AT89LP428 datasheet(17/149 Pages) ATMEL

17 / 149 page

3654A–MICRO–8/09

AT89LP428/828

Figure 5-3.

Two-cycle ALU Operation (Example: ADD A, #data)

5.1

Enhanced Dual Data Pointers

The AT89LP428/828 provides two 16-bit data pointers: DPTR0 formed by the register pair

DPOL and DPOH (82H an 83H), and DPTR1 formed by the register pair DP1L and DP1H (84H

and 85H). The data pointers are used by several instructions to access the program or data

memories. The Data Pointer Configuration Register (DPCF) controls operation of the dual data

pointers (Table 5-4). The DPS bit in DPCF selects which data pointer is currently referenced by

instructions including the DPTR operand. Each data pointer may be accessed at its respective

SFR addresses regardless of the DPS value. The AT89LP428/828 provides two methods for

fast context switching of the data pointers:

• Bit 2 of DPCF is hard-wired as a logic 0. The DPS bit may be toggled (to switch data pointers)

simply by incrementing the DPCF register, without altering other bits in the register

unintentionally. This is the preferred method when only a single data pointer will be used at

one time.

EX:

INC

DPCF

; Toggle DPS

•In some cases, both data pointers must be used simultaneously. To prevent frequent toggling

of DPS, the AT89LP428/828 supports a prefix notation for selecting the opposite data pointer

per instruction. All DPTR instructions, with the exception of JMP @A+DPTR, when prefixed

with an 0A5H opcode will use the inverse value of DPS (DPS) to select the data pointer.

Some assemblers may support this operation by using the /DPTR operand. For example,

the following code performs a block copy within EDATA:

MOV

DPCF, #00H

; DPS = 0

MOV

DPTR, #SRC

; load source address to dptr0

MOV

/DPTR, #DST

; load destination address to dptr1

MOV

R7, #BLKSIZE

; number of bytes to copy

COPY:

MOVX A, @DPTR

; read source (dptr0)

INC

DPTR

; next src (dptr0+1)

MOVX @/DPTR, A

; write destination (dptr1)

INC

/DPTR

; next dst (dptr1+1)

DJNZ R7, COPY

System Clock

Total Execution Time

Fetch Immediate Operand

ALU Operation Execute

Result Write Back

Fetch Next Instruction

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