Draft 6/5/00
Serial Port
A-19
Copyright 2000 by LSI Logic Corporation. All rights reserved.
An external device can use the interrupt indications to initiate a read
cycle. When an interrupt is detected, the individual registers (or multiple
registers) can be read out and the status bits compared against their
previous values to determine any changes. After the interrupt bits have
been read out, the interrupt signals are automatically deasserted.
A mask register bit exists for every status output bit in the MI serial port
Interrupt Mask register so that the interrupt bits can be individually
programmed for each application.
A.11.2 Multiple Register Access
If the MI serial port needs to be constantly polled in order to monitor
changes in status output bits, or if it is desired that all registers be read
or written in a single serial port access cycle, multiple register access
mode can be used. Multiple register access allows access to all registers
in a single MI serial port access cycle. When multiple register access is
enabled, all the registers are read or written when the register address
REGAD[4:0] = 0b11111. This eliminates the need to read or write
registers individually. Multiple register access mode is normally disabled.
To enable it, set the Multiple Register Access Enable (MREG) bit in the
MI serial port Configuration 2 register.
A.11.3 Serial Port Addressing
Tying the MDA[4:0]n pins to the desired value selects the device address
for the MI serial port. MDA[4:0]n share the same pins as the LED
outputs, as shown in
Figure A.7
a. At powerup or reset, the output drivers
are 3-stated for an interval called the power-on reset time. During the
power-on reset interval, the value on these pins is latched into the
device, inverted, and used as the MI serial port address. The LED
outputs are open-drain with internal pullup to V
DD
.
If an LED is to be connected on an LED output, an LED and resistor are
tied to V
DD
as shown in
Figure A.6
b. If a HIGH address is desired, the
LED to V
DD
automatically makes the latched address value a HIGH. If a
LOW value for the address is desired, a 50 K
resistor to GND must be
added as shown in
Figure A.6
b.
If no LEDs are needed on the LED outputs, the selection of addresses
can be done as shown in
Figure A.6
c. If a HIGH address is desired, the
pin should be left floating and the internal pullup pulls the pin HIGH