FDC37B80X SMSC [SMSC Corporation], FDC37B80X Datasheet - Page 125

• Manufacturer Part Number: FDC37B80X
• Description: PC98/99 Compliant Enhanced Super I/O Controller with Keyboard/Mouse Wake-Up
• Manufacturer: SMSC [SMSC Corporation]

Bit 1 of Port 92, the ALT_A20 signal, is used to
force nA20M to the CPU low for support of real
mode compatible software.
externally OR’ed with the A20GATE signal from
the keyboard controller and CPURST to control
the nA20M input of the CPU. Writing a 0 to bit 1
of the Port 92 Register forces ALT_A20 low.
ALT_A20 low drives nA20M to the CPU low, if
A20GATE from the keyboard controller is also
low. Writing a 1 to bit 1 of the Port 92 Register
forces ALT_A20 high. ALT_A20 high drives
nA20M to the CPU high, regardless of the state
of A20GATE from the keyboard controller. Upon
reset, this signal is driven low.
This signal is
125
8042 P12, P16 and P17 Functions
8042
implemented as in a true 8042 part. Reference
the 8042 spec for all timing. A port signal of 0
drives the output to 0. A port signal of 1 causes
the port enable signal to drive the output to 1
within 20-30nsec. After several (# TBD) clocks,
the port enable goes away and for P12, P16 the
internal 90µA pull-up maintains the output signal
as 1, and for P17 an external pull-up maintains
the output signal as 1.
In 8042 mode, the pins can be programmed as
open drain. When programmed in open drain
mode, the port enables do not come into play. If
the port signal is 0 the output will be 0. If the
port signal is 1, the output tristates: an external
pull-up can pull the pin high, and the pin can be
shared i.e., P12 and nSMI can be externally tied
together. In 8042 mode, the pins cannot be
programmed as input nor inverted through the
GP configuration registers.
functions
P12,
P16
and
P17
are