Datasheet

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29.5.2 Transmitting Data
The SCI transmitter is enabled if the TX FUNC bit and the TXENA bit are set to 1. If the TX FUNC bit is
not set, the SCITX pin functions as a general purpose I/O pin rather than as an SCI function pin. Any
value written to the SCITD before TXENA is set to 1 is not transmitted. Both of these control bits allow for
the SCI transmitter to be held inactive independently of the receiver.
SCI waits for data to be written to SCITD, transfers it to SCITXSHF, and transmits it. The flags TXRDY
and TX EMPTY indicate the status of the transmit buffers. That is, when the transmitter is ready for data to
be written to SCITD, the TXRDY bit is set. Additionally, if both SCITD and SCITXSHF are empty, then the
TX EMPTY bit is also set.
User can transmit data by:
1. Polling Transmit Ready Flag
2. Receive Interrupt
3. DMA
In polling method, software can poll for TXRDY bit to go high before writing the data to SCITD register.
CPU is unnecessarily overloaded by doing this Polling method. To avoid this user can use either Interrupt
or DMA method. To use interrupt method SET TX INT bit should be set and to use DMA SET TX DMA bit
should be set. Either an Interrupt or a DMA request is generated the moment TXRDY is set. When the
SCI has completed transmission of all pending frames, the SCITXSHF register and SCITD are empty, the
TXRDY bit is set, and an interrupt/DMA request is generated, if enabled. Because all data has been
transmitted, the interrupt/DMA request should be halted. This can be done by either disabling the transmit
interrupt (CLR TX INT) / DMA request (CLR TX DMA bit) or by disabling the transmitter (clear TXENA bit).
NOTE: The TXRDY flag cannot be cleared by reading the corresponding interrupt offset in the
SCIINTVECT0 or SCIINTVECT1 register.
29.6 SCI Low Power Mode
The SCI can be put in either local or global low-power mode. Global low-power mode is asserted by the
system and is not controlled by the SCI. During global low-power mode, all clocks to the SCI are turned off
so the module is completely inactive.
Local low-power mode is asserted by setting the POWERDOWN bit; setting this bit stops the clocks to the
SCI internal logic and the module registers. Setting the POWERDOWN bit causes the SCI to enter local
low-power mode and clearing the POWERDOWN bit causes SCI to exit from local low-power mode. All
the registers are accessible during local power-down mode as any register access enables the clock to
SCI for that particular access alone.
The wake-up interrupt is used to allow the SCI to exit low-power mode automatically when a low level is
detected on the SCIRX pin and also this clears the POWERDOWN bit. If wake-up interrupt is disabled,
then the SCI immediately enters low-power mode whenever it is requested and also any activity on the
SCIRX pin does not cause the SCI to exit low-power mode.
NOTE: Enabling Local Low-Power Mode During Receive and Transmit
If the wake-up interrupt is enabled and low-power mode is requested while the receiver is
receiving data, then the SCI immediately generates a wake-up interrupt to clear the
powerdown bit and prevents the SCI from entering low-power mode and thus completes the
current reception. Otherwise, if the wake-up interrupt is disabled, then the SCI completes the
current reception and then enters the low-power mode.
1452
Serial Communication Interface (SCI) Module SPNU562–May 2014
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