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:
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 34. 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.
|0000||Disabled. Eye scan is disabled.|
|0x01||Compare. Counts mismatches between the normal sample and the eye scan sample if ES = 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 = 0, and ones when ES= 1.|
|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.|
|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.|
|1x11||Timed Compare. As ES = 001x, but analyses over the time interval specified by ESLEN. Analyses zeroes when ES = 0, and ones when ES = 1.|
When ES = 0, the selected analysis runs continuously. However, when ES = 1, only the number of qualified samples specified by ESLed, as shown in Table 35. In this case, analysis is started by writing a 1 to ESRUN (it is not necessary to set it back to 0). When analysis completes, ESDONE is set to 1.
|ESLen||NUMBER OF SAMPLES ANALYZED|
When ESVO OVR = 1, the ESVO field determines the amount of offset voltage that is applied to the eye scan data samplers associated with rxpi and rxni. The amount of offset is variable between 0 and 300 mV in increments of ~10 mV, as shown Table 36. When ES = 1, ESVO OVR must be 0 to allow the optimized voltage offset to be read back via ESVO.
The phase position of the samplers associated with rxpi and rxni, is controlled to a precision of 1/32UI. When ES is not 00, the phase position can be adjusted forwards or backwards by more than one UI using the ESPO field, as shown in Table 37. In normal use, the range should be limited to ±0.5 UI (+15 to –16 phase steps).
|ESPO||OFFSET (1/32 UI)|