NXP Semiconductors Kinetis KE1xZ256, Reference Manual

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When the LPUART receiver is configured to sample on both edges of the baud rate
clock, the number of segments in each received bit is effectively doubled (from 1 to
OSR×2). The start and data bits are then sampled at OSR, OSR+1 and OSR+2. Sampling
on both edges of the clock must be enabled for oversampling rates of 4× to 7× and is
optional for higher oversampling rates.
The falling edge detection logic continuously looks for falling edges. If an edge is
detected, the sample clock is resynchronized to bit times (unless resynchronization has
been disabled). This improves the reliability of the receiver in the presence of noise or
mismatched baud rates. It does not improve worst case analysis because some characters
do not have any extra falling edges anywhere in the character frame.
In the case of a framing error, provided the received character was not a break character,
the sampling logic that searches for a falling edge is filled with three logic 1 samples so
that a new start bit can be detected almost immediately.
46.4.3.2 Receiver wakeup operation
Receiver wakeup and receiver address matching is a hardware mechanism that allows an
LPUART receiver to ignore the characters in a message intended for a different receiver.
During receiver wakeup, all receivers evaluate the first character(s) of each message, and
as soon as they determine the message is intended for a different receiver, they write
logic 1 to the receiver wake up control bit (LPUART_CTRL[RWU]). When RWU bit
and LPUART_S2[RWUID] bit are set, the status flags associated with the receiver, with
the exception of the idle bit, IDLE, are inhibited from setting, thus eliminating the
software overhead for handling the unimportant message characters. At the end of a
message, or at the beginning of the next message, all receivers automatically force
LPUART_CTRL[RWU] to 0 so all receivers wake up in time to look at the first
character(s) of the next message.
During receiver address matching, the address matching is performed in hardware and the
LPUART receiver will ignore all characters that do not meet the address match
requirements.
Table 46-3. Receiver Wakeup Options
RWU MA1 | MA2 MATCFG WAKE:RWUID Receiver Wakeup
0 0 X X Normal operation
1 0 00 00 Receiver wakeup on
idle line, IDLE flag not
set
1 0 00 01 Receiver wakeup on
idle line, IDLE flag set
Table continues on the next page...
Chapter 46 Low Power Universal Asynchronous Receiver/Transmitter (LPUART)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors 1211