8
8
oscillator, pin 21 (XTAL1) functions as the clock
input.
Emulate Mode
The COM20051 contains a unique feature called
the
Emulate
mode
that
most
8051-based
peripheral
devices
do
not
accommodate.
TheEmulate mode permits developers to access
and program the internal ARCNET core using a
standard low-cost 8032 emulator.
This feature
eliminates the need for expensive dedicated
development equipment needed for other types
of 8051-based peripheral devices. The Emulate
mode is invoked by connecting the nEA pin to
VCC. This causes the internal 80C32 processor
to enter a HI-Z state and changes the state of the
COM20051 pins according to the following table:
Table 1 - Emulate Mode
SIGNAL NAME
EMUL = 0
EMUL = 1
PORT 0
Bidirectional
Bidirectional
PORT 1
Bidirectional
HI-Z (except for pins
designated as interrupt
destinations)
PORT 2
Output
Input
INT0,1
(P3.2, P3.3)
Input
Output
RD/WR
(P3.6, P3.7)
Output
Input
ALE
Output
Input
TX,T0, T1
(P3.1,3.4,3.5)
nPSEN
Output
HI-Z
Address Decoding
The COM20051, as described previously, maps
the ARCNET registers into the 80C32's External
Data Memory space.
This provides system
flexibility because the location of the ARCNET
registers can be located anywhere within the 64K
External Data Memory space.
The precise
location can be resolved with a 256-byte page.
The location of that page in the External Data
Memory space is pointed to by the read/write
Address Decode Register, as shown in Figure 2.
The Address Decode Register is located at
FFFFh of the External Data Memory space. It
holds the upper 8 bits of the 16-bit address at
which the 256 page boundary will start.
This
register must be programmed prior to any access
to the ARCNET core.
The default value is
0000h.
The ARCNET core register page must be
mapped away from the external RAM prior to any
access to the external RAM by the software.
Failure to do this may result in incorrect data
write and read operations to the external RAM as
well as the core registers.
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