Intel 8xC251TB - Manual (Page 14)

8xC251Tx Hardware Description
8
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); F
OSC
/12 and F
OSC
/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).
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-
ing to SBUF1 loads the transmit buffer; reading SBUF1 access the receive
buffer 9BH
SCON1 Serial Port Control 1. Selects the second serial I/O port operating mode.
SCON1 enables and disables the receiver, framing bit error detection, multi-
processor communication, automatic address recognition and the serial port
interrupt bits. 9AH
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
given address for a slave device BAH
BGCON Secondary Serial Port Control. Contains controls to the second serial port
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 8FH
IE1 Interrupt Enable Register 1. Contains the second serial I/O port interrupt
enable bit B1H
IPL0 Interrupt Priority Low Control Register 1. IPL0, together with IPH0,
assigns the second serial I/O port interrupt level from 0 (lowest) to 3 (high-
est) B2H
IPH0 Interrupt Priority High Control Register 1. IPH0, together with IPL0,
assigns the second serial I/O port interrupt level from 0 (lowest) to 3 (high-
est) B3H