UART/Autobaud Functional Description
12-44
In transmit mode, a DMA request is automatically asserted when FIFO is
empty. This request is deasserted when the number of bytes defined by the
threshold level has been written by the system DMA. The DMA request is
again asserted if the FIFO is able to receive the number of bytes defined by
the threshold.
12.5.6 Sleep Mode
Sleep mode is a low-power, enhanced feature of the UART that can be
enabled by writing a 1 to IER[4] (when EFR[4] = 1).
Sleep mode is entered when:
-
Serial RX data input line is idle.
-
TX FIFO and TX shift register are empty.
-
No interrupts are pending except transmit holding register (THR)
interrupts.
Sleep mode is a good way to lower UART power consumption, but this state
can be achieved only when the UART is set in modem mode. Therefore, even
if the UART does not have a functional key role, it must be initialized in a func-
tional mode to take advantage of sleep mode.
In sleep mode, the module clock and baud rate clock are stopped internally.
Since most registers are clocked using these clocks, the power consumption
is greatly reduced. The module wakes up when any change is detected on the
RX line, when data is written to the TX FIFO, or when there is any change in
the state of the modem input pins. An interrupt can be generated on a wake-up
event by setting SCR[4] to 1.
Note:
Writing to the divisor latches, DLL and DLH, to set the baud clock, BCLK,
must not be done during sleep mode. Disable sleep mode using IER[4]
before writing to DLL or DLH.