SAB80C515A-5 Datasheet, PDF (68/108 Pages) Siemens Semiconductor Group – 8-Bit CMOS Single-Chip Microcontroller Family

SAB 80C515A/83C515A-5
Special Function Register XPAGE
Addr. 91_H
XPAGE
The reset value of XPAGE is 00_H.
XPAGE can be set and read by software.
The register XPAGE provides the upper address byte for accesses to XRAM with MOVX @Ri
instructions. If the address formed from XPAGE and Ri is less than the XRAM address range,
then an external access is performed. For the SAB 80C515A the contents of XPAGE must be
greater or equal than F8_H in order to use the XRAM. Of course, the XRAM must be enabled if
it shall be used with MOVX @Ri instructions.
Thus, the register XPAGE is used for addressing of the XRAM; additionally its contents are
used for generating the internal XRAM select. If the contents of XPAGE is less than the XRAM
address range then an external bus access is performed where the upper address byte is
provided by P2 and not by XPAGE!
Therefore, the software has to distinguish two cases, if the MOVX @Ri instructions with paging
shall be used:
a) Access to XRAM:
The upper address byte must be written to XPAGE or P2;
both writes selects the XRAM address range.
b) Access to external memory: The upper address byte must be written to P2; XPAGE will
be loaded with the same address in order to deselect the
XRAM.
Semiconductor Group
6-16