DP83849CVSSLASHNOPB datasheet(74/101 Pages) TI1

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7.3.7 Variance Control Register (VAR_CTRL), Page 2, address 1Ah

The Variance Control and Data Registers provide control and status for the Cable Signal Quality Estimation function.

The Cable Signal Quality Estimation allows a simple method of determining an approximate Signal-to-Noise Ratio for the

100Mb receiver. This register contains the programmable controls and status bits for the variance computation, which

can be used to make a simple Signal-to-Noise Ratio estimation.

7.3.8 Variance Data Register (VAR_DATA), Page 2, address 1Bh

This register contains the 32-bit Variance Sum. The contents of the data are valid only when VAR_RDY is asserted in the

VAR_CTRL register. Upon detection of VAR_RDY asserted, software should set the VAR_FREEZE bit in the VAR_CTRL

reads of this register are required to get the full value.

Table 49. Variance Control Register (VAR_CTRL), address 1Ah

Bit

Bit Name

Default

Description

VAR_RDY

0, RO

Variance Data Ready Status:

Indicates new data is available in the Variance data register. This

bit will be automatically cleared after two consecutive reads ot

VAR_DATA.

14:4

RESERVED

0, RO

RESERVED: Writes ignored, read as 0.

VAR_FREEZE

0, RW

Freeze Variance Registers:

Freeze VAR_DATA register.

This bit is ensures that VAR_DATA register is frozen for software

reads. This bit is automatically cleared after two consecutive reads

of VAR_DATA.

2:1

VAR_TIMER

0, RW

Variance Computation Timer (in ms):

Selects the Variance computation timer period. After a new value

is written, computation is automatically restarted. New variance

Var_Timer = 0 => 2 ms timer (default)

Var_Timer = 1 => 4 ms timer

Var_Timer = 2 => 6 ms timer

Var_Timer = 3 => 8 ms timer

Time units are actually 217 cycles of an 8ns clock, or 1.048576ms.

VAR_ENABLE

0, RW

Variance Enable:

Enable Variance computation. Off by default.

Table 50. Variance Data Register (VAR_DATA), address 1Bh

Bit

Bit Name

Default

Description

15:0

VAR_DATA

0, RO

Variance Data:

Two reads are required to return the full 32-bit Variance Sum value.

Following setting the VAR_FREEZE control, the first read of this

read will return the high 16 bits of Variance data.

DP83849CVS/NOPB Datasheet(PDF) 74 Page - Texas Instruments

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