NXP Semiconductors MC9S12G - Page 671

Serial Communication Interface (S12SCIV5)
MC9S12G Family Reference Manual Rev.1.27
NXP Semiconductors 673
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 writing another byte to the Transmitter buffer (SCIDRH/SCIDRL), while the shift register is still
shifting out the first byte.
To initiate an SCI transmission:
1. Configure the SCI:
a) Select a baud rate. Write this value to the SCI baud registers (SCIBDH/L) to begin the baud
rate generator. Remember that the baud rate generator is disabled when the baud rate is zero.
Writing to the SCIBDH has no effect without also writing to SCIBDL.
b) Write to SCICR1 to configure word length, parity, and other configuration bits
(LOOPS,RSRC,M,WAKE,ILT,PE,PT).
c) Enable the transmitter, interrupts, receive, and wake up as required, by writing to the SCICR2
register bits (TIE,TCIE,RIE,ILIE,TE,RE,RWU,SBK). A preamble or idle character will now
be shifted out of the transmitter shift register.
2. Transmit Procedure for each byte:
a) Poll the TDRE flag by reading the SCISR1 or responding to the TDRE interrupt. Keep in mind
that the TDRE bit resets to one.
b) If the TDRE flag is set, write the data to be transmitted to SCIDRH/L, where the ninth bit is
written to the T8 bit in SCIDRH if the SCI is in 9-bit data format. A new transmission will not
result until the TDRE flag has been cleared.
3. Repeat step 2 for each subsequent transmission.
NOTE
The TDRE flag is set when the shift register is loaded with the next data to
be transmitted from SCIDRH/L, which happens, generally speaking, a little
over half-way through the stop bit of the previous frame. Specifically, this
transfer occurs 9/16ths of a bit time AFTER the start of the stop bit of the
previous frame.
Writing the TE bit from 0 to a 1 automatically loads the transmit shift register with a preamble of 10 logic
1s (if M = 0) or 11 logic 1s (if M = 1). After the preamble shifts out, control logic transfers the data from
the SCI data register into the transmit shift register. A logic 0 start bit automatically goes into the least
significant bit position of the transmit shift register. A logic 1 stop bit goes into the most significant bit
position.
Hardware supports odd or even parity. When parity is enabled, the most significant bit (MSB) of the data
character is the parity bit.
The transmit data register empty flag, TDRE, in SCI status register 1 (SCISR1) becomes set when the SCI
data register transfers a byte to the transmit shift register. The TDRE flag indicates that the SCI data
register can accept new data from the internal data bus. If the transmit interrupt enable bit, TIE, in SCI
control register 2 (SCICR2) is also set, the TDRE flag generates a transmitter interrupt request.