Transmitter Functional Description; Send Break And Queued Idle - Freescale Semiconductor MC9S08PT60 Reference Manual

Functional description
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.

15.4.2 Transmitter functional description

This section describes the overall block diagram for the SCI transmitter, as well as
specialized functions for sending break and idle characters.
The transmitter output (TxD) idle state defaults to logic high, SCI_C3[TXINV] is cleared
following reset. The transmitter output is inverted by setting SCI_C3[TXINV]. The
transmitter is enabled by setting the TE bit in SCI_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 SCI data register (SCI_D).
The central element of the SCI transmitter is the transmit shift register that is 10 or 11 or
12 bits long depending on the setting in the SCI_C1[M] control bit and SCI_BDH[SBNS]
bit. For the remainder of this section, assume SCI_C1[M] is cleared, SCI_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 SCI 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 (SCI_S1[TDRE]) status flag is set to indicate another character may
be written to the transmit data buffer at SCI_D.
Always read SCI_S1 before writing to SCI_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.
Writing 0 to SCI_C2[TE] does not immediately release the pin to be a general-purpose
I/O pin. Any transmit activity in progress must first be completed. This includes data
characters in progress, queued idle characters, and queued break characters.
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NOTE
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
Freescale Semiconductor, Inc.