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5.7.1.5 Cable Signal Quality Estimation

The cable signal quality estimator keeps a simple tracking

of results of the DSP and can be used to generate an

approximate Signal-to-Noise Ratio for the 100Mb receiver.

This information is available to software through the Link

Diagnostics

Registers

Page

Variance

Control

(VAR_CTRL),

address

1Ah

and

Data

(VAR_DATA),

address 1Bh.

The variance computation times (VAR_TIMER) can be

chosen from the set of {2, 4, 6, 8} ms. The 32-bit variance

sum can be read by two consecutive reads of the

VAR_DATA register. This sum can be used to compute an

SNR estimate by software using the following equation:

SNR = 10log10((37748736 * VAR_TIMER) / Variance).

5.7.2 Link Quality Monitor

The Link Quality Monitor allows a method to generate an

alarm when the DSP adaption strays from a programmable

window.

This could occur due to changes in the cable

which could indicate a potential problem.

Software can

program thresholds for the following DSP parameters to be

used to interrupt the system:

— Digital Equalizer C1 Coefficient (DEQ C1)

— Digital Adaptive Gain Control (DAGC)

— Digital Base-Line Wander Control (DBLW)

— Recovered Clock Long-Term Frequency Offset (FREQ)

— Recovered Clock Frequency Control (FC)

Software is expected to read initial adapted values and

then program the thresholds based on an expected valid

range. This mechanism takes advantage of the fact that

the DSP adaption should remain in a relatively small range

once a valid link has been established.

5.7.2.1 Link Quality Monitor Control and Status

Control of the Link Quality Monitor is done through the Link

Quality Monitor Register (LQMR), address 1Dh and the

Link Quality Data Register (LQDR), address 1Bh of the

Link Diagnostics Registers - Page 2. The LQMR register

includes a global enable to enable the Link Quality Monitor

function. In addition, it provides warning status from both

high and low thresholds for each of the monitored parame-

ters. Note that individual low or high parameter threshold

comparisons can be disabled by setting to the minimum or

maximum values.

To allow the Link Quality Monitor to interrupt the system,

the Interrupt must be enabled through the interrupt control

registers, MICR (11h) and MISR (12h).

5.7.2.2 Checking Current Parameter Values

Prior to setting Threshold values, it is recommended that

software check current adapted values.

The thresholds

may then be set relative to the adapted values. The current

adapted values can be read using the LQDR register by

setting the Sample_Param bit [13] of LQDR, address

(1Eh).

For example, to read the DBLW current value:

1. Write 2400h to LQDR (1Eh) to set the Sample_Param

bit and set the LQ_PARAM_SEL[2:0] to 010.

2. Read LQDR (1Eh). Current DBLW value is returned

in the low 8 bits.

5.7.2.3 Threshold Control

The LQDR (1Eh) register also provides a method of pro-

gramming high and low thresholds for each of the four

parameters that can be monitored.

The register imple-

ments an indirect read/write mechanism.

Writes are accomplished by writing data, address, and a

write strobe to the register. Reads are accomplished by

writing the address to the register, and reading back the

value of the selected threshold. Setting thresholds to the

maximum or minimum values will disable the threshold

comparison since values have to exceed the threshold to

generate a warning condition.

Warnings are not generated if the parameter is equal to the

threshold. By default, all thresholds are disabled by setting

to the min or max values. The following table shows the

four parameters and range of values:

Table 17. Link Quality Monitor Parameter Ranges

Parameter

Minimum Value

Maximum Value

Min (2-s comp)

Max (2-s comp)

DEQ C1

-128

+127

0x80

0x7F

DAGC

+255

0x00

0xFF

DBLW

-128

+127

0x80

0x7F

Freq Offset

-128

+127

0x80

0x7F

Freq Control

-128

+127

0x80

0x7F

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