(OSR/2)+2 to make sure this was a real start bit and not merely noise. If at least two of
these three samples are 0, the receiver assumes it is synchronized to a receive character.
If another falling edge is detected before the receiver is considered synchronized, the
receiver restarts the sampling from the first segment.
The receiver then samples each bit time, including the start and stop bits, at (OSR/2),
(OSR/2)+1, and (OSR/2)+2 to determine the logic level for that bit. The logic level is
interpreted to be that of the majority of the samples taken during the bit time. If any
sample in any bit time, including the start and stop bits, in a character frame fails to agree
with the logic level for that bit, the noise flag (LPUART_STAT[NF]) is set when the
received character is transferred to the receive data buffer.
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.
47.3.3.2 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 is
set, the status flags associated with the receiver, with the exception of the idle bit, IDLE,
when LPUART_S2[RWUID] bit is set, 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.
Chapter 47 Low Power Universal asynchronous receiver/transmitter (LPUART)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
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