Extending Reach FPD-Link Interface with Cable Drivers Equalizers
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Extending Reach FPD-Link Interface with Cable Drivers Equalizers
Extending Reach FPD-Link Interface with Cable Drivers Equalizers
National's family embedded clock LVDS SER/DES (FPDlink provides 2-wire serial interface Display applications distances meters. application example illustrated Figure DS90UR241 device. serializes 18-bit color information along with three video control signals (VS, three additional General Purpose (GP0, GP1, GP2) signals into single high-speed FPDLink serial stream. Display example Dual (1280 480) resolution. blanking, required PCLK 40.5 MHz. Thus serial rate times PCLK 1.134 Gbps. applications meters, companion device, DS90UR124, used.
National Semiconductor Application Note 1826 Davor Glisic, John Goldie March 2008
performs Clock Data Recovery (CDR) function recreates wide parallel bus. some applications, longer cables required that beyond capability integrated FPD-Link Line Drivers Receivers. Instead using repeaters which would require circuitry power middle link, long haul cable driver equalizer pair used extend reach cable 100s meters. addition, repeater would normally able restore only signal amplitude would able reduce jitter induced cable losses inter-symbol interference; therefore, repeater approach would provide rather limited extension SER/DES link.
30059501
FIGURE FPD-Link SER/DES Application Example meters less)
Cable Extender Chipset Enables Long Reach Applications SER/DES
DS15BA101 DS15EA101 used Cable Extender chipset enable long reach applications
FPD-Link SER/DES Display Links shown Figure length cable greatly extended more than meters depending upon serial transmission rate also cable characteristics.
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2008 National Semiconductor Corporation
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FIGURE Long Reach FPD-Link SER/DES Application (100 meters more)
Cable Extender Chipset Overview
cable extender chipset consists DS15BA101 DS15EA101 devices. These flexible devices that used with LVDS, LVPECL signaling. DS15BA101 high speed differential buffer with adjustable output amplitude. used cable driving, level translation, signal buffering signal repeating. operates from 1.5+ Gbps. application example, serial line rate roughly Gbps DS15BA101 well suited application. optimal signal amplitude equalization, buffering required. companion device DS15EA101. adaptive equalizer optimized equalizing data transmitted over coaxial, twinax twisted pair cables. operating range equalizer from Mbps 1.5+ Gbps. encoding FPD-Link serial stream, switching rate will remain above Mbps even when sending flat field color data. Equalizer automatically equalizes cable length from zero meters lengths that attenuate signal approximately MHz. important realize that performance DS15EA101's automatic equalization circuitry dependant signal amplitude transmitter cable driver cable launch amplitude. differential cables (twinax twisted pair cables), optimal launch amplitude ±400 (800 mVp-p); coaxial cables, also (VOH-VOL). energy detector circuitry DS15EA101 quantifies energy incoming signal feeds this analog information automatic equalization control circuitry which compares with assumed energy original signal instructs equalizer filter apply certain amount gain high frequency components signal based comparison. deviation from optimal launch amplitude result deviation signal energy from assumed energy original signal either direction will cause equalizer filter assert either excessive insufficient amount gain illustrated
Figure DS15BA101 provide signals with optimal launch amplitude equalization both, differential single ended cables with minimal deviation over process, voltage temperature. This reason FPD-Link Line Driver buffered DS15BA101 application example.
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FIGURE DS15EA101 Output Residual Jitter Function Launch Amplitude Figure shows launch amplitude affects DS15EA101 output residual jitter when equalizing CAT5e cable. clear that having launch amplitude that smaller larger than optimal amplitude (±400 differential cables) results higher output residual jitter. data provided Figure measured using single twisted pair CAT5e cable. other twisted pairs cable were doubly terminated with resistors minimize reflections crosstalk.
