HT85F2240 datasheet(47/147 Pages) HOLTEK

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Rev. 1.10

47 of 147

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A/D Flash 8051 Core Low Pin Count MCU

HT85F2230/HT85F2240

ACC Register – Accumulator

The Accumulator is central to the operation of any microcontroller and is closely related with

operations carried out by the ALU. The Accumulator is the place where all intermediate results

from the ALU are stored. Without the Accumulator it would be necessary to write the result of

each calculation or logical operation such as addition, subtraction, shift, etc., to the Data Memory

resulting in higher programming and timing overheads. Data transfer operations usually involve

the temporary storage function of the Accumulator; for example, when transferring data between

one user-defined register and another, it is necessary to do this by passing the data through the

Accumulator as no direct transfer between two registers is permitted.

B Register

The B register is used as a general purpose register for these devices. It is used during multiplying

and division instructions.

SP Register – Stack Pointer

The Stack Pointer register is 8 bits wide. It denotes the top of the Stack, which is the last used

value. The user can place the Stack anywhere in the internal scratchpad Data Memory by setting

the Stack Pointer to the desired location, although the lower bytes are normally used for working

registers. After a reset, the Stack Pointer is initialised to 07H. This causes the stack to begin at

location 08H. It is used to store the return address of the main program before executing interrupt

routines or subprograms. The SP is incremented before executing a PUSH or CALL instruction and

it is decremented after executing a POP, RET or RETI instruction.

DPL, DPH, DPL1, DPH1 Registers – Data Pointer Registers

The Data Pointer (DPTR) registers, DPL, DPH, DPL1 and DPH1, although having their locations

in normal Data Memory register space, do not actually physically exist as normal registers.

Indirect addressing instructions for Data Memory data manipulation use these Indirect Addressing

Registers and Memory Pointers, in contrast to direct memory addressing, where the actual memory

address is specified. Actions on the DPTR registers will result in no actual read or write operation

to these registers but rather to the memory location specified by their corresponding Memory

Pointer for the MOVX, MOVC or JMP instructions. The DPTR registers can be operated as two

16-bit registers or four individual 8-bit registers. There are two sets of 16-bit Data Pointer register,

DPTR1 and DPTR. The DPTR register is composed of DPL and DPH, while the DPTR1 register is

composed of DPL1 and DPH1. They are generally used to access external code or data space using

instructions such as “MOVC A,@A+DPTR” or “MOVX A,@DPTR” respectively. The selection

of DPTR or DPTR1 is controlled by the DPS0 bit. Setting the DPS0 bit high will select the DPTR1

DPH

DPL

DPH1

DPL1

DPTR

DPTR1

DPS0

Data Memor�

DPTRn Registers Control Block Diagram

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