USB3 interface design: the K28.5 sequence as a test pattern

In NRZ ( non return to zero ) signaling, a series of 1's and 0's are used. The probablity of occurence of a digit is 50%. As a result of this there is a relatively high probability of getting a long series of 0's or 1's in the signal. The spectrum of such a sequence contains low frequency content. Consequently high frequency transmission design can become difficult. In order to alleviate this problem data encoding or scrambling is used. A typical technique ( used in USB3 for example) uses 8b/10b encoding. In this case, an 8 bit word is encoded into a 10 bit word. The extra bits are added to make the number of 0's equal to the number of 1's in a given bit interval. Additionally this encoding can also be used to improve BER. ( But that is another posting!). For different applications, different types of encoding may be used as well as test patterns. One of the test patterns ( an ubiquitous one) is the
K28.5 pattern. This pattern is a composite of a K28.5+ and a K28.5- bit word and can be described as follows: K28+ = 1100000101 and the K28.5-: ( The inverse of K28.5+)=0011111010. The complete pattern is thus: 11000001010011111010. In USB 3 circuit design, this pattern is encountered often. Please visit our website at www.signalpro.biz and the engineer's corner for other interesting articles on wireline communications.