
Serial Communications Interface (SCI)
Data Sheet
M68HC11E Family — Rev. 5
130
Serial Communications Interface (SCI)
For More Information On This Product,
  Go to: www.freescale.com
MOTOROLA
these flags is automatic. Functions that are normally performed in response to the 
status flags also satisfy the conditions of the clearing sequence. 
TDRE and TC flags are normally set when the transmitter is first enabled (TE set 
to 1). The TDRE flag indicates there is room in the transmit queue to store another 
data character in the TDR. The TIE bit is the local interrupt mask for TDRE. When 
TIE is 0, TDRE must be polled. When TIE and TDRE are 1, an interrupt is 
requested. 
The TC flag indicates the transmitter has completed the queue. The TCIE bit is the 
local interrupt mask for TC. When TCIE is 0, TC must be polled. When TCIE is 1 
and TC is 1, an interrupt is requested. 
Writing a 0 to TE requests that the transmitter stop when it can. The transmitter 
completes any transmission in progress before actually shutting down. Only an 
MCU reset can cause the transmitter to stop and shut down immediately. If TE is 
written to 0 when the transmitter is already idle, the pin reverts to its 
general-purpose I/O function (synchronized to the bit-rate clock). If anything is 
being transmitted when TE is written to 0, that character is completed before the 
pin reverts to general-purpose I/O, but any other characters waiting in the transmit 
queue are lost. The TC and TDRE flags are set at the completion of this last 
character, even though TE has been disabled. 
7.9  Receiver Flags 
The SCI receiver has five status flags, three of which can generate interrupt 
requests. The status flags are set by the SCI logic in response to specific conditions 
in the receiver. These flags can be read (polled) at any time by software. Refer to 
Figure 7-10
, which shows SCI interrupt arbitration. 
When an overrun takes place, the new character is lost, and the character that was 
in its way in the parallel RDR is undisturbed. RDRF is set when a character has 
been received and transferred into the parallel RDR. The OR flag is set instead of 
RDRF if overrun occurs. A new character is ready to be transferred into RDR 
before a previous character is read from RDR. 
The NF and FE flags provide additional information about the character in the RDR, 
but do not generate interrupt requests. 
The last receiver status flag and interrupt source come from the IDLE flag. The RxD 
line is idle if it has constantly been at logic 1 for a full character time. The IDLE flag 
is set only after the RxD line has been busy and becomes idle, which prevents 
repeated interrupts for the whole time RxD remains idle. 
F
Freescale Semiconductor, Inc.
n
.