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KS152JB Universal Communications Controller
Technical Specifications
Kawasaki LSI USA, Inc. Page 11 of 120 Ver. 0.9 KS152JB2
2.6 ACCESSING EXTERNAL MEMORY
External Memory is accessed if either of the following two conditions is met
1) The signal
EA is low
2) Whenever the program counter (PC) contains an address greater than 0FFFh.
Accesses to external memory are of two type: External Program Memory accesses and External
Data Memory accesses. External Program Memory is accessed using the
PSEN signal as the read
strobe. External Data Memory is accessed using the
RD or WR pins to strobe the memory.
Fetches from the external Program Memory always use a 16 bit address, while External Data
Memory accesses can have addresses of 8 bit or 16 bit. Whenever a 16 bit address is used, Port 2
is used to emit the higher byte of the address. The point to note is that during external accesses,
port 2 uses strong pullups while emitting 1s. During the time Port 2 does not emit the higher
address byte, it continuously outputs the Port 2 SFR contents, which are not modified by the hard-
ware (unless of course the user writes to the Port 2 SFR). If an 8 bit address is being used, then the
Port 2 SFR contents will be outputed on the port pins throughout the external memory cycle.
The lower byte of the address is always multiplexed with the data byte on Port 0. The ADDR/
DATA bus can drive both the active pullup and pulldown FETs. Thus for external memory
accesses, the port 0 pins are not open-drain outputs and do not need any external pullups. The fall-
ing edge of the ALE signal can be used to store the address on Ports 0 and 2 in an external address
latch. During a write cycle, the data to be written to the external Data Memory is present on Port 0
pins and remains there until after
WR is deactivated. In case of a read access, the data on Port 0
pins is read just before the deactivation of the
RD signal.
During accesses to external memory, the CPU presets the Port 0 SFR to FFh in order to float the
Port 0 pins. Therefore any data that was present in the SFR latch will be lost. If the user, writes
any data other than FFh to Port 0 during an external memory cycle, then the incoming code byte
will be corrupted. Therefore, DO NOT WRITE TO Port 0 if external memory is used.
Table 4:
EBEN
EA Program
Fetch via
PSEN EPSEN Comments
0 0 P0, P2 Active Inactive Addresses 0 - 0FFFFH
0 1 N/A N/A N/A Invalid Combination
1 0 P5, P6 Inactive Addresses 0 - 0FFFFH
1 1 P5, P6
P0, P2
Inactive
Active
Active
Inactive
Addresses 0 - 1FFFH
Addresses 0 - 2000H