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NE83C92A Datasheet(PDF) 6 Page - NXP Semiconductors |
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NE83C92A Datasheet(HTML) 6 Page - NXP Semiconductors |
6 / 9 page Philips Semiconductors Product specification NE83C92 Low-power coaxial Ethernet transceiver 1995 May 1 6 FUNCTIONAL DESCRIPTION The NE83C92 is a low power BiCMOS coaxial Ethernet transceiver which complies with the IEEE 802.3 specification and offers the following features: 1. Low current consumption of typically 15mA when idling and 70mA while transmitting and no collision allows smaller DC-DC converter to be used for the isolated power supply. (No external pull-down resistors.) 2. Automatic selection of the AUI connector (for remote MAU) instead of the direct local (Thin) coaxial connection is possible by automatically placing the AUI drivers in high-impedance state when the local coaxial cable is disconnected. This eliminates the need for changing a jumper position on the Ethernet board when selecting either one of the connections. (Automatic selection of the local (Thin) connection is done by disconnecting the AUI cable and reconnecting the local coaxial one, which allows the NE83Q92 to automatically activate itself.) 3. High-efficiency AUI drivers for the RX ± and CD± ports automatically power down when idling and are powered up when a receive signal is detected. This is very important/useful for power sensitive applications such as lap-top computers or PCMCIA cards. 4. The NE83C92 advanced AUI driver (RX ± and CD±) design does require external pull-down resistors (500 Ω) to drive a terminated (78 Ω) AUI cable. However, these drivers will operate correctly without the external resistors for integrated/local MAU applications where no AUI cable is used. Hence, they can be retro-fitted into existing 8392 designs with or without external pull-down resistors depending on the application. An extra current of 7mA/output (for 500 Ω resistors) would be generated, by these resistors, regardless of whether the transceiver is idle or responding to traffic. Receiver Functions The receiver consists of an input buffer, a cable equalizer, a 4-pole Bessel low pass filter, a squelch circuit and a differential line driver. The buffer provides high input resistance and low input capacitance to minimize loading and reflections on the coaxial cable. The equalizer is a high pass filter that compensates for the low pass effect of the coaxial cable and results in a flatband response over all signal frequencies to minimize signal distortion. The 4-pole Bessel low pass filter extracts the average DC voltage level on the coaxial cable for use by the receiver squelch and collision detection circuits. The receiver squelch circuit prevents noise on the coaxial cable from falsely triggering the receiver in the absence of a true signal. At the beginning of a packet, the receiver turns on when the DC level from the low pass filter exceeds the DC squelch threshold and the received packet has started with a 01 bit sequence with acceptable timing parameters. For normal signal levels this will take less than 500ns, or 5 bits. However, at the end of a packet, a fast receiver turn off is needed to reject both dribble bits on the coaxial cable and spurious responses due to settling of the on-chip bandpass filter. This is accomplished by an AC timing circuit that disables the receiver if the signal level on the coaxial cable remains high for typically 250ns and only enables the receiver again after approximately .5 µs. Figures 3 and 4 illustrate receiver timing. The differential line driver provides typically +900mV signals to the DTE with less than 7ns rise and fall times. When in idle state (no received signal) its outputs provide <20mV differential voltage offset to minimize DC standing current in the isolation transformer. Transmitter Functions The transmitter has differential inputs and an open collector current driver output. The differential input common mode voltage is established by the CTI and should not be altered by external circuitry. Controlled rise and fall times of 25ns (+5ns) minimize higher harmonic components in the transmitted spectrum, while matching of these rise and fall times to typically 2ns minimizes signal jitter. The drive current levels of the CTI are set by an on-chip bandgap voltage reference and an external 1% resistor. An on-chip isolation diode is provided to reduce the transmitter’s coaxial cable load capacitance. For Thin Ethernet applications, no further external isolation diode is required, since the NE83C92 meets the capacitive loading specifications. For Ethernet applications a further external diode should be added to reduce loading capacitance. The transmitter squelch circuit ensures that the transmitter can only be enabled if the transmitted packet begins with a 01 bit sequence where the negative-going differential signals are typically greater than 225mV in magnitude and 25ns in duration. The transmitter will be disabled at the end of a packet if there are no negative going signals of greater than 225mV for more than typically 150ns. Figure 5 illustrates transmitter timing. Collision Functions The collision detection scheme implemented in the NE83C92 is receive mode detection, which detects a collision between any two stations on the network with certainty at all times, irrespective of whether or not the local DTE is producing one of the colliding signals. This is the only detection scheme allowed by the IEEE 802.3 standard for both repeater and non-repeater nodes. The collision circuitry consists of the 4-pole Bessel low pass filter, a comparator, a precision voltage reference that sets up the collision threshold, a heartbeat generator, a 10MHz oscillator, and a differential line driver. The collision comparator monitors the DC level at the output of the low pass filter and enables the line driver if it is more negative than the collision threshold. A collision condition is indicated to the DTE by a 10MHz oscillation signal at the CD outputs and typically occurs within 700ns of the onset of the collision. The collision signal begins with a negative-going pulse and ends with a continuous high-to-idle state longer than 170ns. Figure 6 illustrates collision timing. At the end of every transmission, the heartbeat generator creates a pseudo collision to ensure that the collision circuitry is properly functioning. This pseudo collision consists of a 1 µs burst of 10MHz oscillation at the line driver outputs approximately 1 µs after the end of the transmission. The heartbeat function can be disabled externally by connecting the HBE (heartbeat enable) to VEE. This allows the CTI to be used in repeater applications. Figure 7 illustrates heartbeat timing. Jabber Functions The jabber timer monitors the transmitter and inhibits transmission if it is active for longer than typically 30ms. The jabber circuit then enables the collision outputs for the remainder of the data packet and for typically 450ns (unjab time) after it has ended. At this point the transmitter becomes uninhibited. Figure 8 illustrates jabber timing. |
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