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Figure 75.
Transmit FIFO DMA Request Generation (1 Space)
TX buffer max
Programmable
threshold
Zero byte
DMA active periods,this
does not represent the
DMA signalling
6.6.6
Sleep Mode
UART Modes
SPRU760B
1 space
In UART modes, sleep mode is enabled by writing a 1 to IER[4] (when EFR[4]
= 1).
Sleep mode is entered when
The serial data input line, RX, is idle.
-
The TX FIFO and TX shift register are empty.
-
The RX FIFO is empty.
-
There are no interrupts pending except THR interrupts.
-
Sleep mode is a good way to lower power consumption of the UART, but this
state can be achieved only when the UART is set in modem mode. Therefore,
even if the UART has no functional key role, it must be initialized in a functional
mode to take advantage of sleep mode.
In sleep mode, the module clock and baud rate clock are stopped internally.
Because 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; if data is written to the TX FIFO, it occurs with any
change in the state of the modem input pins. An interrupt is generated on a
wake-up event by setting SCR[4] to 1.
Characters transmitted from
the UART
Example, DMA disabled to illustrate the
end of the transfer and the tx buffer
emptying
Serial Interfaces
UARTs
Time
189
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