DS83C520 datasheet(11/43 Pages) MAXIM

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

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16kB internal program space. If software reconfigures the ROMSIZE register to 4kB (0000h–0FFFh) in

the current state, the device will immediately jump to external program execution because program code

from 4kB to 16kB (1000h–3FFFh) is no longer located on-chip. This could result in code misalignment

and execution of an invalid instruction. The recommended method is to modify the ROMSIZE register

from a location in memory that will be internal (or external) both before and after the operation. In the

above example, the instruction which modifies the ROMSIZE register should be located below the 4kB

(1000h) boundary, so that it will be unaffected by the memory modification. The same precaution should

be applied if the internal program memory size is modified while executing from external program

memory.

Off-chip memory is accessed using the multiplexed address/data bus on P0 and the MSB address on P2.

While serving as a memory bus, these pins are not I/O ports. This convention follows the standard 8051

method of expanding on-chip memory. Off-chip ROM access also occurs if the

EA pin is a logic 0. EA

overrides all bit settings. The

PSEN signal goes active (low) to serve as a chip enable or output enable

when Ports 0 and 2 fetch from external ROM.

Figure 2. ROM Memory Map

DATA MEMORY ACCESS

Unlike many 8051 derivatives, the DS87C520/DS83C520 contain on-chip data memory. They also

contain the standard 256 bytes of RAM accessed by direct instructions. These areas are separate. The

MOVX instruction accesses the on-chip data memory. Although physically on-chip, software treats this

area as though it was located off-chip. The 1kB of SRAM is between address 0000h and 03FFh.

Access to the on-chip data RAM is optional under software control. When enabled by software, the data

SRAM is between 0000h and 03FFh. Any MOVX instruction that uses this area will go to the on-chip

RAM while enabled. MOVX addresses greater than 03FFh automatically go to external memory through

Ports 0 and 2.

ROM SIZE IGNORED

ROM SIZE ADJUSTABLE

DEFAULT = 16kB

DS83C520 Datasheet(PDF) 11 Page - Maxim Integrated Products