EMCH 367
Fundamentals of Microcontrollers
Example SCI TRANSMISSION
EXAMPLE SCI TRANSMISSION
OBJECTIVE
This example has the following objectives:

Review the use of serial communication interface (SCI) for transmission

Illustrate the sending of a character through SCI
PROGRAM SCI_TRANSMIT
This program is an example of SCI transmission. The character T is being sent to SCDR when TDRE is
set.
Instructions
a)
Load REGBAS in reg. X
b)
Initialize BAUD=9600
c)
Initialize 8-bit word
d)
Initialize TE and RE
e)
Load character 'T' in AccB
f)
Check if TDRE is set
g)
Store accB to SCDR
h)
Branch back to (v)
i)
SWI
Flowchart
Code
START
Initialize REGBAS in reg X
Initialize SCI:
BAUD = 9600
8-bit word
TE=1, RE=1
LABEL2
Load ‘T’ in accB
Check if TDRE is set
LABEL3 Load SCSR into accA
AND with mask %10000000
Loop if zero
LDX
#REGBAS
LDAA
#%00110000
STAA
BAUD,X
LDAA
#%00000000
STAA
SCCR1,X
LDAA
#%00001100
STAA
SCCR2,X
LABEL2 LDAB
#'T'
* Send the character to the terminal
LABEL3 LDAA
SCSR,X
ANDA
#%10000000
BEQ
LABEL3
* You are here when the transmission reg. is
empty
STAB
SCDR,X
BRA
LABEL2
SWI
Store AccB to SCDR
SWI
FLOWCHART AND CODE
The program flowchart is show to the right of the program instructions. Note the initialization block,
which contains reg. X initialization and SCI initialization. After initialization, the character T is loaded into
accB. Then, the status of TDRE (transmission data register empty) flag is checked in a loop. When
TDRE is found set, the loop is exited and the contents of accB is stored in SCDR (serial communication
data register). This operation automatically resets TDRE. Now, the program loops back to the beginning
and tries to send again.
The essential code for the program is shown to the right of the program flowchart. This essential code
was incorporated into the standard asm template to generate the file SCI_transmit.asm.
EXECUTION
Open THRSim11. Close the Commands window. View serial registers. View serial receiver. Open and
assemble SCI_transmit.asm. Set breakpoint at STAB SCDR,X instruction and at SWI. Reset registers.
Set standard labels (Label/Set Standard Labels). Set display of accA to binary. Set display of BAUD,
SCCR1, SCCR2, SCSR to binary. Arrange windows for maximum benefit: Reset, A, B, X. Press the
RESET button. Your screen should look like this:
Dr. Victor Giurgiutiu
Page 12/12/2016
EMCH 367
Fundamentals of Microcontrollers
Example SCI TRANSMISSION
Note that the reset value of SCSR is %11000000, i.e., it has the flags TDRE and TC set. This indicates
that the transmitter is ready to transmit.
a) Press the RUN button. The program should loop on LABEL3 and exit the loop when the
condition TDRE=1 is satisfy. This condition is checked with the mask %10000000. The mask is
applied on the value found in SCSR. When bit 7 in SCSR is set, the mask senses it and the
condition for exiting the loop is satisfied. Since TDRE=1 from the reset, the loop is passed
through and exited.
Dr. Victor Giurgiutiu
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EMCH 367
Fundamentals of Microcontrollers
Example SCI TRANSMISSION
b) When the ‘check TDRE’ loop is exited, the program gets to the line
STAB
SCDR,X
This line is a breakpoint. At this moment, the screen looks like this:
c) Step manually to the next line, which stores accB into SCDR. This store operation automatically
resets TDRE. The screen looks like this:
Dr. Victor Giurgiutiu
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EMCH 367
Fundamentals of Microcontrollers
Example SCI TRANSMISSION
Note that SCSR is %00000000. This proves that TDRE has been reset by the storing to SCDR. Also
note that the value $54 from accB has not yet arrived in SCDR. (The reason for this is that the simulator
has an internal delay when running SCI in manual mode.)
d) Press RUN again. The program loops several times on LABEL3 before exiting. While looping,
you can see SCSR=%00000000, i.e., TDRE=0. When exiting, SCSR=%10000000, i.e.,
TDRE=1. When the program pause, the screen looks like this:
Note that SCSR=%10000000, i.e., TDRE=1. The transmission of the character was done, and SCI
is ready to transmit again.
Dr. Victor Giurgiutiu
Page 42/12/2016
EMCH 367
Fundamentals of Microcontrollers
Example SCI TRANSMISSION
e) Press RUN again 2 times. Note that the character T has appeared in the serial receiver window.
Your screen capture looks like this:
f)
Press RUN again. You get an additional character in the serial receiver window. Your screen
capture looks like this:
Dr. Victor Giurgiutiu
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EMCH 367
Fundamentals of Microcontrollers
WHAT YOU HAVE LEARNED
In this example, you have learned:

The use of serial communication interface (SCI) for transmission

The sending of a character through SCI

New words and notations: SCSR, SCDR, TDRE.
Dr. Victor Giurgiutiu
Page 62/12/2016
Example SCI TRANSMISSION
EMCH 367
Fundamentals of Microcontrollers

Dr. Victor Giurgiutiu
Page 72/12/2016
Example SCI TRANSMISSION
EMCH 367
Dr. Victor Giurgiutiu
Fundamentals of Microcontrollers
Page 1
Example SCI TRANSMISSION
2/12/2016