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SN74GTL16612ADGGR Scheda tecnica(PDF) 9 Page - Texas Instruments |
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SN74GTL16612ADGGR Scheda tecnica(HTML) 9 Page - Texas Instruments |
9 / 15 page www.ti.com APPLICATION INFORMATION GTL Background Information Input Characteristics GTL Output Characteristics VTT VTT RTT RTT To Other Inputs Input Output GTL Bus SN54GTL16612A, SN74GTL16612A 18-BIT LVTTL-TO-GTL+ UNIVERSAL BUS TRANSCEIVERS SCES187D – JANUARY 1999 – REVISED JULY 2005 GTL was approved as JEDEC standard JESD 8-3 in 1993 and originally was created as a reduced-swing I/O driver technology to support high-speed buses and backplanes. The GTL bus is designed to work with low voltage swings. The input buffer works like an analog comparator rather than like an inverter, which allows the GTL inputs to switch quickly without needing to be driven rail to rail. GTL drivers were designed to pull a 1.2-V signal down to 0.4 V when switched on. This, however, placed the reference voltage for the input comparator at 0.8 V, which made it susceptible to ground-bounce noise. A variant of GTL, called GTL+, is being used to address this noise-margin concern. The GTL+ termination voltage is raised to 1.5 V, with the driver pulling down to a VOL of 0.5 V. This moved the reference voltage to 1 V and out of the range of most ground bounces. TI GTL devices operate at, and are specified for, both GTL and the improved-noise-margin GTL+ standard. However, the 'GTL16612A devices deviate from this history. They are designed with slow rising and falling edges, to offer significant system frequency improvement in heavily loaded backplanes. They are AC specified only at GTL+ because most applications are moving to this improved-noise-margin standard; they operate at either GTL or GTL+. Devices named GTL or GTLP indicate reduced voltage-swing operation at a VTT of 1.2 V (GTL standard) or 1.5 V (improved-noise-margin GTL+ standard). Fast-edge GTL devices are best for point-to-point or lower-frequency backplanes. Slow-edge GTL devices extend backplane operations to cover even higher frequencies. The input characteristics are identical on both A and B ports. Both ports are very high impedance and have an input diode to provide protection against high negative-voltage spikes. The input diode conducts and prevents more sensitive components from being destroyed as the result of electrostatic discharges or line reflections. The principle of the GTL bus is based on open-drain drivers, as shown in Figure 3. Figure 3. GTL Bus: An Open-Drain Bus The devices actively drive the bus low, whereas, the termination voltage source (VTT) pulls it high. Only the pullup resistor (RTT), which usually is of a low resistance, limits the current. The pullup resistor value should match the fully loaded backplane impedance, not the trace impedance, to provide an optimum termination of the bus and avoid line reflections. The resistance of the GTL output is in the range of a few ohms. However, in the high state, the output transistor is in the high-impedance state. RTT needs to be greater than 25 Ω at GTL+ signal levels, not to exceed the 'GTL16612A absolute maximum output current of 80 mA, and should be greater than 50 Ω at GTL+ signal levels, not to exceed the recommended output current limit of 34 mA. 9 |
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