The operational range of the asynchronous reception depends on the accuracy of the internal baud-rate clock, the rate of the incoming frames, and the frame size (in number of bits). In addition, the operational range of the receiver is depending on the difference between the received bit rate and the internally generated baud rate. If the baud rate of an external transmitter is too high or too low compared to the internally generated baud rate, the receiver will not be able to synchronize the frames to the start bit.
There are two possible sources for a mismatch in baud rate: First, the reference clock will always have some minor instability. Second, the baud-rate generator cannot always do an exact division of the reference clock frequency to get the baud rate desired. In this case, the BAUD register value should be set to give the lowest possible error. Refer to Clock Generation – Baud-Rate Generator for details.
Recommended maximum receiver baud-rate errors for various character sizes are shown in the table below.
D (Data bits+Parity) | RSLOW [%] | RFAST [%] | Max. total error [%] | Recommended max. Rx error [%] |
---|---|---|---|---|
5 | 94.12 | 107.69 | +5.88/-7.69 | ±2.5 |
6 | 94.92 | 106.67 | +5.08/-6.67 | ±2.0 |
7 | 95.52 | 105.88 | +4.48/-5.88 | ±2.0 |
8 | 96.00 | 105.26 | +4.00/-5.26 | ±2.0 |
9 | 96.39 | 104.76 | +3.61/-4.76 | ±1.5 |
10 | 96.70 | 104.35 | +3.30/-4.35 | ±1.5 |
The following equations calculate the ratio of the incoming data rate and internal receiver baud rate: