HT95R25 datasheet(7/51 Pages) HOLTEK

7 / 51 page

HT95R25

Rev. 1.00

March 6, 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.

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 can neither be read from nor

written to. The activated level is indexed by the Stack

Pointer, SP, which can also neither be read from nor

written to. 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 by a return instruction, RET or , 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.

Mode

Program Counter Bits

b13

b12

b11

b10

Initial Reset

00000000000000

External Interrupt

00000000000100

Timer/Event Counter 0

Overflow

00000000001000

Timer/Event Counter 1

Overflow

00000000001100

Peripheral Interrupt

00000000010000

RTC Interrupt

00000000010100

Multi-Function Interrupt

00000000011000

Skip

Program Counter + 2 (Within current bank)

Loading PCL

PC13 PC12 PC11 PC10 PC9 PC8

Jump, Call Branch

#13

#12

#11

#10

Return from Subroutine

S13

S12

S11

S10

Program Counter

Note:

PC13~PC8: Current Program Counter bits

@7~@0: PCL bits

#13~#0: Instruction code address bits

S13~S0: Stack register bits

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