HT48R005 datasheet(13/62 Pages) HOLTEK

13 / 62 page

HT48R005/HT46R005

Rev. 1.00

November 29, 2011

Program Counter

During program execution, the Program Counter is used

to keep track of the address of the next instruction to be

executed. It is automatically incremented by one each

time an instruction is executed except for instructions,

such as

²JMP² or ²CALL² that demand a jump to a

non-consecutive Program Memory address. Note that

the Program Counter width varies with the Program

Memory capacity depending upon which device is se-

lected. However, it must be noted that only the lower 8

bits, known as the Program Counter Low Register, are

directly addressable by user.

When executing instructions requiring jumps to

non-consecutive addresses such as a jump instruction,

a subroutine call, interrupt or reset, etc., the

microcontroller manages program control by loading the

required address into the Program Counter. For condi-

tional skip instructions, once the condition has been

met, the next instruction, which has already been

fetched during the present instruction execution, is dis-

carded and a dummy cycle takes its place while the cor-

rect instruction is obtained.

Device

Program Counter

High Byte

PCL Register

HT48R005

HT46R005

PC9~PC8

PCL7~PCL0

The lower byte of the Program Counter, known as the

Program Counter Low register or PCL, is available for

program control and is a readable and writeable regis-

ter. By transferring data directly into this register, a short

program jump can be executed directly, however, as

only this low byte is available for manipulation, the

jumps are limited to the present page of memory, that is

256 locations. When such program jumps are executed

it should also be noted that a dummy cycle will be in-

serted.

The lower byte of the Program Counter is fully accessi-

ble under program control. Manipulating the PCL might

cause program branching, so an extra cycle is needed

to pre-fetch. Further information on the PCL register can

be found in the Special Function Register section.

Stack

This is a special part of the memory which is used to

save the contents of the Program Counter only. The

stack is neither part of the Data or Program Memory

space, and is neither readable nor writeable. The acti-

vated level is indexed by the Stack Pointer, SP, and is

neither readable nor writeable. At a subroutine call or in-

terrupt acknowledge signal, the contents of the Program

Counter are pushed onto the stack. At the end of a sub-

routine or an interrupt routine, signaled by a return in-

struction, RET or RETI, the Program Counter is restored

to its previous value from the stack. After a device reset,

the Stack Pointer will point to the top of the stack.

If the stack is full and an enabled interrupt takes place,

the interrupt request flag will be recorded but the ac-

knowledge signal will be inhibited. When the Stack

Pointer is decremented, by RET or RETI, the interrupt

will be serviced. This feature prevents stack overflow al-

lowing the programmer to use the structure more easily.

However, when the stack is full, a CALL subroutine in-

struction can still be executed which will result in a stack

overflow. Precautions should be taken to avoid such

cases which might cause unpredictable program

branching.

Arithmetic and Logic Unit

- ALU

The arithmetic-logic unit or ALU is a critical area of the

microcontroller that carries out arithmetic and logic op-

erations of the instruction set. Connected to the main

microcontroller data bus, the ALU receives related in-

struction codes and performs the required arithmetic or

logical operations after which the result will be placed in

the specified register. As these ALU calculation or oper-

ations may result in carry, borrow or other status

changes, the status register will be correspondingly up-

dated to reflect these changes. The ALU supports the

following functions:

· Arithmetic operations: ADD, ADDM, ADC, ADCM,

SUB, SUBM, SBC, SBCM, DAA

· Logic operations: AND, OR, XOR, ANDM, ORM,

XORM, CPL, CPLA

· Rotation RRA, RR, RRCA, RRC, RLA, RL, RLCA,

RLC

· Increment and Decrement INCA, INC, DECA, DEC

· Branch decision, JMP, SZ, SZA, SNZ, SIZ, SDZ,

SIZA, SDZA, CALL, RET, RETI

P r o g r a m C o u n t e r

S t a c k L e v e l 1

S t a c k L e v e l 2

P r o g r a m

M e m o r y

T o p o f

S t a c k

P o i n t e r

B o t t o m

o f S t a c k

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