The USC Andrew and Erna Viterbi School of Engineering USC Ming Hsieh Department of Electrical Engineering University of Southern California

EE 459Lx - Embedded System Design Laboratory

MC908JL16 Baud Rate Calculator

The SCI (Serial Communications Interface) of the MC908JL16 derives the data transfer rate (baud rate) from the clock input to the microcontroller. The clock frequency is divided down by various divisor values to reach the desired baud rate. In many cases the choice of baud rate is determined by the requirements of the device the microcontroller is communicating with. The JL16 can only derive 26 different baud rates from any clock frequency. As a result, the choice of what frequency oscillator to use it often determined by what baud rate is required for the SCI port.

The clock frequency is first divided by four to reach the "bus clock" frequency. It is then divided by the "Prescalar Divisor" (PD). This divisor is determined by a two bit value (SCBR_SCP) that can take on values from 0 to 3 and results in divisor values of 1, 3, 4 or 13. Next it is divided by the "Baud Rate Divisor" (BD). This is determined by a three bit value (SCBR_SCR) that can take on values from 0 to 7 and results in divisor values of 1, 2, 4, 8, 16, 32, 64 or 128. The clock is further divided by a fixed divisor of 64 to reach the final baud rate.

To determine what baud rates are available for a specific frequency oscillator, enter the frequency of the oscillator in the box below (in Hz.) and click on the Submit button. The resulting table will show the values that must be entered into the SCBR_SCP and SCBR_SCR register bits to select that baud rate.

Enter oscillator frequency (in Hz.):