MC68HC05C9A Datasheet, PDF (55/124 Pages) Freescale Semiconductor, Inc – Microcontrollers

9.4 SCI Transmitter Features
Features of the SCI transmitter include:
• Transmit data register empty flag
• Transmit complete flag
• Send break
SCI Transmitter Features
9.5 Functional Description
A block diagram of the SCI is shown in Figure 9-1. Option bits in serial control register1 (SCCR1) select
the wakeup method (WAKE bit) and data word length (M bit) of the SCI. SCCR2 provides control bits that
individually enable the transmitter and receiver, enable system interrupts, and provide the wakeup enable
bit (RWU) and the send break code bit (SBK). Control bits in the baud rate register (BAUD) allow the user
to select one of 32 different baud rates for the transmitter and receiver.
Data transmission is initiated by writing to the serial communications data register (SCDR). Provided the
transmitter is enabled, data stored in the SCDR is transferred to the transmit data shift register. This
transfer of data sets the transmit data register empty flag (TDRE) in the SCI status register (SCSR) and
generates an interrupt (if transmitter interrupts are enabled). The transfer of data to the transmit data shift
register is synchronized with the bit rate clock (see Figure 9-2). All data is transmitted least significant bit
first. Upon completion of data transmission, the transmission complete flag (TC) in the SCSR is set
(provided no pending data, preamble, or break is to be sent) and an interrupt is generated (if the transmit
complete interrupt is enabled). If the transmitter is disabled, and the data, preamble, or break (in the
transmit data shift register) has been sent, the TC bit will be set also. This will also generate an interrupt
if the transmission complete interrupt enable bit (TCIE) is set. If the transmitter is disabled during a
transmission, the character being transmitted will be completed before the transmitter gives up control of
the TDO pin.
OSC FREQ
(f_OSC)
SCP0–SCP1
SCR0–SCR2
÷2
BUS FREQ
(f_OP)
SCI PRESCALER
SELECT
CONTROL
SCI RATE
SELECT
CONTROL
SCI RECEIVE
CLOCK (RT)
N
M
Figure 9-2. Rate Generator Division
SCI TRANS
÷ 16
CLOCK (TX)
When SCDR is read, it contains the last data byte received, provided that the receiver is enabled. The
receive data register full flag bit (RDRF) in the SCSR is set to indicate that a data byte has been
transferred from the input serial shift register to the SCDR; this will cause an interrupt if the receiver
interrupt is enabled. The data transfer from the input serial shift register to the SCDR is synchronized by
the receiver bit rate clock. The OR (overrun), NF (noise), or FE (framing) error flags in the SCSR may be
set if data reception errors occurred.
An idle line interrupt is generated if the idle line interrupt is enabled and the IDLE bit (which detects idle
line transmission) in SCSR is set. This allows a receiver that is not in the wakeup mode to detect the end
of a message, or the preamble of a new message, or to re-synchronize with the transmitter. A valid
character must be received before the idle line condition or the IDLE bit will not be set and idle line
interrupt will not be generated.
MC68HC05C9A Advance Information Data Sheet, Rev. 5.1
Freescale Semiconductor
55