HT45F23 datasheet(21/152 Pages) HOLTEK

21 / 152 page

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 selected. 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 conditional skip instructions, once the condition has

been met, the next instruction, which has already been fetched during the present instruction

execution, is discarded and a dummy cycle takes its place while the correct instruction is obtained.

Program Counter

Program Counter High Byte

PCL Register

PC10~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 register. 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 inserted.

The lower byte of the Program Counter is fully accessible 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 organized into 6 levels and is neither part of the Data or Program Memory space, and is

neither readable nor writeable. The activated level is indexed by the Stack Pointer, SP, and is neither

readable nor writeable. At a subroutine call or interrupt acknowledge signal, the contents of the

Program Counter are pushed onto the stack. At the end of a subroutine or an interrupt routine, signaled

HT45F23/HT45F43

8-Bit Flash MCU with Op Amps & Comparators

Rev. 1.20

September 15, 2011

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

S t a c k L e v e l 3

S t a c k L e v e l 6

P r o g r a m

M e m o r y

T o p o f S t a c k

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

F e t c h I n s t . 1

E x e c u t e I n s t . 1

F e t c h I n s t . 2

F l u s h P i p e l i n e

D E L A Y :

M O V A , [ 1 2 H ]

C A L L D E L A Y

C P L [ 1 2 H ]

N O P

E x e c u t e I n s t . 2

F e t c h I n s t . 3

F e t c h I n s t . 6

E x e c u t e I n s t . 6

F e t c h I n s t . 7

Instruction Fetching

HT45F23 Datasheet(PDF) 21 Page - Holtek Semiconductor Inc

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