24.9.2 Asynchronous Data Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in Double Speed mode. The figure below shows the sampling of the data bits and the parity bit. Each of the samples is given a number that is equal to the state of the recovery unit.

The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three samples in the center of the received bit: If two or all three center samples (those marked by their sample number inside boxes) have high levels, the received bit is registered to be a logic '1'. If two or all three samples have low levels, the received bit is registered to be a logic '0'. This majority voting process acts as a low pass filter for the incoming signal on the RxDn pin. The recovery process is then repeated until a complete frame is received, including the first stop bit. The receiver only uses the first stop bit of a frame.

The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit of the next frame.

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is registered to have a logic '0' value, the Frame Error (UCSRnA.FE) flag will be set.

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 taken at the point marked (A) in the figure above. For Double Speed mode, the first low level must be delayed to (B). (C) marks a stop bit of full length. The early start bit detection influences the operational range of the receiver.