LM3S818 Luminary Micro, Inc, LM3S818 Datasheet - Page 232

• Manufacturer Part Number: LM3S818
• Description: Lm3s818 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

Universal Asynchronous Receivers/Transmitters (UARTs)
12.2.3
12.2.4
12.2.5
232
Data Transmission
Data received or transmitted is stored in two 16-byte FIFOs, though the receive FIFO has an extra
four bits per character for status information. For transmission, data is written into the transmit
FIFO. If the UART is enabled, it causes a data frame to start transmitting with the parameters
indicated in the UARTLCRH register. Data continues to be transmitted until there is no data left in
the transmit FIFO. The BUSY bit in the UART Flag (UARTFR) register (see page 240) is asserted
as soon as data is written to the transmit FIFO (that is, if the FIFO is non-empty) and remains
asserted while data is being transmitted. The BUSY bit is negated only when the transmit FIFO is
empty, and the last character has been transmitted from the shift register, including the stop bits.
The UART can indicate that it is busy even though the UART may no longer be enabled.
When the receiver is idle (U0Rx or U1Rx is continuously 1) and the data input goes Low (a start bit
has been received), the receive counter begins running and data is sampled on the eighth cycle of
Baud16 (described in “Transmit/Receive Logic” on page 230).
The start bit is valid if U0Rx or U1Rx is still low on the eighth cycle of Baud16, otherwise a false
start bit is detected and it is ignored. Start bit errors can be viewed in the UART Receive Status
(UARTRSR) register (see page 238). If the start bit was valid, successive data bits are sampled on
every 16th cycle of Baud16 (that is, one bit period later) according to the programmed length of
the data characters. The parity bit is then checked if parity mode was enabled. Data length and
parity are defined in the UARTLCRH register.
Lastly, a valid stop bit is confirmed if U0Rx or U1Rx is High, otherwise a framing error has
occurred. When a full word is received, the data is stored in the receive FIFO, with any error bits
associated with that word.
FIFO Operation
The UART has two 16-entry FIFOs; one for transmit and one for receive. Both FIFOs are
accessed via the UART Data (UARTDR) register (see page 236). Read operations of the
UARTDR register return a 12-bit value consisting of 8 data bits and 4 error flags while write
operations place 8-bit data in the transmit FIFO.
Out of reset, both FIFOs are disabled and act as 1-byte-deep holding registers. The FIFOs are
enabled by setting the FEN bit in UARTLCRH (page 244).
FIFO status can be monitored via the UART Flag (UARTFR) register (see page 240) and the
UART Receive Status (UARTRSR) register. Hardware monitors empty, full and overrun
conditions. The UARTFR register contains empty and full flags (TXFE, TXFF, RXFE and RXFF bits)
and the UARTRSR register shows overrun status via the OE bit.
The trigger points at which the FIFOs generate interrupts is controlled via the UART Interrupt
FIFO Level Select (UARTIFLS) register (see page 247). Both FIFOs can be individually
configured to trigger interrupts at different levels. Available configurations include 1/8, 1/4, 1/2, 3/4
and 7/8. For example, if the 1/4 option is selected for the receive FIFO, the UART generates a
receive interrupt after 4 data bytes are received. Out of reset, both FIFOs are configured to trigger
an interrupt at the 1/2 mark.
Interrupts
The UART can generate interrupts when the following conditions are observed:
Overrun Error
Break Error
Parity Error
Framing Error
Preliminary
February 6, 2007