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DS83C520-MCL Datasheet(PDF) 11 Page - Maxim Integrated Products |
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DS83C520-MCL Datasheet(HTML) 11 Page - Maxim Integrated Products |
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11 / 43 page  DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers 11 of 43 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 |
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