I/O Interfaces & Buses
What is the role of the clock and data recovery (CDR) circuit in a high-speed serial receiver?
- AIt amplifies a clock sent over a separate wire by the transmitter
- BIt extracts clock timing from the transitions of the received data and decides each bit at that timing
- CIt detects data errors and asks the transmitter to resend
Show answer and explanation
Answer: B. It extracts clock timing from the transitions of the received data and decides each bit at that timing
Sending the clock on a separate wire would introduce skew relative to the data. So in high-speed serial links, a CDR circuit watches where the data switches between 0 and 1, generates a clock of matching frequency and phase with a PLL or similar, and samples the data in the middle of each bit. This requires the data to contain transitions often enough, which line coding guarantees.
- A. Most high-speed serial links do not send the clock on a separate wire.
- B. Correct. No dedicated clock wire is needed.
- C. Retransmission is a higher layer's job.