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PRELIMINARY 13
8XC251SA/SB/SP/SQ HIGH-PERFORMANCE CHMOS MICROCONTROLLER
Table 7. Memory Signal Selections (RD1:0)
RD1:0
P1.7/CEX/
A17/WCLK
P3.7/RD#/A16 PSEN# WR# Features
0 0 A17 A16 Asserted for
all addresses
Asserted for writes to
all memory locations
256-Kbyte external
memory
0 1 P1.7/CEX4/
WCLK
A16 Asserted for
all addresses
Asserted for writes to
all memory locations
128-Kbyte external
memory
1 0 P1.7/CEX4/
WCLK
P3.7 only Asserted for
all addresses
Asserted for writes to
all memory locations
64-Kbyte external
memory. One
additional port pin.
1 1 P1.7/CEX4/
WCLK
RD# asserted
for addresses
≤ 7F:FFFFH
Asserted for
≥ 80:0000H
Asserted only for
writes to MCS 51
microcontroller data
memory locations.
64-Kbyte external
memory. Compatible
with MCS 51 micro-
controllers.