MYSON

TECHNOLOGY

MTD492

MTD492 Revision 3.5 01/07/1997

5/8

3.3. Collision Detection Path

In 10Base2 and 10Base5, a collision condition is determined by the DC level on the cable. The DC average

is extracted by a low pass filter. The output of the filter is compared with an internally set threshold (Vcd) to

determine the collision condition. The internal threshold is set for the reception-collision mode. By

definition, MTD492 will detect collision conditions caused by more than 2 stations simultaneously

transmitting.

The collision detection threshold can be shifted by applying a voltage to the CDS pin. This is typically done

by a resistor divider network between GND and VEE to implement the transmission-collision mode

detection. Transmission-collision differs from reception-collision in that transmission-collision will only

detect a 2-station collision if it is transmitting.

Once a collision is detected, MTD492 turns on the CD+/- pins. These 2 pins are also driven by source

followers, thus requiring external pull-down resistors. However, the followers are disabled during idle time,

and CD+/- will be pulled down to VEE and no current will flow through the external resistors.

Because Vcd is always larger than Vcs, when a collision occurs the reception path will also be turned on.

MTD492 implements an internal timer of 250 msec to detect the extended collision period. When a collision

condition period extends beyond this timer, the reception path is turned off. Any interruption of the collision

will reset the timer.

3.4. Reference and Control

The internal reference of MTD492 is generated by an on-chip bandgap circuit. An external resistor

connected between RR+ and RR- pins is used to set the reference current level. Typically, a precision 1K

Ohm resistor should be used. RR+ is internally shorted to GND, while RR- is biased at around -1.25V.

The heartbeat and jabber functions of MTD492 are implemented in the transmission state machine. The

heartbeat function is controlled by the HBE pin. MTD492 also provides a test mode that disables the jabber

function also controlled by the HBE pin. HBE has a 3-level input. It is biased internally at around -4.5V

(0.5Vee). When HBE is pulled high by a low value resistor (<4.7K), the heartbeat function is enabled; when

it is pulled low, the heartbeat is disabled. If it is driven to the mid-level or left floating, the jabber is disabled.

The exit of the jabber disable state takes about 0.5 sec (jabber reset time).

4.0 ABSOLUTE MAXIMUM RATINGS

DC Supply Voltage(VEE)

-12V

Input Voltage

GND+0.3 to VEE-0.3V

Storage Temperature

Ambient Operating Temperature

ESD Protection except for Pin14

2000V