Mitsubishi Motors DS5000TK User Manual
Page 121
USER’S GUIDE
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121
SM0 = 0 and SM1 = 1 corresponds to the value SCON.7
= 0 and SCON.6 = 1. In addition the since the applica-
tion requires receiving data, the serial receiver must be
enabled. This is done by setting the REN bit at SCON.4
to a logic 1. The remaining bits in SCON can be written
to 0. Thus the value for SCON is 01010000b or 50h.
SCON – 98h
SM0
SM1
SM2
REN
TB8
RB8
TI
RI
0
1
0
1
0
0
0
0
This application uses the serial interrupt. It serves two
purposes. First, the software knows when a byte has
been sent, so it knows when another can be written.
Second, after the 256 bytes have been transmitted, the
PC will respond. It is not know when this will occur and
the software may have other tasks to attend. Therefore
the serial interrupt will inform the microcontroller when
the confirmation code has been received by the serial
port. This example will enable only the serial Interrupt. It
will be set for high priority since in a real system, other
interrupts might be enabled.
IE – 0A8h
EA
–
–
ES
ET1
EX1
ET0
EX0
1
0
0
1
0
0
0
0
To enable interrupts, the EA bit must be set. In addition,
the setting the ES bit turns on the Serial Interrupt. Thus
the value 10010000b or 90h will enable serial interrupts.
Note that although a timer will ultimately be used to gen-
erate serial baud rates, the timer interrupt is not used.
As mentioned above, this example will use a high prior-
ity for the serial interrupt. This is done as follows.
IP – 0B8h
RWT
–
–
PS
PT1
PX1
PT0
PX0
0
0
0
1
0
0
0
0
The serial port interrupt has been set to a high priority by
setting the PS bit to a logic 1. Thus the serial port is con-
figured for high priority by writing a 00010000b or 10h to
the IP register at location 0B8h.
The serial port is now configured. The only remaining
task is to set the correct baud rate. The example stated
above that the communication rate would be either 9600
or 19,200 baud. To generate baud rates in Serial Mode
1, the Timer 1 is used. The serial port uses the Timer 1
overflow, then divides this frequency by either 16 or 32
to generate the internal baud rate clock. Each time the
Timer 1 value increments past 0FFh is considered an
overflow. Due to the formula used for generating baud
rates shown below, the 11.0592 MHz crystal assumed
for this example generates good baud rate values. This
is the most convenient and commonly used choice for
generating baud rates. Other convenient values are
7.3728 MHz and 1.8432 MHz.
To get 9600 bits per second, the baud rate generator
must create an interval af 1/9600 seconds = 1.0416 ms.
To get 19,200 bits per second, the interval is 1/19200 =
520.83
µ
s. Note that the timers count up, so the value
that the timer starts from must be selected to generate
the desired interval. The following values are useful:
1/11.0592 MHz = t
CLK
=
90.4 *10
–9
Timer runs at 12 t
CLK
per count = 1.085 * 10
–6
Time out = (256 – Timer start value) * 12 t
CLK
=
(256 – Timer start value) * 1.085 * 10
–6
serial port Baud rate clock = 1/baud rate = (16
or 32) * Time out
Whether 16 or 32 is used in the baud rate generator is
determined by the SMOD bit at PCON.7. 16 is used for
SMOD=1, and 32 is used for SMOD=0. This is com-
monly referred to by the expression (2
SMOD
)/32. Since
SMOD is either 0 or 1, this value is either 1/32 or 2/32
respectively.
The user selects the time–out value and the setting for
SMOD to set the baud rate. This is done as follows.
Baud Rate
+
2
SMOD
32
X
1
12t
CLK
* (256
*
TH1)