Special Function Registers
To ensure successful operation of the microcontroller, certain internal registers are implemented
in the Data Memory area. These registers ensure correct operation of internal functions such as
timers, interrupts, watchdog, etc., as well as external functions such as I/O data control. The loca
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tion of these registers within the Data Memory begins at the address ²00H². Any unused Data
Memory locations between these special function registers and the point where the General Pur
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pose Memory begins is reserved for future expansion purposes, attempting to read data from
these locations will return a value of ²00H².
Indirect Addressing Registers - IAR, IAR0, IAR1
The method of indirect addressing allows data manipulation using memory pointers instead of the
usual direct memory addressing method where the actual memory address is defined. Any action
on the Indirect Addressing Registers will result in corresponding read/write operations to the mem
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ory location specified by the corresponding memory pointer. For the HT48R10A-1/HT48C10-1
and HT48R30A-1/HT48C30-1 devices, one Indirect Addressing Register, IAR, and one Memory
Pointer, MP, is provided. For the HT48R50A-1/HT48C50-1, HT48R70A-1/HT48C70-1 and
HT48RU80/HT48CU80 devices, two Indirect Addressing Registers, IAR0 and IAR1, and two Mem
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ory Pointers, MP0 and MP1, are provided. Note that these Indirect Addressing Registers are not
physically implemented and that reading the Indirect Addressing Registers directly will return a re
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sult of ²00H² and writing to the registers indirectly will result in no operation.
Memory Pointers - MP, MP0, MP1
For the HT48R10A-1/HT48C10-1 and HT48R30A-1/HT48C30-1 devices, one memory pointer
known as MP is provided, whereas for the HT48R50A-1/HT48C50-1, HT48R70A-1/ HT48C70-1
and HT48RU80/ HT48CU80 devices, two memory pointers known as MP0 and MP1 are provided.
These memory pointers are physically implemented in the Data Memory and can be manipulated
in the same way as normal registers providing a convenient way with which to address and track
data. When any operation to the relevant Indirect Addressing Registers is carried out, the actual
address that the microcontroller is directed to is the address specified by the related Memory
Pointer.
Note
For the HT48R10A-1/HT48C10-1 and HT48R30A-1/HT48C30-1 devices, bit 7 of the memory
pointers are not implemented. However, it must be noted that when the memory pointers in these
devices are read, a value of ²1² will be read.
The following example for the HT48R10A-1/HT48C10-1 or HT48R30A-1/HT48C30-1 devices
shows how to clear a section of four RAM locations already defined as locations adres1 to adres4.
data .section ¢data¢
adres1 db ?
adres2 db ?
adres3 db ?
adres4 db ?
block db ?
code .section at 0 ¢code¢
org 00h
Chapter 1 Hardware Structure
31
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