Intel 8XC251SP Second Serial I/O Port Special Function Registers, Mnemonic Description Address

8xC251Tx Hardware Description

Table 5. Second Serial I/O Port Special Function Registers

Mnemonic	Description	Address
SBUF1	Serial Buffer 1. Two separate registers comprise the SBUF1 register. Writ-	9BH
	ing to SBUF1 loads the transmit buffer; reading SBUF1 access the receive
	buffer
SCON1	Serial Port Control 1. Selects the second serial I/O port operating mode.	9AH
	SCON1 enables and disables the receiver, framing bit error detection, multi-
	processor communication, automatic address recognition and the serial port
	interrupt bits.
SADDR1	Serial Address 1. Defines the individual address for a slave device	AAH
SADEN1	Serial Address Enable 1. Specifies the mask byte that is used to define the	BAH
	given address for a slave device
BGCON	Secondary Serial Port Control. Contains controls to the second serial port	8FH
	including the double baud rate bit, read/write access to the SCON1.7 bit as
	well as bits to control Timer1 or 2 overflow as the baud rate generator for
	reception and transmission
IE1	Interrupt Enable Register 1. Contains the second serial I/O port interrupt	B1H
	enable bit
IPL0	Interrupt Priority Low Control Register 1. IPL0, together with IPH0,	B2H
	assigns the second serial I/O port interrupt level from 0 (lowest) to 3 (high-
	est)
IPH0	Interrupt Priority High Control Register 1. IPH0, together with IPL0,	B3H
	assigns the second serial I/O port interrupt level from 0 (lowest) to 3 (high-
	est)

The second serial I/O port interrupt is enabled by setting the ES1 bit in the IE1 register. The priority of the second serial I/O port’s interrupt is set to one of four levels by programming the IPL1.0 and IPH1.0 bits in the IPL1 and IPH1 registers, respectively. The second serial I/O port is last in the interrupt polling sequence (see Chapter 6 of the 8xC251SA, 8xC251SB, 8xC251SP, 8xC251SQ Embedded Microcontroller User’s Manual (272795) for details of the interrupt system). The second serial I/O port's Interrupt Service Routine Vector Address is FF:0043H.

When the second serial I/O port is used, the alternate functions of RXD1 and TXD1 can no longer be used. Specifically, the PCA can no longer be clocked by an external clock input since ECI now functions as RXD1. The PCA can, however, be clocked by one of three other methods. They consist of two fixed frequencies (fixed in relation to the Oscillator frequency); FOSC/12 and FOSC/4 and Timer 0 overflow.

The other consequence of using the second serial I/O port is Module 0 of the PCA can now be used only as a 16 bit Software Timer. The 16-bit Capture, High Speed Output and Pulse Width Modulation modes are no longer available to Module 0 as these modes require the use of CEX0 (which, when the second serial I/O port is in operation, functions as TXD1).

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