UART — Universal asynchronous receiver/transmitter

Listed here are the main features of UART.

The UART implements support for the following features:

• Full-duplex operation
• Automatic flow control
• Parity checking and generation for the 9^th data bit

As illustrated in Figure 1, the UART uses the TXD and RXD registers directly to transmit and receive data. The UART uses one stop bit.

The different signals RXD, CTS (Clear To Send, active low), RTS (Request To Send, active low), and TXD associated with the UART are mapped to physical pins according to the configuration specified in the PSELRXD, PSELCTS, PSELRTS, and PSELTXD registers respectively.

If a value of 0xFFFFFFFF is specified in any of these registers, the associated UART signal will not be connected to any physical pin. The PSELRXD, PSELCTS, PSELRTS, and PSELTXD registers and their configurations are only used as long as the UART is enabled, and retained only for the duration the device is in ON mode. PSELRXD, PSELCTS, PSELRTS and PSELTXD must only be configured when the UART is disabled.

To secure correct signal levels on the pins by the UART when the system is in OFF mode, the pins must be configured in the GPIO peripheral as described in Pin configuration.

Only one peripheral can be assigned to drive a particular GPIO pin at a time. Failing to do so may result in unpredictable behavior.

The UART shares registers and other resources with other peripherals that have the same ID as the UART.

Therefore, you must disable all peripherals that have the same ID as the UART before the UART can be configured and used. Disabling a peripheral that has the same ID as the UART will not reset any of the registers that are shared with the UART. It is therefore important to configure all relevant UART registers explicitly to ensure that it operates correctly.

See the Instantiation table in Instantiation for details on peripherals and their IDs.

A UART transmission sequence is started by triggering the STARTTX task.

Bytes are transmitted by writing to the TXD register. When a byte has been successfully transmitted the UART will generate a TXDRDY event after which a new byte can be written to the TXD register. A UART transmission sequence is stopped immediately by triggering the STOPTX task.

If flow control is enabled a transmission will be automatically suspended when CTS is deactivated and resumed when CTS is activated again, as illustrated in Figure 2. A byte that is in transmission when CTS is deactivated will be fully transmitted before the transmission is suspended. For more information, see Suspending the UART.

A UART reception sequence is started by triggering the STARTRX task.

The UART receiver chain implements a FIFO capable of storing six incoming RXD bytes before data is overwritten. Bytes are extracted from this FIFO by reading the RXD register. When a byte is extracted from the FIFO a new byte pending in the FIFO will be moved to the RXD register. The UART will generate an RXDRDY event every time a new byte is moved to the RXD register.

When flow control is enabled, the UART will deactivate the RTS signal when there is only space for four more bytes in the receiver FIFO. The counterpart transmitter is therefore able to send up to four bytes after the RTS signal is deactivated before data is being overwritten. To prevent overwriting data in the FIFO, the counterpart UART transmitter must therefore make sure to stop transmitting data within four bytes after the RTS line is deactivated.

The RTS signal will first be activated again when the FIFO has been emptied, that is, when all bytes in the FIFO have been read by the CPU, see Figure 3.

The RTS signal will also be deactivated when the receiver is stopped through the STOPRX task as illustrated in Figure 3. The UART is able to receive four to five additional bytes if they are sent in succession immediately after the RTS signal has been deactivated. This is possible because the UART is, even after the STOPRX task is triggered, able to receive bytes for an extended period of time dependent on the configured baud rate. The UART will generate a receiver timeout event (RXTO) when this period has elapsed.

To prevent loss of incoming data the RXD register must only be read one time following every RXDRDY event.

To secure that the CPU can detect all incoming RXDRDY events through the RXDRDY event register, the RXDRDY event register must be cleared before the RXD register is read. The reason for this is that the UART is allowed to write a new byte to the RXD register, and therefore can also generate a new event, immediately after the RXD register is read (emptied) by the CPU.

As indicated in occurrence 2 in the figure, the RXDRDY event associated with byte B is generated first after byte A has been extracted from RXD.

The UART can be suspended by triggering the SUSPEND task.

SUSPEND will affect both the UART receiver and the UART transmitter, i.e. the transmitter will stop transmitting and the receiver will stop receiving. UART transmission and reception can be resumed, after being suspended, by triggering STARTTX and STARTRX respectively.

Following a SUSPEND task, an ongoing TXD byte transmission will be completed before the UART is suspended.

When the SUSPEND task is triggered, the UART receiver will behave in the same way as it does when the STOPRX task is triggered.