ICS1893 datasheet(51/152 Pages) ICST

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Chapter 7

Functional Blocks

ICS1893 Rev C 6/6/00

June, 2000

ICS1893 - Release

7.5

Functional Block: 10Base-T Operations

When configured for 10Base-T mode, the ICS1893 MAC/Repeater Interface can be configured to provide

either a 10M MII (Media Independent Interface) or a 10M Serial Interface. The Twisted-Pair Interface is

automatically configured to provide a two-level, Manchester-encoded signal at the voltage levels specified

in the ISO/IEC standard. (For more information on the Twisted-Pair Interface, see Section 6.5,

“Twisted-Pair Interface”.)

The 10Base-T and 100Base-TX operations differ as follows. 10Base-T operations are fundamentally

simpler than 100Base-TX operations. The data rate is slower, requiring less encoding than 100Base-TX

operations. In addition, the bandwidth requirements (and therefore the line attenuation issues) are not as

severe as with 100-MHz operations. Consequently, when an ICS1893 is set for 10Base-T operations, it

requires fewer internal circuits in contrast to 100Base-TX operations. (For an overview of 10Base-T

operations, see Section 5.6, “10Base-T Operations”.).

7.5.1

10Base-T Operation: Manchester Encoder/Decoder

During data transmission the ICS1893 acquires data from its MAC/Repeater Interface in either 4-bit nibbles

or as a serial bit stream. The ICS1893 converts this data into a Manchester-encoded signal for presentation

to its MDI, as required by the ISO/IEC specification.

In a Manchester-encoded signal, all logic:

• Ones are:

– Positive during the first half of the bit period

– Negative during the second half of the bit period

• Zeros are:

– Negative during the first half of the bit period

– Positive during the second half of the bit period

During 10Base-T data reception, a Manchester Decoder translates the serial bit stream obtained from the

Twisted-Pair Receiver (MDI) into an NRZ bit stream. The Manchester Decoder then passes the data to the

MAC/Repeater Interface in either serial or parallel format, depending on the interface configuration.

Manchester-encoded signals have the following advantages:

• Every bit period has an encoded clock.

• The split-phase nature of the signal always provides a zero DC level regardless of the data (that is, there

is no baseline wander phenomenon).

The primary disadvantage in using Manchester-encoded signals is that it doubles the data rate, making it

operationally prohibitive for 100-MHz operations.

7.5.2

10Base-T Operation: Clock Synthesis

The ICS1893 synthesizes the clocks required for synchronizing data transmission. In 10Base-T mode, the

MAC/Repeater Interface can provide either a 10M MII (Media Independent Interface) or a 10M Serial

Interface. When the ICS1893 is configured to support a:

• 10M MII interface, the ICS1893 synthesizes a 2.5-MHz clock for nibble-wide transactions

• 10M Serial Interface to the MAC/repeater, the ICS1893 synthesizes a 10-MHz clock

7.5.3

10Base-T Operation: Clock Recovery

The ICS1893 recovers its receive clock from the Manchester-encoded data stream obtained from its

Twisted-Pair Receiver using a phase-locked loop (PLL). The ICS1893 then uses this recovered clock for

synchronizing data transmission between itself and the MAC/repeater. Receive-clock PLL acquisitions begin

with reception of the MAC Frame Preamble and continue as long as the ICS1893 is receiving data.

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