14us
6us
8042
P20
KRST
KBDRST
KRST_GA20
Bit 2
P92
nALT_RST
Bit 0
Pulse
Gen
14us
Note: When Port 92 is disabled,
writes are ignored and reads
return undefined values.
6us
KRESET Generation
Bit 1 of Port 92, the ALT_A20 signal, is used to
force nA20M to the CPU low for support of real
of A20GATE from the keyboard controller. Upon
reset, this signal is driven low.
mode compatible software.
This signal is
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
8042 P17 Functions
8042 function P17 is 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 the
internal 90µA 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
134
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