Lexicon MPX-1 - Page 50

ICW3
The 8259A will only accept ICW3 if programmed in the
cascade mode (ICW'. SNGL = 0). ICW3 is used for
specific programming within the cascade mode. Bit
definition of ICW3 differs depending on whether the
8259A is a master or a slave. Definition of the ICW3 bits
is as follows:
SFNM: The SFNM bit designates selection of the
special fully nested mode which is used in
conjunction with the cascade mode. Only the
master should be programmed in the special
fully nested mode to assure a truly fully nested
structure among the slave IR inputs. If SFNM
is set to a " the special fully nested mode is
aUF:
:;;Iilve. vvnen MI;:' I:;; :;;el 10 a 1, me lS4!Ol:lA
operates as the master; when MIS is 0, it
operates as a slave. If not programmed in the
buffered mode, the state of the MIS bit is
ignored.
The aUF bit is used to designate operation in
the
buffered mode, thus controlling the use of
the SPIEN pin. 101 aUF IS set to a " the buffered
mode IS programmed and SPIEN is used as a
transceiver enable output. If aUF is 0, the buf·
fered mode isn't programmed and SPIEN is
used for master/slave selection. Note if ICW4
isn't programmed, SPIEN is used for masterl
slave selection.
51
fOllowS:
LO-L2: The LO-L2
bits are used to designate an inter·
rupt level (0-7) to be acted upon for the opera·
tion selected by the EOI, SL, and R bits of
OCW2. The level designated will either be
used to reset a specific ISR bit or to set a
specific priority. The LO-L2 bits are enabled or
disabled by the SL bit.
EOI: The EOI bit is used for all end of interrupt com-
mands (not automatic end of interrupt mode).
If set to a " a form of an end of interrupt com-
mand will be executed depending on the state
of the SL and R bits. If EOI is 0, an end of inter-
rupt command won't be executed.