Chapter 13 Serial Communications Interface (S12SCIV2) Block Description
Freescale Semiconductor
MC9S12C-Family / MC9S12GC-Family
397
Rev 01.24
13.4.3
Transmitter
Figure 13-11. Transmitter Block Diagram
13.4.3.1
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).
13.4.3.2
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
Tx output
signal, 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 flag by
PE
PT
H
8
7
6
5
4
3
2
1
0
L
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
LOOP
RSRC
CLOCK
TE
TO
CONTROL
RXD
LOOPS
TXD
Summary of Contents for MC9S12C Family
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