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FIGURE Line Driver Output, Equalizer Input, Equalizer Output Patterns Figure illustrates three important pattern locations example application. serial signal repeated level shifted Cable Driver. pattern this location optimal launch amplitude very jitter. After losses long twisted pair cable, pattern essentially closed. signal amplitude attenuated jitter excessive. Equalizer provides gain that compensates cable losses also repeats level shifts recovered pattern. Jitter greatly reduced, signal swing also recovered. FPD-Link receives this signal, performs operation, provides parallel output display. lowest jitter performance. Coupling capacitors should 0.1F greater (i.e. 1F), standard values. DS15BA101 device requires external resistor optimal launch amplitude. value recommended differential applications. DS15BA101 outputs also require resistors pulled VDD. three resistors need have better tolerance ensure minimal variation launch amplitude. long reach cable coupled both ends provide maximum isolation source sink ends cable faults. link also compatible with coupling encoding provided data FPD-Link SER. DS15EA101 device requires external filter shown recommended datasheet. differential outputs include internal pull resistors. Thus common-mode offset outside range DS90UR124 inputs this link needs coupled compatibility reasons. Termination should placed close inputs minimize resulting stub lengths. detailed connection diagram shown Figure
Detailed Connection Diagram
FPD-Link typically loaded with DC100 load output coupled receiving device. line also terminated with termination close DS15BA101 inputs possible. This done minimize resulting stub length. coupling used isolate offset voltage FPD-Link driver (1.2V) from DS15BA101 input which optimized higher input offset
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FIGURE Extended Reach FPD-Link SER/DES Connection Diagram link tested either employing parallel BERT test equipment around this application using on-chip BIST mode DS90UR241/124 chipset. device configured output PRBS stream that detects sets status flag indicate PASS error free transmission. Please refer DS90UR241/124 datasheet details. Figure illustrates cable attenuation unit length various twisted pair cables. This data used idea what kind attenuation signals will experience system. Let's say, CAT5e cable employed FPD-Link application, DVGA application operating 40.5 MHz. serial rate 1.134 Gbps corresponding Nyquist frequency MHz. Based data Figure CAT5e loss about -0.595 dB/m. CAT5e link would attenuate sinusoidal signal about that sinusoidal signal amplitude after CAT5e, amplitude would attenuated Interestingly enough, amplitude square wave would roughly same transmitted over CAT5e. This could proven performing (Discrete Fourier Transform) ideal square wave attenuating each harmonic based Figure data. this application note, important realize that signal amplitudes very small when transmitted over long cables. Small signal amplitudes imply that even small amounts noise corrupt data. equalizer, such DS15EA101 wide band, high gain, analog amplifier-filter that will boost high frequency components attenuated signal regardless whether been corrupted not; thus importance designing noise. Application note AN-1347 details about Layout techniques that ensure well isolated, noise environment adaptive equalizers. This discussion also brings another important cable parameter, near crosstalk (NEXT), type system noise that occurs when more signals being transmitted over same cable bi-directional manner.
Design Considerations
While factors such pattern characteristics, SER/DES characteristics, clock characteristics, connector cable characteristics, power supply noise system noise characteristics important aspects design involving SER/ devices, long reach applications require special attention cable characteristics. There three twisted pair cable parameters that need considered when using cable long reach applications: cable attenuation, multi-pair applications: near-end crosstalk pair-to-pair skew also need taken into account. Cable attenuation measure decrease signal strength along length transmission line. dominant parts skin effect dielectric loss. While skin effect directly proportional square root frequency, dielectric loss directly proportional frequency. Cable attenuation major jitter contributor when transmitting signals over long cables, thus need equalization.
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FIGURE Twisted Pair Cable Attenuation
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ommended operating conditions DS90UR241/124 chipset between TxCLKs (PCLK) MHz. direct connection typically limited meters less. This shown small operating Figure lines chart were obtained from bench test using external parallel BERT test equipment. shows operating margin obtained different types twisted pair cable interconnects with DS15BA101 DS15EA101 devices employed extending length FPD-Link serial link. example DVGA application operating 40.5 MHz, meter lengths possible with higher loss CAT5e cabling. using lower loss cable with individually shielded twisted pairs (CAT7), length extended almost meters. WVGA application, PCLK range, lengths meters possible depending upon cable selected.
FIGURE Twisted Pair Cable NEXT Figure illustrates NEXT various category cables. unshielded twisted pair (UTP) cables, NEXT increases quickly with frequency leaves little room cable attenuation, therefore maximum cable length bi-directional transmission much shorter verses unidirectional transmission such example application. cables with individually shielded foiled twisted pairs (FTP) have much better NEXT performance allow bidirectional transmission frequencies beyond GHz. rule thumb, signal noise ratio attenuation crosstalk ratio cabling industry term), needs 15-20 bi-directional transmission desired. example, back DVGA application where have FPD-Link application over CAT5e cable that attenuates signal want back channel using same cable. back channel would have signal that does couple primary channel more than 45-50 assuming same amplitude both channels. signal that would satisfy this requirement would have have significant frequency components above approximately MHz. pair-to-pair skew parameter that should overlooked when timing relationship between signals being transmitted same twisted cable critical. Novice CAT5e users find themselves surprised when they discover that pair-to-pair skew large (typically meters (TIA/EIA-568-B.2). This high number nanoseconds when dealing with signals running hundreds megabits second. FPD-Link SER/DES example impacted pair-to-pair skew entire serialized single pair thus dismissed.
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FIGURE Far, Fast CAT5e CAT7 Twisted Pair Cables
Conclusion
Using DS15BA101 DS15EA101 Cable Driver Equalizer allows long reach cables used between FPD-Link SER/DES devices. Equalizer provides gain compensate cable losses. Armed with adaptive equalizer's high gain capability good design practices addressed this application note, long reach FPDLink applications designed with ease. long reach capability useful many industrial display applications where Display located great distance from source.
Far, Fast?
Next, bench data collected illustrate operating boundaries part under tested conditions. rec-
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Extending Reach FPD-Link Interface with Cable Drivers Equalizers
Notes
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