Asynchronous Modes (Modes 1, 2, And 3); Mode 1 - Intel 8XC196K Series User Manual

8XC196K x , J x , CA USER'S MANUAL
7.3.2

Asynchronous Modes (Modes 1, 2, and 3)

Modes 1, 2, and 3 are full-duplex serial transmit/receive modes, meaning that they can transmit
and receive data simultaneously. Mode 1 is the standard 8-bit, asynchronous mode used for nor-
mal serial communications. Modes 2 and 3 are 9-bit asynchronous modes typically used for in-
terprocessor communications (see "Multiprocessor Communications" on page 7-8). In mode 2,
the serial port sets an interrupt pending bit only if the ninth data bit is set. In mode 3, the serial
port always sets an interrupt pending bit upon completion of a data transmission or reception.
When the serial port is configured for mode 1, 2, or 3, writing to SBUF_TX causes the serial port
to start transmitting data. New data placed in SBUF_TX is transmitted only after the stop bit of
the previous data has been sent. A falling edge on the RXD input causes the serial port to begin
receiving data if RXD is enabled. Disabling RXD stops a reception in progress and inhibits fur-
ther receptions. (See "Programming the Control Register" on page 7-8.)
7.3.2.1

Mode 1

Mode 1 is the standard asynchronous communications mode. The data frame used in this mode
(Figure 7-4) consists of ten bits: a start bit (0), eight data bits (LSB first), and a stop bit (1). If
parity is enabled, a parity bit is sent instead of the eighth data bit, and parity is checked on recep-
tion.
Stop Start D0 D1 D2 D3 D4 D5 D6 D7 Stop
8 Bits of Data or 7 Bits of Data
with Parity Bit
10-Bit Frame
A0245-02
Figure 7-4. Serial Port Frames for Mode 1
The transmit and receive functions are controlled by separate shift clocks. The transmit shift
clock starts when the baud rate generator is initialized. The receive shift clock is reset when a start
bit (high-to-low transition) is received. Therefore, the transmit clock may not be synchronized
with the receive clock, although both will be at the same frequency.
The transmit interrupt (TI) and receive interrupt (RI) flags in SP_STATUS are set to indicate com-
pleted operations. During a reception, both the RI flag and the RI interrupt pending bit are set just
before the end of the stop bit. During a transmission, both the TI flag and the TI interrupt pending
bit are set at the beginning of the stop bit. The next byte cannot be sent until the stop bit is sent.
7-6