DS87C520/DS83C520
MEMORY RESOURCES
Like the 8051, the DS87C520/DS83C520 uses three memory areas. The total memory configuration of
the DS87C520/DS83C520 is 16 kB of ROM, 1 kB of data SRAM and 256 bytes of scratchpad or direct
RAM. The 1 kB of data space SRAM is read/write accessible and is memory mapped. This on-chip
SRAM is reached by the MOVX instruction. It is not used for executable memory. The scratchpad area is
256 bytes of register mapped RAM and is identical to the RAM found on the 80C52. There is no conflict
or overlap among the 256 bytes and the 1 kB as they use different addressing modes and separate
instructions.
OPERATIONAL CONSIDERATION
The erasure window of the windowed CERDIP should be covered without regard to the
programmed/unprogrammed state of the EPROM. Otherwise, the device may not meet the AC and DC
parameters listed in the datasheet.
PROGRAM MEMORY ACCESS
On-chip ROM begins at address 0000h and is contiguous through 3FFFh (16 kB). Exceeding the
maximum address of on-chip ROM will cause the device to access off-chip memory. However, the
maximum on-chip decoded address is selectable by software using the ROMSIZE feature. Software can
cause the DS87C520/DS83C520 to behave like a device with less on-chip memory. This is beneficial
when overlapping external memory, such as Flash, is used. The maximum memory size is dynamically
variable. Thus a portion of memory can be removed from the memory map to access off-chip memory,
then restored to access on-chip memory. In fact, all of the on-chip memory can be removed from the
memory map allowing the full 64 kB memory space to be addressed from off-chip memory. ROM
addresses that are larger than the selected maximum are automatically fetched from outside the part via
Ports 0 and 2. A depiction of the ROM memory map is shown in Figure 2.
The ROMSIZE register is used to select the maximum on-chip decoded address for ROM. Bits RMS2,
RMS1, RMS0 have the following affect.
RMS2
0
0
0
0
1
1
1
1
RMS1
0
0
1
1
0
0
1
1
RMS0
0
1
0
1
0
1
0
1
Maximum on-chip
ROM Address
0 kB
1 kB/03FFh
2 kB/07FFh
4 kB/0FFFh
8 kB/1FFFh
16 kB (default)/3FFFh
Invalid - reserved
Invalid - reserved
The reset default condition is a maximum on-chip ROM address of 16 kB. Thus no action is required if
this feature is not used. When accessing external program memory, the first 16 kB would be inaccessible.
To select a smaller effective ROM size, software must alter bits RMS2-RMS0. Altering these bits
requires a Timed Access procedure as explained later.
Care should be taken so that changing the ROMSIZE register does not corrupt program execution. For
example, assume that a DS87C520/DS83C520 is executing instructions from internal program memory
near the 12 kB boundary (~3000h) and that the ROMSIZE register is currently configured for a 16 kB
internal program space. If software reconfigures the ROMSIZE register to 4 kB (0000h-0FFFh) in the
current state, the device will immediately jump to external program execution because program code
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DALLAS [ DALLAS SEMICONDUCTOR ]
