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iSBC 80/30
First, reset the USART chip by writing a Command in-
struction to location ED (or EF). The Command instruc-
tion must have bit 6 set (lR6
=
1); all other bits are
immaterial.
NOTE
This reset procedure should be used only if the
USART has been completely initialized, or if
the initialization procedure has reached the
point that the USART is ready to receive a
Command word. For example, if the reset com-
mand is written when the initialization sequence
calls for a sync character, then subsequent pro-
gramming will be in error.
Next, write a Mode instruction word to the USART. (See
figures 3 -1 through 3 -4.) A typical subroutine for writing
both Mode and Command instructions is given in table
3-3.
If the USART is programmed for the synchronous mode,
write one or two sync characters depending on the trans-
mission format.
Finally, write a Command instruction word to the
USART. Refer to figure 3-5 and table 3-3.
3-13. OPERATION
Normal operating procedures use data I/O read and write,
status read, and Command instruction write operations.
Programming and addressing procedures for the above are
summarized in following paragraphs.
NOTE
After the USART has been initialized, always
check the status of the TXRDY bit prior to
writing data or writing a new command word to
the USART. The TXRDY bit must be true to
prevent overwriting and subsequent loss of
command or data words. The TXRDY bit is
Programming Information
inactive until initialization has been completed;
therefore, do not check TXRDY until after the
command word, which concludes the initializa-
tion procedure, has been written.
Prior to any operating change, a new command word must
be written with command bits changed as appropriate.
(Refer to figure 3-5 and table 3-3.)
ADDRESS
ED
ED
ED
ED
EC
ED
EC
ED
:~
.~
RESET
MODE INSTRUCTION
SYNC CHARACTER 1
SYNC CHARACTER
2
COMMAND INSTRUCTION
DATA 1/0
:~
COMMAND INSTRUCTION
DATA 1/0
::~
COMMAND INSTRUCTION
}
SYNC MODE
ONLY·
*The second sync character is skipped if Mode instruction
has programmed USART to single character internal
sync mode. Both sync characters are skipped if Mode
instruction has programmed USART to async mode.
Figure 3-6. Typical USART
611·6
Initialization and Data I/O Sequence
3-14.
DATA INPUT/OUTPUT. For data receive or
transmit operations perform a read or w.rite, respectively,
to the USART. Tables 3-4 and 3-5 provide examples of
typical character read and write subroutines.
During normal transmit operation, the USARTgenerates a
Transmit Ready (TXRDY) signal that indicates that the
Table 3-3. Typical USART Mode or Command Instruction Subroutine
;CMD2 OUTPUTS CONTROL WORD TO USART.
;USES-A, STAT; DESTROYS-NOTHING.
CMD2:
EXTRN
STAT
PUSH
CALL
ANI
JZ
POP
OUT
RET
END
PSW'
STAT
1
LP
PSW
OED
;CHECK TXRDY
;TXRDY MUST BE TRUE
;ENTER HERE FOR INITIALIZATION
3-5
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