SMSC COM20020 用户手册

4

1-3

1-3

Address
0-2

A0/nMUX,
A1,A2/ALE

Input.  On a non-multiplexed bus, these
signals are directly connected to the low bits
of the host address bus.  On a multiplexed
address/data bus, A0/nMUX is tied low, A1
is left open, and A2 is tied to the Address
Latch Enable signal of the host.  A1 is
connected to an internal pull-up resistor.

4-11

4-6,8-12

Data 0-7

AD0-AD2,
D3-D7

Input/Output.  On a non-multiplexed bus,
these signals are used as the data lines for
the device.  On a multiplexed address/data
bus, AD0-AD2 act as the address lines
(latched by ALE) and as the low data lines
for the device.  D3-D7 are always used for
data only.  These signals are connected to
internal pull-up resistors.

23

27

nRead/nData
nStrobe

nRD/nDS

Input.  On a 68XX-like bus, this active low
signal is issued by the microcontroller as the
data strobe signal to strobe the data onto
the bus.  On a 80XX-like bus, this active low
signal is issued by the microcontroller to
indicate a read operation.  In this case, a
logic "0" on this pin, when the COM20020 is
accessed, enables data from the device to
the data bus to be read by the
microcontroller.  

22

26

nWrite/
Direction

nWR/DIR

Input.  On a 68XX-like bus, this signal is
issued by the microcontroller as the
Read/nWrite signal to determine the
direction of data transfer.  In this case, a
logic "1" selects a read operation, while a
logic "0" selects a write operation.  In this
case, data is actually strobed by the nDS
signal.  On an 80XX-like bus, this active low
signal is issued by the microcontroller to
indicate a write operation.  In this case, a
logic "0" on this pin, when the COM20020 is
accessed, enables data from the data bus to
be written to the device.