NXP Semiconductors MKW01Z128, Reference Manual

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Writing 0 to UART_C2[TE] does not immediately release the pin to be a general-purpose
I/O pin. Any transmit activity in progress must first be completed. This includes data
characters in progress, queued idle characters, and queued break characters.
33.4.2.1 Send break and queued idle
UART_C2[SBK] 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 times
including the start and stop bits. A longer break of 13 bit times can be enabled by setting
UART_S2[BRK13]. Normally, a program would wait for UART_S1[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 UART_C2[SBK]. This action queues a break character to be sent as
soon as the shifter is available. If UART_C2[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 Freescale Semiconductor UART, the break
characters are received as 0s in all eight data bits and a framing error (UART_S1[FE] =
1) occurs.
When idle-line wake-up 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
UART_S1[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 UART_C2[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 UART_C2[TE] is cleared, the UART
transmitter never actually releases control of the TxD pin. If there is a possibility of the
shifter finishing while UART_C2[TE] is cleared, set the general-purpose I/O controls so
the pin shared with TxD is an output driving a logic 1. This ensures that the TxD line
looks like a normal idle line even if the UART loses control of the port pin between
writing 0 and then 1 to UART_C2[TE].
The length of the break character is affected by the UART_S2[BRK13] and
UART_C1[M] as shown below.
Table 33-3. Break character length
BRK13 M SBNS Break character length
0 0 0 10 bit times
0 0 1 11 bit times
0 1 0 11 bit times
0 1 1 12 bit times
1 0 0 13 bit times
1 0 1 14 bit times
Table continues on the next page...
Functional description
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
662 Freescale Semiconductor, Inc.