BIOS BUFFER
The chip contains one 245 type buffer that can be
transmission from the RD bus to the SD bus or
from the SD bus to the RD bus. The direction of
the transfer is controlled by nROMOE. The enable
input, nROMCS, can be used to disable the
used for a BIOS Buffer. If the BIOS buffer is not
used, then nROMCS must be tied high or pulled
up to Vcc with a resistor so as not to interfere with
the boot ROM.
This function allows data
transfer
and
isolate
the
buses.
nROMCS
nROMOE
DESCRIPTION
L
L
H
L
H
X
RD[0:7] data to SD[0:7] bus
SD[0:7] data to RD[0:7]
Isolation
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.
RD Bus Functionality
The following cases described below illustrate the
use of the RD Bus.
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 programmed as
Case 3: nROMCS as GPIO function. (nROMCS
internally tied to VDD.) The RD bus floats - cannot
use as a bus. Any pin can be programmed as an
alternate function.
alternate function.
These 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 4: nROMCS and nROMOE as GPIO
function. Same as Case 3.
Case 5: Parallel IRQ enabled; RD Bus pins,
nROMOE, nROMCS are used as IRQ pins.
Case 2: nROMOE as GPIO 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
120
SMSC [ SMSC CORPORATION ]
