Datasheet

Chapter 8 Serial Communication Interface (SCIV3)
MC9S12E128 Data Sheet, Rev. 1.07
262 Freescale Semiconductor
8.4.4 Transmitter
Figure 8-13. Transmitter Block Diagram
8.4.4.1 Transmitter Character Length
The SCI transmitter can accommodate either 8-bit or 9-bit data characters. The state of the M bit in SCI
control register 1 (SCICR1) determines the length of data characters. When transmitting 9-bit data, bit T8
in SCI data register high (SCIDRH) is the ninth bit (bit 8).
8.4.4.2 Character Transmission
To transmit data, the MCU writes the data bits to the SCI data registers (SCIDRH/SCIDRL), which in turn
are transferred to the transmitter shift register. The transmit shift register then shifts a frame out through
the TXD pin, after it has prefaced them with a start bit and appended them with a stop bit. The SCI data
registers (SCIDRH and SCIDRL) are the write-only buffers between the internal data bus and the transmit
shift register.
The SCI also sets a flag, the transmit data register empty flag (TDRE), every time it transfers data from
the buffer (SCIDRH/L) to the transmitter shift register. The transmit driver routine may respond to this
PE
PT
H876543210L
11-BIT TRANSMIT SHIFT REGISTER STOP START
T8
TDRE
TIE
TCIE
SBK
TC
PARITY
GENERATION
MSB
SCI DATA REGISTERS
LOAD FROM SCIDR SHIFT ENABLE
PREAMBLE (ALL ONES)
BREAK (ALL 0s)
TRANSMITTER CONTROL
M
INTERNAL BUS
SBR12–SBR0
BAUD DIVIDER
÷ 16
TDRE INTERRUPT REQUEST
TC INTERRUPT REQUEST
BUS
CLOCK
TE
SCTXD
LOOPS
LOOP
RSRC
CONTROL
TO RECEIVER