Section 14 Serial I/O - Mitsubishi DS907x SIP User Manual

USER'S GUIDE
SECTION 14: SERIAL I/O
FUNCTION DESCRIPTION
The Secure Microcontroller, like the 8051, includes a
powerful Serial I/O (UART) port capable of both syn-
chronous and asynchronous communication. The baud
rate and time–base source is fully programmable. The
serial port uses P3.0 as Receive Data (RXD) and P3.1
Transmit Data (TXD). Note that no special action other
than enabling the function (i.e. writing a logic 1 to the
corresponding port pins) is required to make these pins
become the serial port. The serial port is capable of full
duplex operation in asynchronous mode and half–du-
plex operation in synchronous mode.
The serial port consists of a receive shift register,
receive buffer, transmit shift register and control logic.
An incoming serial word from an external source is
shifted into the receive shift register one bit at a time.
Bits are shifted at the baud rate, which is programmable.
The baud rate must be programmed by user's software
to match the incoming frequency or the serial data will
be unintelligible. Once the word is received, the serial
port transfers it into the receive buffer. At this time, the
serial port can receive another byte into its shift register.
Once a word is received, the software should read the
receive buffer before another word is completely
received. The serial port will automatically transfer the
new word into the receive buffer regardless of whether
software has read the old value. This destroys the data
that had been present from the previous word. At 9600
SERIAL PORT OPERATING MODES Table 14–1
MODE SYNC/ASYNC
MODE 0
MODE 1
MODE 2
MODE 3
The serial port has four modes (Mode 0–3) of operation
as shown in Table 14–1. Mode 0, is a synchronous
mode. This means that the microcontroller serial port
will supply a clock to synchronize the data I/O shifting.
One clock pulse is generated per bit. The external
device that is communicating with the micro must also
use this clock. This mode is typically used with serial
peripherals. Synchronous mode is generally capable of
050396 109/173
BAUD CLOCK
SYNC 12 t
CLK
ASYNC Timer 1
Overflow
ASYNC 32 t
CLK
64 t
CLK
ASYNC Timer 1
Overflow
baud, receiving an asynchronous word takes 1.04 ms.
Thus software must read a received word within 1.04 ms
or it may be overwritten by another incoming word.
The transmit shift register has no buffer. Software writes
into the shift register and the word is immediately shifted
out. Thus software must wait until the serial word is
shifted out before writing another to transmit. Both the
receive buffer and the transmit shift register are located
in the SFR map. Furthermore, they reside at the same
address called SBUF (99h). Reading SBUF automati-
cally transfers the word out of the serial receive buffer.
Writing to SBUF automatically transfers a byte into the
transmit shift register. Serial Port operation is controlled
via the SCON (98h) register.
Each serial port function (receive and transmit) is capa-
ble of generating an interrupt. If enabled, the receive
function interrupts the CPU when a word has been
shifted in. This indicates that software should read the
receive buffer. The serial port will set the RI flag bit in the
SCON.0 location to indicate the source of the interrupt.
The serial port will also generate an interrupt when it has
completely shifted out a word. This indicates that
another word can be transmitted. The serial port will set
the TI flag bit at SCON.1 to indicate the source of the
interrupt. Remember that the Serial Interrupt vectors to
location 23h regardless of the source. The ISR must
determine the cause of the interrupt from the flags men-
tioned above.
DATA BITS
8
8
or 9
9
a higher speed communication speeds than the
asynchronous modes. It generates its speed as a fixed
function of 12 microcontroller oscillator clocks per bit.
Mode 1 is a 10–bit asynchronous mode using 8–bit
words, one start bit, and one stop bit. The time base is
generated from the Timer 1 overflow and is therefore
fully programmable. A user simply loads the timer with a
110
9TH DATA BIT
START/STOP
FUNCTION
None None
1 Start None
1 Stop
1 Start 0, 1, or parity
1 Stop
1 Start 0, 1, or parity
1 Stop