SAB80C517A-N18 Siemens Semiconductor Group, SAB80C517A-N18 Datasheet - Page 21

• Manufacturer Part Number: SAB80C517A-N18
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: Siemens Semiconductor Group

Accesses to XRAM
Because the XRAM is used in the same way as external data memory the same instruction
types must be used for accessing the XRAM.
Note: If a reset occurs during a write operation to XRAM, the effect on XRAM depends on the
Accesses to XRAM using the DPTR
There are a Read and a Write instruction from and to XRAM which use one of the 16-bit DPTR
for indirect addressing. The instructions are:
MOVX A,
MOVX
Normally the use of these instructions would use a physically external memory. However, in the
SAB 80C517A the XRAM is accessed if it is enabled and if the DPTR points to the XRAM
address space (DPTR
Accesses to XRAM using the Registers R0/R1
The 8051 architecture provides also instructions for accesses to external data memory range
which use only an 8-bit address (indirect addressing with registers R0 or R1). The instructions
are:
MOVX A,
MOVX
In application systems, either a real 8-bit bus (with 8-bit address) is used or Port 2 serves as
page register which selects pages of 256-byte. However, the distinction, whether Port 2 is
used as general purpose I/O or as "page address" is made by the external system design. From
the device’s point of view it cannot be decided whether the Port 2 data is used externally as
address or as I/O data!
Hence, a special page register is implemented into the SAB 80C517A to provide the possibility
of accessing the XRAM also with the MOVX @Ri instructions, i.e. XPAGE serves the same
function for the XRAM as Port 2 for external data memory.
Semiconductor Group
cycle which the reset is detected at (MOVX is a 2-cycle instruction):
Reset detection at cycle 1:
Reset detection at cycle 2:
@DPTR (Read)
@DPTR, A (Write)
@Ri (Read)
@Ri, A (Write)
F800
H
).
The new value will not be written to XRAM. The old value
is not affected.
The old value in XRAM is overwritten by the new value.
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SAB 80C517A/83C517A-5
1994-05-01