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UPSD323X Datasheet(PDF) 26 Page - STMicroelectronics |
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UPSD323X Datasheet(HTML) 26 Page - STMicroelectronics |
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26 / 176 page  µPSD323X 26/176 External RAM. Table 10 shows a list of the Data Transfer instructions that access external Data Memory. Only indirect addressing can be used. The choice is whether to use a one-byte address, @Ri, where Ri can be either R0 or R1 of the se- lected register bank, or a two-byte address, @DTPR. Note: In all external Data RAM accesses, the Ac- cumulator is always either the destination or source of the data. Lookup Tables. Table 11 shows the two instruc- tions that are available for reading lookup tables in Program Memory. Since these instructions access only Program Memory, the lookup tables can only be read, not updated. The mnemonic is MOVC for “move constant.” The first MOVC instruction in Table 11 can accommo- date a table of up to 256 entries numbered 0 through 255. The number of the desired entry is loaded into the Accumulator, and the Data Pointer is set up to point to the beginning of the table. Then: MOVC A, @A+DPTR copies the desired table entry into the Accumula- tor. The other MOVC instruction works the same way, except the Program Counter (PC) is used as the table base, and the table is accessed through a subroutine. First the number of the desired en-try is loaded into the Accumulator, and the subroutine is called: MOV A , ENTRY NUMBER CALL TABLE The subroutine “TABLE” would look like this: TABLE: MOVC A , @A+PC RET The table itself immediately follows the RET (re- turn) instruction is Program Memory. This type of table can have up to 255 entries, numbered 1 through 255. Number 0 cannot be used, because at the time the MOVC instruction is executed, the PC contains the address of the RET instruction. An entry numbered 0 would be the RET opcode it- self. Table 10. Data Transfer Instruction that Access External Data Memory Space Table 11. Lookup Table READ Instruction Address Width Mnemonic Operation 8 bits MOVX A,@Ri READ external RAM @Ri 8 bits MOVX @Ri,A WRITE external RAM @Ri 16 bits MOVX A,@DPTR READ external RAM @DPTR 16 bits MOVX @DPTR,a WRITE external RAM @DPTR Mnemonic Operation MOVC A,@A+DPTR READ program memory at (A+DPTR) MOVC A,@A+PC READ program memory at (A+PC) |
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