LAN8808D datasheet(37/80 Pages) MICROCHIP

37 / 80 page

LAN8808D

DS00005833C-page 37

Ideally, each receive data sample would be the maximum distance from the compare thresholds, with error values of 0.

But because of noise and imperfection in real applications, the sampled data may be off from its ideal. The closer to the

compare threshold, the worse the signal quality.

The slicer error is a measurement of how far the processed data is off from its ideal location. The largest instantaneous

slicer error for 1000BASE-T is ±32. The largest instantaneous slicer error for 100BASE-TX is ±64.

A higher absolute slicer error means a degraded signal receiving condition.

5.11.2

NON OPEN ALLIANCE LOW PASS FILTERED ERROR

With this method, the slicer error is converted into an absolute value and then filtered by a programmable low pass filter.

This is similar to taking the average of absolute slicer error over a long moving time window.

This mode is enabled by setting the sqi_enable bit in the Dynamic Channel Quality (DCQ) Configuration register.

For each data sample, the difference between the absolute slicer error (scaled by x2 (before squaring) for

1000BASE-T) and the current filtered value is added back into the current filtered value. The sqi_squ_mode_en bit in

the DCQ Configuration register is used to square the (scaled) slicer error.

The sqi_kp field in the DCQ Configuration register sets the weighting of the add back as a divide by 2^sqi_kp, effectively

setting the filter bandwidth. As the sqi_kp value is increased, the weighing is decreased, and the mean slicer error value

takes a longer time to settle to a stable value. Also, as the sqi_kp value is increased, there will be less variation in the

mean slicer error value reported.

The filtered error value is saved every 1.0 ms (125,000 symbols).

In order to capture the current error value, the DCQ Read Capture bit in the DCQ Configuration register needs to be

written as a high with the desired cable pair specified in the DCQ Channel Number field of the same register. The DCQ

Read Capture bit immediately self-clears and the result is available in the Mean Slicer Error register.

A software based lookup table (derived empirically in lab conditions) may be used to report a Signal Quality Index (SQI)

number.

5.11.3

OPEN ALLIANCE TC1/TC12 DCQ MEAN SQUARE ERROR

This section defines the implementation of section 6.1.1 of the TC1 and TC12 specifications. The PHY can provide

detailed information of the dynamic signal quality by means of an MSE value. This mode is enabled by setting the

sqi_enable bit in the DCQ Configuration register.

With this method, the slicer error is converted into a squared value and then filtered by a programmable low pass filter.

This is similar to taking the average of absolute slicer error over a long moving time window.

For each data sample, the difference between the absolute slicer error (scaled by x2 (before squaring) for 1000BASE-

T) and the current filtered value is added back into the current filtered value.

The sqi_squ_mode_en bit in the DCQ Configuration register must be set to choose square mode.

The sqi_kp field in the DCQ Configuration register sets the weighting of the add back as a divide by 2^sqi_kp, effectively

setting the filter bandwidth. As the sqi_kp value is increased, the weighing is decreased, and the mean slicer error value

takes a longer time to settle to a stable value. Also, as the sqi_kp value is increased, there will be less variation in the

mean slicer error value reported.

The scale611 field in the DCQ Configuration register is used to set a divide by factor (divide by 2^scale611) such that the

Mean Squared Error (MSE) value is linearly scaled to the range of 0 to 511. If the divide by factor is too small, the MSE

value is capped at a maximum of 511.

The filtered error value is saved every 1.0 ms (125,000 symbols).

Note:

All registers references in this section are in Extended Page 1.

Note:

All registers references in this section are in Extended Page 1.

LAN8808D Datasheet(PDF) 37 Page - Microchip Technology

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