This slide discusses the need for timers to be used with UARTs. Clock accuracy is important for proper UART communication, with a worst case scenario of Tx being +2% and Rx being –2%; the difference is 4%. The absolute timing error between the transmitter and receiver to avoid a bit error is 5%. Then split that and give half to the TX and half to the RX, which leads to the required timing accuracy of +/- 2.5% for each side. Because the oscillator is guaranteed to +/-2%, that leaves 0.5% to handle baud rate generation error (error due to a non-integer number of timer clocks per UART bit). If baud rates differ more than this, then the received Bytes can have bit errors. The Silicon Labs internal oscillators have accuracy specifications over temperature of 2% that allow them to be used to generate the baud rate clocks without the use of an external crystal. The internal clock used for the baud rate clock is divided down to a lower rate for use as the baud rate clock. The example provided shows the 24.5 MHz clock source relative to the UART baud rate required for the application.