Configurable pulse amplitude modulation clock data recovery

摘要

Embodiments include systems and methods for using generalized pulse amplitude modulation (PAM-X) signaling with an at-rate not-return-to-zero (NRZ) clock data recovery (CDR) system. Some implementations include dual-mode signaling for an at-rate CDR (e.g., using standard NRZ signaling at lower operating frequencies and pseudo-NRZ signaling derived from PAM-X signaling at higher operating frequencies. Embodiments derive an apparent direction of signal transition from PAM-X signaling. The direction can be used to calculate pseudo-NRZ values. For example, when the PAM-X signal transitions in an upward direction, a pseudo-current NRZ value and a pseudo-previous NRZ value of ‘−1’ and ‘+1’ can be generated, respectively. An at-rate NRZ CDR can use the pseudo-NRZ values and a derived error value to make an offset determination. The offset determination can then be used to offset a generated clock signal in the CDR system.

主权项

1. A system for at-rate clock data recovery (CDR), the system comprising:
a clock generator circuit disposed to generate a clock signal according to a received reference clock signal; a PAM (pulse amplitude modulation) to NRZ (not-return-to-zero) circuit disposed to:
sample a received data signal over a plurality of sampling times according to the generated clock signal to generate a plurality of PAM-X values;derive an apparent direction of movement of the data signal and an error value from the plurality of PAM-X values; andcalculate a pseudo-current NRZ value and a pseudo-previous NRZ value according the derived apparent direction of the data signal; and an at-rate CDR circuit disposed to generate an offset determination as a function of the pseudo-current NRZ value, the pseudo-previous NRZ value, and the error value.