UPSD323X datasheet(26/176 Pages) STMICROELECTRONICS

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µPSD323X

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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

bank,

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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