Intel 386 - Manual (Page 301)

Intel386™ EX EMBEDDED MICROPROCESSOR USER’S MANUAL
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11.2.7 External UART Support
Many PC compatible applications may need to support COM3 and COM4 serial ports. Since the
integrated serial ports are mapped to I/O addresses that support only COM1 and COM2, an inter-
face to support an external serial I/O unit has been included. The master ICU interrupt inputs IR3
and IR4 may be brought out to package pins as INT8 (muxed with P3.1/TMROUT1) and INT9
(muxed with P3.0/TMROUT0), respectively.
In order to select between the internal SIO units and the external SIO units, use the OUT2 bit in
the modem control register (MCR). In normal user mode (with no loopback) clear the OUT2 bit
to enable external SIO support, and set OUT2 to enable internal SIO support. Doing this, com-
bined with the correct settings of the P3CFG.1:0 and INTCFG.6:5 bits connect the INT8 and
INT9 pins to IR3 and IR4 of the master ICU, respectively. Note that the reset state of P3CFG and
INTCFG enables SIOINT and disconnects OUT2 gating. See Chapter 5, “DEVICE CONFIGU-
RATION” (Tables 5-1 and 5-2) for more details on how to select this option.