Intel Microcontroller, 80C196NU, 8XC196NP manual Serial Port Frames for Mode

8XC196NP, 80C196NU USER’S MANUAL

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 8-8.)

8.3.2.1Mode 1

Mode 1 is the standard asynchronous communications mode. The data frame used in this mode (Figure 8-4) 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 8-4. 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.

Use caution when connecting more than two devices with the serial port in half-duplex (i.e., with one wire for transmit and receive). The receiving processor must wait for one bit time after the RI flag is set before starting to transmit. Otherwise, the transmission could corrupt the stop bit, causing a problem for other devices listening on the link.

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