46.6.1 RXEDGIF description
S2[RXEDGIF] is set when an active edge is detected on the RxD pin. Therefore, the
active edge can be detected only when in two wire mode. A RXEDGIF interrupt is
generated only when S2[RXEDGIF] is set. If RXEDGIE is not enabled before
S2[RXEDGIF] is set, an interrupt is not generated.
46.6.1.1 RxD edge detect sensitivity
Edge sensitivity can be software programmed to be either falling or rising. The polarity
of the edge sensitivity is selected using S2[RXINV]. To detect the falling edge,
S2[RXINV] is programmed to 0. To detect the rising edge, S2[RXINV] is programmed
to 1.
Synchronizing logic is used prior to detect edges. Prior to detecting an edge, the receive
data on RxD input must be at the deasserted logic level. A falling edge is detected when
the RxD input signal is seen as a logic 1 (the deasserted level) during one module clock
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.
46.6.1.2 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.
46.6.1.3 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).
46.7 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.
Chapter 46 Universal Asynchronous Receiver/Transmitter (UART)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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