The receiver input is inverted by setting LPUART_STAT[RXINV]. The receiver is
enabled by setting the LPUART_CTRL[RE] bit. Character frames consist of a start bit of
logic 0, seven to ten data bits (msb or lsb first), and one or two stop bits of logic 1. For
information about 7-bit, 9-bit or 10-bit data mode, refer to
. For the
remainder of this discussion, assume the LPUART is configured for normal 8-bit data
mode.
After receiving the stop bit into the receive shifter, and provided the receive data register
is not already full, the data character is transferred to the receive data register and the
receive data register full (LPUART_STAT[RDRF]) status flag is set. If
LPUART_STAT[RDRF] was already set indicating the receive data register (buffer) was
already full, the overrun (OR) status flag is set and the new data is lost. Because the
LPUART receiver is double-buffered, the program has one full character time after
LPUART_STAT[RDRF] is set before the data in the receive data buffer must be read to
avoid a receiver overrun.
When a program detects that the receive data register is full (LPUART_STAT[RDRF] =
1), it gets the data from the receive data register by reading LPUART_DATA. Refer to
for details about flag clearing.
46.4.3.1 Data sampling technique
The LPUART receiver supports a configurable oversampling rate of between 4× and 32×
of the baud rate clock for sampling. The receiver starts by taking logic level samples at
the oversampling rate times the baud rate to search for a falling edge on the RXD serial
data input pin. A falling edge is defined as a logic 0 sample after three consecutive logic
1 samples. The oversampling baud rate clock divides the bit time into 4 to 32 segments
from 1 to OSR (where OSR is the configured oversampling ratio). When a falling edge is
located, three more samples are taken at (OSR/2), (OSR/2)+1, and (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 received 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.
Functional description
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
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NXP Semiconductors
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