DAC38RF80IAAVR datasheet(57/155 Pages) TI1

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SLASEA3C – DECEMBER 2016 – REVISED JULY 2017

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8.3.14 Error Counter

All receive channels include a 12-bit counter for accumulating pattern verification errors. This counter is

accessible via the ECOUNT IEEE1500 Char field. It is an essential part of the eye scan capability (see the Eye

Scan section).

The counter increments once for every cycle that the TESTFAIL bit is detected. The counter does not increment

when at its maximum value (i.e., all 1s). When an IEEE1500 capture is performed, the count value is loaded into

the ECOUNT scan elements (so that it can be scanned out), and the counter is then reset, provided NCOR is set

high.

ECOUNT can be used to get a measure of the bit error rate. However, as the error rate increases, it becomes

less accurate due to limitations of the pattern verification capabilities. Specifically, the pattern verifier checks

multiple bits in parallel (as determined by the Rx bus width), and it is not possible to distinguish between 1 or

more errors.

8.3.15 Eye Scan

All receive channels provide features which facilitate mapping the received data eye or extracting a symbol

response. A number of fields accessible via the IEEE1500 Char scan chain allow the required low level data to

be gathered. The process of transforming this data into a map of the eye or a symbol response must then be

performed externally, typically in software.

The basic principle used is as follows:

•

Enable dedicated eye scan input samplers, and generate an error when the value sampled differs from the

normal data sample;

•

Apply a voltage offset to the dedicated eye scan input samplers, to effectively reduce their sensitivity;

•

Apply a phase offset to adjust the point in the eye that the dedicated eye scan data samples are taken;

•

Reset the error counter to remove any false errors accumulated as a result of the voltage or phase offset

adjustments;

•

Run in this state for a period of time, periodically checking to see if any errors have occurred;

•

Change voltage and/or phase offset, and repeat.

Alternatively, the algorithm can be configured to optimize the voltage offset at a specified phase offset, over a

specified time interval.

Eye scan can be used in both synchronous and asynchronous systems, while receiving normal data traffic. The

IEEE1500 Char fields used to directly control eye scan and symbol response extraction are ES, ESWORD, ES

BIT SELECT, ESLEN, ESPO, ESVO, ESVO OVR, ESRUN and ESDONE. Eye scan errors are accumulated in

ECOUNT.

The required eyescan mode is selected via the ES field, as shown in Table 29. When enabled, only data from

the bit position within the 20-bit word specified via ES BIT SELECT is analyzed. In other words, only eye scan

errors associated with data output at this bit position will accumulate in ECOUNT. The maximum legal ES BIT

SELECT is 10011.

Table 29. Eye Scan Mode Selection

ES[3:0]

EFFECT

0000

Disabled. Eye scan is disabled.

0x01

Compare. Counts mismatches between the normal sample and the eye scan sample if ES[2] = 0, and matches

otherwise.

0x10

Compare zeros. As ES = 0x01, but only analyses zeros, and ignores ones.

0x11

Compare ones. As ES = 0x01, but only analyses ones, and ignores zeroes.

0100

Count ones. Increments ECOUNT when the eye scan sample is a 1.

1x00

Average. Adjusts ESVO to the average eye opening over the time interval specified by ESLEN. Analyses zeroes when

ES[2] = 0, and ones when ES[2]= 1.

1001

1110

Outer. Adjusts ESVO to the outer eye opening (i.e. lowest voltage zero, highest voltage 1) over the time interval

specified by ESLEN. 1001 analyses zeroes, 1110 analyses ones.

1010

1101

Inner. Adjusts ESVO to the inner eye opening (i.e. highest voltage zero, lowest voltage 1) over the time interval

specified by ESLEN. 1010 analyses zeroes, 1101 analyses ones.

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