HT86B05 datasheet(25/79 Pages) HOLTEK

25 / 79 page

HT86BXX/HT86BRXX

Rev. 1.60

October 20, 2009

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.

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 writable 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 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 has 8 levels and is neither part of the data nor part

of the program space, and is neither readable nor

writable. The activated level is indexed by the Stack

Pointer, SP, and is neither readable nor writable. At a

subroutine call or interrupt acknowledge signal, the con-

tents of the Program Counter are pushed onto the stack.

At the end of a subroutine or an interrupt routine, sig-

naled by a return instruction,

²RET² or ²RETI², the Pro-

gram 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.

Mode

Program Counter

*12

*11

*10

Initial Reset

0000000000000

External Interrupt

0000000000100

Timer 0 Overflow

0000000001000

Timer 1 Overflow

0000000001100

Timer 2 Overflow

0000000010000

Timer 3 Overflow

0000000010100

Skip

Program Counter + 2

Loading PCL

*12

*11

*10

Jump, Call Branch

#12

#11

#10

Return from Subroutine

S12

S11

S10

Program Counter

Note:

*12~*0: Program counter bits

S12~S0: Stack register bits

#12~#0: Instruction code bits

@7~@0: PCL bits

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 8

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

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