In your datasheet MFRC522 Standard performance MIFARE and NTAG frontend you have referred to Table 10. Selectable UART transfer speeds with Accuracy with positive and negative values with only one remark saying The resulting transfer speed error is less than 1.5 % for all described transfer speeds. what is meant by +/- mark on the values and what is btter speed to choose if both fluctuate from - to + values around 9.6 kBd what is the law that our calculation is based on for - and + value I mean an error is an error? i.e. a lost bit, or not? I just do not understand - and + values???