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An-5013 gtlp in btl applicationsGTLP in BTL Applications
Today’s high performance systems require fast edge rates Because GTLP is an evolutionary step from the previous and smooth transitions with minimal ringing, overshoot/ designs in BTL and ECL, it shares many of the conceptual undershoot, and other signal integrity issues. The Gunning features and performance characteristics that multi-drop Transceiver Logic Plus (GTLP) family offers a portfolio of applications require. Since the GTLP family provides bi- translation devices and clock drivers that are not only capa- directional translation of LVTTL signal levels and GTLP sig- ble of the fast HL/LH transitions demanded for high speed nal levels, there are varied requirements when driving or backplanes, but also feature a unique edge rate control receiving signals from either of its interface ports. Some of characteristic that ensures smoother transitions. By tuning the desirable features of the GTLP port driver include high the backplane with proper termination resistors and refer- drive capability, low output impedance, controlled output ence voltage techniques, GTLP can act as a drop-in electri- edge rates and live insertion characteristics. It must also cal solution for Backplane Transceiver Logic (BTL) as well provide a completely different set of characteristics for the as support high-speed backplane systems up to 125 MHz LVTTL driver interface such as medium drive levels, and beyond. GTLP’s real advantages occur when the devices are operated at their recommended normal operat-ing specifications. This application note illustrates the abil- Specifications
ity of GTLP to perform in existing BTL applications andevaluates GTLP port interface equivalence with most fami- One of the noted differences between BTL and GTLP is the output and input signal. As depicted in Figure 1, BTL andGTLP differ in their drive and threshold levels. This may Background
seem like a major design problem, when in fact GTLP canoperate at the I/O levels specified by BTL. At these levels, BTL and GTLP signaling can be seen as derivations of the drive characteristics of two technologies look very com- ECL/PECL signaling. Each of them addresses “open col- patible. The output IOL characteristics of BTL and GTLP lector” or “open drain” control of a multi-drop signaling envi- can be plotted together. The curves in Figure 2 show that ronment with the benefit of a pull-up termination for output GTLP has stronger drive at the critical low output voltage high signal levels. The benefit of “open drain” technology is levels. The stronger drive will provide more noise margin that the output is either sinking current or in high imped- ance state (3-STATE) but never sources current. It can alsoprovide translation from TTL/LVTTL signaling.
The evolution from ECL to BTL to GTLP was an effort tosolve power consumption problems in earlier applicationswhile providing near-equivalent performance. ECL is apure bipolar design and BTL designs are BiCMOS. GTLPwas the first all CMOS solution and provides the mostpower savings of the two previous device designs.
FIGURE 1. Input/Output Signal Level Comparison
2001 Fairchild Semiconductor Corporation Specifications (Continued)
Using GTLP at BTL voltage levels results in a maximumfrequency similar to BTL of about 40 MHz. When used atthe recommended voltage levels, however, GTLP’s outputcontrol circuitry allows operation at frequencies exceeding100 MHz.
By controlling the device’s output edge rate, much lessovershoot and undershoot characteristics are seen thanwith fast signal transitions in medium to high performancebackplanes. The ability to set different rise and fall timescompensates for various loads. Therefore the output edgerate control feature coupled with an adequate terminationscheme allows the system designer to fine tune the outputedge for optimum performance. Such features are foundon the GTLP16T1655.
FIGURE 2. Output I
Proper termination of the system is important for optimizing OL Characteristics
performance. The backplane impedance will vary from sys- From a datasheet point of view, GTLP and BTL provide dif- tem to system depending on stub lengths, number of slots, ferent reference from which they derive their specifications.
slot pitch, and types of connectors used. The recom- BTL follows the standards set by various standards’ bodies mended termination technique for GTLP is a Thevenin ter- and its specifications are set by those. GTLP is more flexi- mination. This termination scheme uses two resistors in ble in which standard it needs to set its specification. Ini- parallel, one at each end of the backplane. The resulting tially GTLP followed an established specification set by parallel value of the resistor equals half that of the trans- Intel in their microprocessor interface designs. Intel set mission line. This type of termination reduces overshoot these specifications in an effort to optimize on-board signal and improves the noise margin of the system, both of integrity characteristics. However, off-board applications which are critical for incident wave operation.
remain more “open” in nature, and the flexibility of theGTLP output allows it to interface to a number of interfacesignal levels.
Interface signals in today’s high-speed systems often have The output structure of GTLP is designed for a 50Ω termi- to contend with edge rates becoming faster and refections, nation tied at each end of the backplane. This parallel ter- ringing, crosstalk, and ground bounce becoming more evi- mination configuration is a 25Ω equivalent load. Single dent. Interface signal levels, through their noise margins, ended termination in a bi-directional backplane will result in become important in ensuring system stability. GTLP non-incident wave switching which can cause signal integ- devices, such as the GTLP6C816, offer comparable noise rity problems. With a 25Ω Thevenin equivalent load, the margins to its BTL predecessors. The combination of a static current demand with GTLP is 34 mA. Since BTL typi- 50mV threshold margin coupled with a wider V cally has load impedances of 19.5Ω (39Ω/2 for parallel ter- margin means GTLP can operate within BTL signal level mination), the IOL is 51mA. Table 1 shows the relationship specifications. Figure 1 shows the lower noise margin to be between VOL and IOL with each technology.
400mV for GTLP compared to 375mV for BTL.
TABLE 1. IOL Demand
BTL and GTLP both need a termination voltage (V voltage pin (VREF) which is an input threshold voltage used to reference an incoming BTL/GTLP signal. As a starting Note 1: IOL = (VTT − VOL)/RT
point for prototype designs, Fairchild recommends that Many factors, both internal and external, affect the power VREF be generated from VTT with the use of a simple volt- consumption of a device: (1) number of outputs simulta- age-divider circuit. By tying VREF to VTT, at any point that a neously switching, (2) operating frequency of the system, signal falls below the threshold region, incident wave (3) junction temperature, and (4) capacitive load. The out- switching can still be achieved by lowering the VTT which put power of both GTLP and BTL are supplied by an exter- consequently lowers VREF. By centering the noise level nal output voltage supply (VTT). When calculating the static between VREF and VTT, the maximum noise immunity can high power consumption of the device, the device power is minimal during a static high event. This is because the out- put is in the high impedance state, and essentially zero cur- uses an external VREF pin, and it is recommended that it be When considering static low drive current, the output power is at the worst case point. As seen in Table 2, GTLP usesless power than its predecessor BTL does. The powerrequirement is increased when considering that two 8-bitBTL devices are needed for one 16-bit GTLP device.
TABLE 2. Static and Dynamic Power Consumption
Replacing BTL with GTLP
GTLP can be used in either high-speed new designs,where signal integrity and smooth edge transitions are cru- cial, or as an electrical drop-in solution for an existing BTL slot. The BTL family has many advantages over older CMOS and TTL logic families when designing large back- planes, and GTLP can be used in much higher perfor- mance backplanes than BTL while offering many of the BTL Appli
Note 2: P = VCC*ICC + n* (VOL*IOL) where n = number of outputs
same characteristics. GTLP has the capability of operating The dynamic power consumption comparison between at a wide range of voltage levels without sacrificing signal BTL and GTLP can be shown graphically as a load line cal- integrity. GTLP can maintain adequate noise margin since culation. The IOL curves in Figure 3 show this comparison.
VREF is derived from VTT. This flexibility allows GTLP to The dynamic power consumption of the two technologies are approximately the same given the same operating con- Many BTL transceivers use a live insertion pin (LI) to sup- ditions, and when GTLP is used at its recommended oper- port hot swapping of cards without disabling the system.
ating conditions (1.0V for VREF) it uses much less power.
This is necessary in some applications when changing or As previously discussed, GTLP offers twice the number of adding daughter cards to a backplane. When the LI pin is bits per device than the BTL technology does.
connected to VCC, the output Schottky diode is reversebiased which minimizes bus loading by reducing thecapacitance.
GTLP also supports live insertion applications. Forinstance, the GTLP16T1655 16-bit universal transceiverhas a VCCbias pin which forces 1V on the output pins. Bypre-biasing the output pins to 1V, the device will preventthe backplane from rail-to-rail voltage swings which causessystem problems upon live insertion of a daughter card.
Today’s high performance systems require fast edge rates
and smooth transitions with minimal ringing, overshoot/
undershoot, and other signal integrity issues. The GTLP
family offers a portfolio of translation devices and clock
drivers that are not only capable of the fast HL/LH transi-
FIGURE 3. IOL Characteristics with AC Load Line
tions demanded for high speed backplanes, but also fea-ture a unique edge rate control characteristic that ensuressmoother transitions.
By tuning the backplane with proper termination resistorsand reference voltage techniques, GTLP can act as a drop-in electrical solution for BTL as well as support high-speedbackplane systems up to 125 MHz and beyond. GTLP’sreal advantages occur when the devices are operated attheir recommended normal operating specifications.
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied andFairchild reserves the right at any time without notice to change said circuitry and specifications.
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