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8XC196NP, 80C196NU USER’S MANUAL
13-2
13.2 EXTERNAL MEMORY INTERFACE SIGNALS
Table 13-2 describes the external memory interface signals. For some signals, the pin has an al-
ternate function (shown in the Multiplexed With column). In some cases the alternate function is
a port signal (e.g., P2.7). Chapter 7, “I/O Ports,” describes how to configure a pin for its I/O port
function and for its special function. In other cases, the signal description includes instructions
for selecting the alternate function.
Table 13-2. External Memory Interface Signals
Name Type Description
Multiplexed
With
A15:0 I/O System Address Bus
These address lines provide address bits 15–0 during the entire
external memory cycle during both multiplexed and demultiplexed
bus modes.
—
A19:16 I/O Address Lines 16–19
These address lines provide address bits 16–19 during the entire
external memory cycle, supporting extended addressing of the
1 Mbyte address space.
NOTE: Internally, there are 24 address bits; however, only 20
address lines (A19:0) are bonded out. The internal address
space is 16 Mbytes (000000–FFFFFFH) and the external
address space is 1 Mbyte (00000–FFFFFH). The device
resets to FF2080H in internal ROM or F2080H in external
memory.
EPORT.3:0
AD15:0 I/O Address/Data Lines
The function of these pins depend on the bus size and mode. When
a bus access is not occurring, these pins revert to their I/O port
function.
16-bit Multiplexed Bus Mode:
AD15:0 drive address bits 0–15 during the first half of the bus cycle
and drive or receive data during the second half of the bus cycle.
8-bit Multiplexed Bus Mode:
AD15:8 drive address bits 8–15 during the entire bus cycle. AD7:0
drive address bits 0–7 during the first half of the bus cycle and either
drive or receive data during the second half of the bus cycle.
16-bit Demultiplexed Mode:
AD15:0 drive or receive data during the entire bus cycle.
8-bit Demultiplexed Mode:
AD7:0 drive or receive data during the entire bus cycle. AD15:8 drive
the data that is currently on the high byte of the internal bus.
—