SAF-C502-2R20N Siemens Semiconductor Group, SAF-C502-2R20N Datasheet - Page 23

• Manufacturer Part Number: SAF-C502-2R20N
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: Siemens Semiconductor Group

Additional On-Chip RAM - XRAM
The SAB-C502 contains another 256byte of On-Chip RAM additional to the 256bytes internal RAM.
This RAM is called XRAM (‘eXtended RAM’) in this document.
The additional ON-Chip RAM is logically located in the external data memory range. The highbyte
of the XRAM address range startaddress is programmable by SFR XCON (94 H ). The reset value of
XCON is 0F8 H (that is, XRAM address range F800H H … F8FF H ).
The contents of the XRAM is not affected by a reset. After power up the contents is undefined, while
it remains unchanged during and after reset as long as the power supply is not turned off. The
XRAM is controlled by SFR SYSCON as shown in table 8.
Table 8
Control of the XRAM
Because of the XRAM is used in the same way as external data memory the same instruction types
must be used for accessing the XRAM. A general overview gives table 9.
Table 9
Accessing the XRAM
Note: When writing the page address (in page mode) at Port2 the value is also written in XPAGE.
The behaviour of Port0/Port2 and RD/WR during MOVX accesses is shown in table 10.
Semiconductor Group
(page mode)
Instruction
XMAP1
R0/R1
DPTR
using
However when writing XPAGE the value at PORT2 is not changed!
0
0
1
SFR SYSCON
MOVX @ DPTR,A
MOVX A @DPTR
XMAP0
MOVX A, @Ri
MOVX@Ri,A
Instruction
1
0
0
Description
Resetvalue. Access to XRAM is disabled. When cleared it can
be set again only by a reset
XRAM enabled
XRAM enabled. The signals RD and WR are activated during
accesses to XRAM
Remarks
Normally the use of these instructions would use a
physically external memory. However, in the SAB-C502
the XRAM is accessed if it is enabled.
Normally Port 2 serves as page register. However, the
distinction, whether Port 2 is as general purpose I/O or
as “page address” is made by the external design.
Hence a special SFR XPAGE is implemented the serve
the same function for the XRAM as Port 2 for external
data memory.
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C502