SWRU626 December 2025 CC3501E , CC3551E
The UART module provides four I/O signals to be routed to the GPIOs. The following signals are selected through the GPIOnPCFG registers in the IOMUX module.
CTS and RTS lines are active low.
IOMUX must be configured before enabling the UART to avoid unwanted transitions on the input being processed as UART signals. When IOMUX is configured as UART-specific I/Os (RXD, CTS, TXD, or RTS), IOMUX sets static output driver enable to the GPIO (output driver enable = 1 for output TXD and RTS and output driver enable = 0 for inputs RXD and CTS).
To enable and initialize the UART, use the following steps:
This section discusses the steps required to use a UART module. For this example, the UART clock is assumed to be 80MHz, and the desired UART configuration is the following:
The first thing to consider when programming the UART is the BRD because the UART.IBRD and UART.FBRD registers must be written before the UART.LCRH register. The BRD can be calculated using the equation described in Section 17.3.2.
The result of previous equation indicates that the UART.IBRD[15:0] DIVINT bit field must be set to 43 decimal or 0x2B.
The previous equation calculates the value to be loaded into the UART.FBRD register.
With the BRD values available, the UART configuration is written to the module in the following order: