Writing 0 to CTRL[TE] does not immediately disable the transmitter. The current
transmit activity in progress must first be completed (that could include a data character,
idle character or break character), although the transmitter will not start transmitting
another character.
48.3.2.1 Send break and queued idle
The LPUART_CTRL[SBK] bit sends break characters originally used to gain the
attention of old teletype receivers. Break characters are a full character time of logic 0,
10-bit to 12-bit times including the start and stop bits. A longer break of 13-bit times can
be enabled by setting LPUART_STAT[BRK13]. Normally, a program would wait for
LPUART_STAT[TDRE] to become set to indicate the last character of a message has
moved to the transmit shifter, write 1, and then write 0 to the LPUART_CTRL[SBK] bit.
This action queues a break character to be sent as soon as the shifter is available. If
LPUART_CTRL[SBK] remains 1 when the queued break moves into the shifter,
synchronized to the baud rate clock, an additional break character is queued. If the
receiving device is another LPUART, the break characters are received as 0s in all data
bits and a framing error (LPUART_STAT[FE] = 1) occurs.
A break character can also be transmitted by writing to the LPUART_DATA register
with bit 13 set and the data bits clear. This supports transmitting the break character as
part of the normal data stream and also allows the DMA to transmit a break character.
When idle-line wakeup is used, a full character time of idle (logic 1) is needed between
messages to wake up any sleeping receivers. Normally, a program would wait for
LPUART_STAT[TDRE] to become set to indicate the last character of a message has
moved to the transmit shifter, then write 0 and then write 1 to the LPUART_CTRL[TE]
bit. This action queues an idle character to be sent as soon as the shifter is available. As
long as the character in the shifter does not finish while LPUART_CTRL[TE] is cleared,
the LPUART transmitter never actually releases control of the LPUART_TX pin.
An idle character can also be transmitted by writing to the LPUART_DATA register with
bit 13 set and the data bits also set. This supports transmitting the idle character as part of
the normal data stream and also allows the DMA to transmit a break character.
The length of the break character is affected by the LPUART_STAT[BRK13],
LPUART_CTRL[M], LPUART_BAUD[M10] and LPUART_BAUD[SNBS] bits as
shown below.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1324 NXP Semiconductors
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