floats - cannot use as a bus. Any pin can be
programmed as an alternate function.
RD Bus Functionality
The following four cases described below
illustrate the use of the RD Bus.
Case 4: nROMCS and nROMOE as alternate
function. Same as Case 3.
Case 1: nROMCS and nROMOE as original
function. The RD bus can be used as the RD
bus or one or more RD pins can be
8042 Functions
programmed as alternate function.
These
The second alternate function for pins 113-118
are the 8042 functions P12-P17. These are
implemented as in a true 8042 part. Reference
the 8042 specification 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.
alternate functions behave as follows: if in RD to
SD mode, any value on RDx will appear on SDx;
if in SD to RD mode, SDx will not appear on
RDx, RDx gets the alternate function value.
Note: In this case, nROMCS=0, nROMOE=1.
Case 2: nROMOE as alternate function
(nROMOE internally tied to ground). In this
case, the RD bus is a unidirectional bus (read
only) controlled by nROMCS. If nROMCS = 0,
the values on RD0-7 appear on SD0-7. If
nROMCS = 1, the RD bus is disabled, and
nothing appears on the SD bus. Note: any RD
bus pin can be programmed as an alternate
function, however, if nROMCS=0, then anything
on the RD bus will appear on the SD bus.
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.
Case 3: nROMCS as alternate function
(nROMCS internally tied to VDD.) The RD bus
119
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