cycle, and then a logic 0 (the asserted level) during the next cycle. A rising edge is
detected when the input is seen as a logic 0 during one module clock cycle and then a
logic 1 during the next cycle.
38.7.1.2 Clearing RXEDGIF interrupt request
Writing a logic 1 to S2[RXEDGIF] immediately clears the RXEDGIF interrupt request
even if the RxD input remains asserted. S2[RXEDGIF] remains set if another active edge
is detected on RxD while attempting to clear S2[RXEDGIF] by writing a 1 to it.
38.7.1.3 Exit from low-power modes
The receive input active edge detect circuit is still active on low power modes (Wait and
Stop). An active edge on the receive input brings the CPU out of low power mode if the
interrupt is not masked (S2[RXEDGIF] = 1).
38.8 DMA operation
In the transmitter, S1[TDRE] can be configured to assert a DMA transfer request. In the
receiver, S1[RDRF], can be configured to assert a DMA transfer request. The following
table shows the configuration field settings required to configure each flag for DMA
operation.
Table 38-15. DMA configuration
Flag
Request enable bit
DMA select bit
TDRE
TIE = 1
TDMAS = 1
RDRF
RIE = 1
RDMAS = 1
When a flag is configured for a DMA request, its associated DMA request is asserted
when the flag is set. When S1[RDRF] is configured as a DMA request, the clearing
mechanism of reading S1, followed by reading D, does not clear the associated flag. The
DMA request remains asserted until an indication is received that the DMA transactions
are done. When this indication is received, the flag bit and the associated DMA request is
cleared. If the DMA operation failed to remove the situation that caused the DMA
request, another request is issued.
Chapter 38 Universal Asynchronous Receiver/Transmitter(UART)
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
Freescale Semiconductor, Inc.
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