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UART communications require the transmitter and receiver, which typically derive baud
rates from independent clock sources, to use the same baud rate. Allowed tolerance on
this baud frequency depends on the details of how the receiver synchronizes to the
leading edge of the start bit and how bit sampling is performed.
The MCU resynchronizes to bit boundaries on every high-to-low transition. In the worst
case, there are no such transitions in the full 10- or 11-bit or 12-bittime character frame
so any mismatch in baud rate is accumulated for the whole character time. For a
Freescale UART system whose bus frequency is driven by a crystal, the allowed baud
rate mismatch is about ±4.5 percent for 8-bit data format and about ±4 percent for 9-bit
data format. Although baud rate modulo divider settings do not always produce baud
rates that exactly match standard rates, it is normally possible to get within a few percent,
which is acceptable for reliable communications.
33.4.2 Transmitter functional description
This section describes the overall block diagram for the UART transmitter, as well as
specialized functions for sending break and idle characters.
The transmitter output (TxD) idle state defaults to logic high, UART_C3[TXINV] is
cleared following reset. The transmitter output is inverted by setting UART_C3[TXINV].
The transmitter is enabled by setting the TE bit in UART_C2. This queues a preamble
character that is one full character frame of the idle state. The transmitter then remains
idle until data is available in the transmit data buffer. Programs store data into the
transmit data buffer by writing to the UART data register (UART_D).
The central element of the UART transmitter is the transmit shift register that is 10 or 11
or 12 bits long depending on the setting in the UART_C1[M] control bit and
UART_BDH[SBNS] bit. For the remainder of this section, assume UART_C1[M] is
cleared, UART_BDH[SBNS] is also cleared, selecting the normal 8-bit data mode. In 8-
bit data mode, the shift register holds a start bit, eight data bits, and a stop bit. When the
transmit shift register is available for a new UART character, the value waiting in the
transmit data register is transferred to the shift register, synchronized with the baud rate
clock, and the transmit data register empty (UART_S1[TDRE]) status flag is set to
indicate another character may be written to the transmit data buffer at UART_D.
NOTE
Always read UART_S1 before writing to UART_D to allow
data to be transmitted.
If no new character is waiting in the transmit data buffer after a stop bit is shifted out the
TxD pin, the transmitter sets the transmit complete flag and enters an idle mode, with
TxD high, waiting for more characters to transmit.
Chapter 33 Universal asynchronous receiver/transmitter (UART1 and UART2)
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc.
661
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