The low-frequency mode is selected when UCOS16 = 0. This mode allows the generation of baud rates from low-frequency clock sources (for example, 9600 baud from a 32768-Hz crystal). By using a lower input frequency, the power consumption of the module is reduced. Using this mode with higher frequencies and higher Prescaler settings causes the majority votes to be taken in an increasingly smaller window and, thus, decreases the benefit of the majority vote.
In low-frequency mode, the baud-rate generator uses one Prescaler and one modulator to generate bit clock timing. This combination supports fractional divisors for a baud-rate generation. In this mode, the maximum USCI baud rate is one-third of the UART source clock frequency BRCLK. The timing for each bit is shown in Figures 36-10. For each bit received, a majority vote is taken to determine the bit value. These samples occur at the N/2 – 1/2, N/2, and N/2 + 1/2 BRCLK periods, where N is the number of BRCLKs per BITCLK.
Modulation is based on the UCBRSx setting (see Table 36-2). A 1 in the table indicates that m = 1 and the corresponding BITCLK period is one BRCLK period longer than a BITCLK riod with m = 0. The modulation wraps around after eight bits but restarts with each new start bit.