SAB80C515A-5 Datasheet, PDF (67/108 Pages) Siemens Semiconductor Group – 8-Bit CMOS Single-Chip Microcontroller Family

SAB 80C515A/83C515A-5
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
cycle which the reset is detected at (MOVX is a 2-cycle instruction):
Reset detection at cycle 1: The new value will not be written to XRAM. The old value
is not affected.
Reset detection at cycle 2: The old value in XRAM is overwritten by the new value.
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, @DPTR (Read)
MOVX
@DPTR, A (Write)
Normally the use of these instructions would use a physically external memory. However, in the
SAB 80C515A the XRAM is accessed if it is enabled and if the DPTR points to the XRAM
address space (DPTR ≥ F800_H).
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, @Ri (Read)
MOVX
@Ri, A (Write)
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 80C515A 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
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