ADE7566 datasheet(72/136 Pages) AD

72 / 136 page

ADE7566/ADE7569

Preliminary Technical Data

Rev. PrA | Page 72 of 136

BASIC 8052 REGISTERS

Program Counter (PC)

The program counter holds the two byte address of the next

instruction to be fetched. The PC is initialized with 0x00 at reset

and is incremented after each instruction is performed. Note

that the amount added to the PC depends on the number of

bytes in the instruction, so the increment can range from 1 byte

to 3 bytes. The program counter is not directly accessible to the

user but can be directly modified by CALL and JMP instructions

that change which part of the program is active.

Instruction Register (IR)

The instruction register holds the opcode of the instruction

being executed. The opcode is the binary code that results from

assembling an instruction. This register is not directly accessible

to the user.

There are four banks that each contain 8 byte-wide registers for a

total of 32 bytes of registers. These registers are convenient for

temporary storage of mathematical operands. An instruction

involving the accumulator and a register can be executed in 1 clock

cycle, as opposed to 2 clock cycles to perform an instruction

involving the accumulator and a literal or a byte of general-purpose

RAM. The register banks are located in the first 32 bytes of RAM.

The active register bank is selected by the RS0 and RS1 bits in

the Program Status Word SFR (PSW, 0xD0).

Accumulator

The accumulator is a working register, storing the results of

many arithmetic or logical operations. The accumulator is used

in more than half of the 8052 instructions where it is usually

referred to as A. The program status register (PSW) constantly

monitors the number of bits that are set in the accumulator to

determine if it has even or odd parity. The accumulator is stored

in the SFR space (see Table 52).

B Register

The B register is used by the multiply and divide instructions,

MUL AB and DIV AB to hold one of the operands. Because

the B register is not used for many instructions, it can be used

as a scratchpad register such as those in the register banks.

The B register is stored in the SFR space (see Table 52).

Program Status Word (PSW)

The PSW register reflects the status of arithmetic and logical

operations through carry, auxiliary carry, and overflow flags.

The parity flag reflects the parity of the accumulator contents,

which can be helpful for communication protocols. The PSW

bits are described in Table 53. The Program Status Word SFR

(PSW, 0xD0) is bit addressable.

Data Pointer (DPTR)

The data pointer is made up of two 8-bit registers: DPH (high

byte) and DPL (low byte). These provide memory addresses for

internal code and data access. The DPTR can be manipulated as

a 16-bit register (DPTR = DPH, DPL), or as two independent

8-bit registers (DPH, DPL). See Table 55 and Table 56.

The ADE7566/ADE7569 support dual data pointers. See the

Dual Data Pointers section.

Stack Pointer (SP)

The stack pointer keeps track of the current address of the top

of the stack. To push a byte of data onto the stack, the stack

pointer is incremented and the data is moved to the new top of

the stack. To pop a byte of data off of the stack, the top byte of

data is moved into the awaiting address, and the stack pointer is

decremented. The stack is a last in, first out (LIFO) method of

data storage because the most recent addition to the stack is the

first to come off it.

The stack is utilized during CALL and RET instructions to keep

track of the address to move into the PC when returning from

the function call. The stack is also manipulated when vectoring

for interrupts to keep track of the prior state of the PC.

The stack resides in the internal extended RAM, and the

SP register holds the address of the stack in the extended RAM

(XRAM). The advantage of this solution is that the stack is

segregated to the internal XRAM. The use of the general-

purpose RAM can be limited to data storing, and the use of the

extended internal RAM can be limited to the stack pointer. This

separation limits the chance of data RAM corruption when the

stack pointer overflows in data RAM.

Data can still be stored in XRAM by using the MOVX command.

To change the default starting address for the stack, move a

value into the stack pointer (SP). For example, to enable the

extended stack pointer and initialize it at the beginning of the

XRAM space, use the following code:

MOV

SP,#00H

0x00

0xFF

0x00

256 BYTES OF

ON-CHIP XRAM

DATA + STACK

256 BYTES OF

RAM

(DATA)

Figure 68. Extended Stack Pointer Operation

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