COM90C66 datasheet(13/76 Pages) SMSC

13 / 76 page

TRANSMISSION MEDIA INTERFACE

The right half of Figure 2 illustrates the

COM90C66 interface to the transmission media

used to connect the node to the network. The

HYC9058/68/88 may be used to drive the

media.

During transmission, the COM90C66

transmits a logic "1" by generating two 100 nS

non-overlapping negative pulses, nPULSE1 and

nPULSE2. These signals are sent to the LAN

Driver, which in turn creates a 200 nS dipulse

signal on the media. A logic "0" is transmitted

by the absence of the two negative pulses, that

is, the nPULSE1 and nPULSE2 outputs remain

high, therefore there is an absence of a dipulse.

During reception the 200 nS dipulse appearing

on the media is coupled through the RF

transformer of the LAN Driver. A positive pulse

at the RXIN pin of the COM90C66 is interpreted

as a logic "1". Again, if no dipulse is present,

the COM90C66 interprets a logic "0".

Typically, RXIN pulse occur at multiples of 400

nS. The COM90C66 can tolerate distortion of

plus or minus 100 nS and still correctly capture

the RXIN pulses.

During Reset, the transmitter portion of the

COM90C66 is disabled and the nPULSE1 and

nPULSE2 pins are inactive high.

The COM90C66 includes the nTXLED and

nBSLED signals which, when tied to LEDs,

provide indication of transmit and board access

activity. In addition, it is possible for the user to

completely

disable

the

transmitter

through

software. These two unique features represent

two

the

improvements

made

the

diagnostics of the device.

Please see the

Improved Diagnostics section of this document

for further detail.

FUNCTIONAL DESCRIPTION

MICROSEQUENCER

The

COM90C66

contains

internal

microsequencer

which

performs

all

the

control operation necessary to carry out the

ARCNET protocol.

It consists of a clock

generator, a 554 x 8 ROM, a program counter,

two instruction registers, an instruction decoder,

no-op

generator,

jump

logic,

and

reconfiguration logic.

The COM90C66 derives a 5 MHz and a 2.5 MHz

clock from the external crystal.

These clocks

provide the rate at which the instructions are

executed within the COM90C66.

The 5 MHz

clock is the rate at which the program counter

operates, while the 2.5 MHz clock is the rate at

which

the

instructions

are

executed.

The

microprogram is stored in the ROM and the

instructions are fetched and then placed into the

instruction registers. One register holds the op

code, while the other holds the immediate data.

Once the instruction is fetched, it is decoded

by the internal instruction decoder, at which

point the COM90C66 proceeds to execute the

instruction.

When

no-op

instruction

encountered, the microsequencer enters a timed

loop, in which case the program counter is

temporarily stopped until the loop is complete.

When a jump instruction is encountered, the

program counter is loaded with the jump

address from the ROM.

The COM90C66

contains an internal reconfiguration timer which

interrupts the microsequencer if it has timed out.

At this point the program counter is cleared,

after which the MYRECON bit of the Diagnostic

Status Register is set.

COM90C66 Datasheet(PDF) 13 Page - SMSC Corporation

COM90C66 Datasheet(HTML) 13 Page - SMSC Corporation

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