Information

48.4.1.1 Transmitter character length
The UART transmitter can accommodate either 8, 9, or 10-bit data characters. The state
of the C1[M] and C1[PE] bits and the C4[M10] bit determine the length of data
characters. When transmitting 9-bit data, bit C3[T8] is the ninth bit (bit 8).
48.4.1.2 Transmission bit order
When the S2[MSBF] bit is set, the UART automatically transmits the MSB of the data
word as the first bit after the start bit. Likewise the LSB of the data word is transmitted
immediately preceding the parity bit (or the stop bit if parity is not enabled). All
necessary bit ordering is handled automatically by the module hence the format of the
data written to the D register for transmission is completely independent of the
S2[MSBF] setting.
48.4.1.3 Character transmission
To transmit data, the MCU writes the data bits to the UART transmit buffer using UART
data registers (C3[T8]/D). Data in the transmit buffer is then in turn transferred to the
transmitter shift register as needed. The transmit shift register then shifts a frame out
through the transmit data output signal after it has prefaced it with any required start and
stop bits. The UART data registers (C3[T8] and D) provide access to the transmit buffer
structure.
The UART also sets a flag, the transmit data register empty flag (S1[TDRE]) and
generates interrupt or DMA request (C5[TDMAS]), whenever the number of datawords
in the transmit buffer is equal to or less than the value indicated by the
TWFIFO[TXWATER]. The transmit driver routine may respond to this flag by writing
additional datawords to the transmit buffer using (C3[T8]/D) as space permits.
See Application information for specific programing sequences.
Setting the C2[TE] bit automatically loads the transmit shift register with a preamble of
10 logic 1s (if C1[M] = 0), 11 logic 1s (if C1[M] = 1 and C4[M10] = 0), or 12 logic 1s (if
C1[M] = 1, C4[M10] = 1, C1[PE] = 1). After the preamble shifts out, control logic
transfers the data from the UART data register into the transmit shift register. The
transmitter automatically transmits the correct start bit and stop bit before and after the
dataword.
Functional description
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
1336 Freescale Semiconductor, Inc.