HT46R92 datasheet(8/61 Pages) HOLTEK

8 / 61 page

HT46R92

Rev. 1.10

November 5, 2008

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 can be read nor written to. By trans-

ferring 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 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 organised into 8 levels and is neither part of the

data nor part of the program space, and is neither be

read nor written to. The activated level is indexed by the

Stack Pointer, SP, and can neither be read 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 rou-

tine, signaled by a return instruction, 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.

Mode

Program Counter Bits

b10

Initial Reset

0000000000

External Interrupt

0000000100

Timer/Event Counter Overflow

0000001000

Time Base Interrupt

0000001100

A/D Converter Interrupt

0000010000

Skip

Program Counter + 2

Loading PCL

PC10

PC9

PC8

Jump, Call Branch

#10

Return from Subroutine

S10

Program Counter

Note:

PC10~PC8: Current Program Counter bits

@7~@0: PCL bits

#10~#0: Instruction code address bits

S10~S0: Stack register bits

The Program Counter is 11 bits wide, i.e. from b10~b0.

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

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