27.3.3.2.2 Data Recovery

As with clock recovery, the data recovery unit samples at a rate 8 or 16 times faster than the baud rate depending on whether it is running in Double-Speed or Normal mode, respectively. The figure below shows the sampling process for reading a bit in a received frame.

A majority voting technique is, like with clock recovery, used on the three center samples for deciding the logic level of the received bit. The process is repeated for each bit until a complete frame is received.

The data recovery unit will only receive the first Stop bit while ignoring the rest if there are more. If the sampled Stop bit is read ‘0’, the Frame Error flag will be set. The figure below shows the sampling of a Stop bit. It also shows the earliest possible beginning of the next frame’s Start bit.

A new high-to-low transition indicating the Start bit of a new frame can come right after the last of the bits used for majority voting. For Normal-Speed mode, the first low-level sample can be at the point marked (A) in the figure above. For Double-Speed mode, the first low level must be delayed to point (B), being the first sample after the majority vote samples. Point (C) marks a Stop bit of full length at the nominal baud rate.