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ML4642 Scheda tecnica(PDF) 11 Page - Micro Linear Corporation |
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ML4642 Scheda tecnica(HTML) 11 Page - Micro Linear Corporation |
11 / 15 page 11 ML4642 Before data will be transmitted to the RXOUT pins of the DTE ports it must meet the unsquelch requirements for the RXIN receiver circuitry. The squelch circuitry at the RXIN differential receiver input performs the same function as that of the TXIN squelch circuitry using the same noise rejection criteria. COLLISION There are two conditions that constitute a collision from the point of view of the ML4642: a) If data is received at the TXIN inputs of both DTE ports simultaneously a local collision occurs within the ML4642. b) If the CDIN input is active at any time other than the inter-packet gap window allowed for the SQE Test function described below. In either of the above circumstances it is necessary for the ML4642 to drive the CDOUT pairs on both DTE ports with the collision signal. The collision signal consists of a 10 MHz +/– 15% square wave matching the AUI specifications and capable of driving a 78ý load. The collision signal shall turn on within 2 bit times of the origination of the collision condition and shall turn off within 2–5 bit times after the collision condition subsides. During a collision condition there are two sources for data to be transmitted to TXOUT, TXIN1 and TXIN2. The highest priority source for data to be transmitted to TXOUT is the TXIN1 receiver. For example if TXIN2 begins transmission then TXIN1 turns on, the collision oscillator will turn on and TXOUT will switch from TXIN2 to TXIN1. If the collision ends by TXIN1 turning off first, TXOUT will switch from TXIN1 to TXIN2, and 2–5 bit times later the collision oscillator will turn off. The MAU port’s CDIN receiver contains squelch circuitry to prevent noise from causing the erroneous detection of a collision signal. A signal on the CDIN pair will not be considered active until it exceeds the same squelch requirements as those of the TXIN receivers. LOOPBACK The loopback function allows the ML4642 to emulate a coaxial transceiver by propagating the TXIN data back out the RXOUT pair of the same DTE port that is sourcing the data as well as the RXOUT pair of the idle DTE port. This allows the Ethernet controller sending the data to monitor its transmit packets and detect network faults. The loopback function is enabled at both DTE ports when the RXLED pin is tied to ground, or 0.6 volts. SQE TEST FUNCTION The Signal Quality Error (SQE) Test function allows the DTE to determine whether or not the collision detection circuitry is functional. After each transmission, during the inter-packet gap time, the collision signal will be activated on the CDOUT pair of the same port as the TXIN pair which received the packet, for typically 1 µs. The SQE function will not be activated on DTE ports of the ML4642 which are in the Jabber state. The SQE function is enabled on both DTE ports when the RXLED/ LPBK/SQE pin is grounded. JABBER The jabber function prevents a babbling transmitter from loading down the network. Within the ML4642 is a jabber timer on each TXIN receiver. Each timer starts at the beginning of a received packet and resets at the end of each packet. If a packet lasts longer than 7 to 20ms the jabber logic disables its corresponding TXIN receiver (thus preventing its data from being retransmitted) and generates a collision signal on the babbling port’s CDOUT pair. When the TXIN pair finally goes idle, a second timer measures 0.5 seconds of idle on TXIN prior to re-enabling the receiver and turning off the collision signal. If the TXIN pair becomes active again before the 0.5 seconds has expired, the timer is reset and measures another 0.5 seconds of idle time. The jabber function can be disabled on both ports by tying the JAB1/JABD pin to ground. LED DRIVERS The ML4642 has six LED driver pins. Each DTE port has a transmit LED and a jabber LED and the MAU port has a receive LED. Additionally, there is a collision LED which indicates the presence of a collision condition. All LED drivers are active low 10mA current sources. The TXLED, RXLED, and CDLED outputs have 50ms pulse stretchers on them to enable the LEDs to be visible. The JLED outputs do not have pulse stretchers on them because their conditions occur long enough for the LEDs to be visible. Two of the ML4642 LED outputs serve as configuration pins as well. RXLED/LPBK/SQE and JAB1/JDIS may be tied through a resistor to VCC, tied through a resistor and a LED to VCC or grounded. Additionally RXLED/LPBK/SQE may be tied to a specific voltage. When these pins are grounded or tied to a 0.6 Volts they become configuration inputs. Otherwise when tied high they become status outputs. CASCADING THE ML4642 FOR 4 AND 8 PORT DESIGNS The configurability of such functions as loopback, jabber, and SQE allows ease of cascading multiple ML4642 chips for larger fan-out designs. Figure 10 shows a four port AUI Multiplexer design. For a type 0 configuration both jabber and transmit LEDs are available on a per port basis for status. The RXLED/LPBK/SQE pins are tied through a resistor to 5 volts, and CDLED is wire OR’ED with the other chip for one collision detect status LED per system. There is also only one receive LED status output which is displayed in a type 2 configuration. This particular pin in FUNCTIONAL DESCRIPTION |
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