ds. Fi s p o o ypes a e
expec ed o p o ide synch oniza ion in he ange o he mi-
c osecond in a compac and cheap de ice ha would sub-
s i u e expensi e compu e -based solu ions o dedica ed GPS
ecei e s.
ACKNOWLEDGMENT
This wo k has been pa ially suppo ed by he PROFIT-
MITC PTC FIT-330100-2006-60 p ojec and he Andalusian
Regional Go e nmen ’s EXC-2005-TIC-1023 p ojec .
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