CS8952 datasheet(23/82 Pages) CIRRUS

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CS8952

CrystalLAN™ 100BASE-X and 10BASE-T Transceiver

Table3. 4B5B CodeViolation Decoding

3.1.1.2

100 Mb/s Loopback

One of two internal 100BASE-TX loopback modes

can be selected. Local loopback redirects the

TXD[3:0] input data to RXD[3:0] data outputs

through the 4B5B coders and scramblers. Local

loopback is selected by asserting pin LPBK, by set-

ting the LPBK bit (bit 14) in the Basic Mode Con-

trol Register (address 00h) or by setting bits 8 and

11 in the Loopback, Bypass, and Receiver Error

Mask Register (address 18h) as shown in Table 4.

Remote loopback redirects the analog line interface

inputs to the analog line driver outputs. Remote

loopback is selected by setting bit 9 in the Loop-

back, Bypass, and Receiver Error Mask Register

(address 18h) as shown in Table 4.

When changing between local and non-loopback

modes, the data on RXD[3:0] will be undefined for

approximately 330 µs.

3.1.2

100BASE-X Repeater Application

The CS8952 provides two low latency modes for

repeater applications. These are selected by assert-

ing either pin BPALIGN or BP4B5B. Both pins

have the effect of bypassing the 4B5B encoder and

decoder. Bypassing the coders decreases latency,

and uses a 5-bit wide parallel code group interface

on pins RXD[4:0] and TXD[4:0] instead of the 4-

bit wide MII nibble interface on pins RXD[3:0] and

TXD[3:0]. In repeater mode, pin RX_ER is rede-

fined as the fifth receive data bit (RXD4), and pin

TX_ER is redefined as the fifth transmit data bit

(TXD4).

BPALIGN can also be selected by setting bit 12 in

Loopback, Bypass, and Receiver Error Mask Reg-

ister (address 18h). BP4B5B can be selected by set-

ting bit 14 of the same register.

Pin BPALIGN causes more of the CS8952 to be

bypassed than the BP4B5B pin. BPALIGN also by-

passes the scrambler/descrambler, and the NRZI to

NRZ converters (see Figure 1). Also, for repeater

applications, pin REPEATER should be asserted to

redefine the function of the CRS (carrier sense) pin.

The REPEATER function may also be invoked by

setting bit 12 in the PCS Sublayer Configuration

For repeater applications, the RX_EN pin can be

used to gate the receive data pins (RXD[4:0],

01000

0110 or 0101 (Note 2)1100

01100

0110 or 0101 (Note 2)1101

10000

0110 or 0101 (Note 2)1110

11001

0110 or 0101 (Note 2)

1111

1. CONTROL code groups become violations when found in the data portion of the frame.

2. Invalid code groups are mapped to 5h unless the Code Error Report select bit in the Loopback,

Bypass, and Receiver Error Mask Register (address 18h) is set, in which case invalid code groups are

mapped to 6h.

Code Violations (RX_ER = 1 or TX_ER = 1)

Name

5-bit Symbol

Normal Mode 4-bit

Nibble

Error Report

Mode 4-bit

Nibble

Comments

Remote

Loopback

(bit 9)

PMD

Loopback

(bit 8)

Function

No Loopback

Local Loopback (toward MII)

Remote Loopback (toward line)

Operation is undefined

Table 4.

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