HT85F2270 datasheet(29/226 Pages) HOLTEK

29 / 226 page

Rev. 1.00

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Standard 8051 8-Bit Flash MCU

HT85F2260/HT85F2270/HT85F2280

15 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 “JMP” or “CALL” that demand a jump to a

non-consecutive Program Memory address.

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.

16 Stack

This is a special part of the memory which is used to save the contents of the Program Counter

only. The stack is located in the 256 byte Data memory; therefore, the depth can be extended

up to 256 levels. 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 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 location 0x07, the top of the stack.

Note that if the data memory has been used as the stack area, it should not be used as general

purpose Data RAM.

Pr og ra m Co un te r

St ac k Le ve l 1

St ac k Le ve l 2

St ac k Le ve l 3

St ac k Le ve l 256

Pr og ra m

Me mo ry

T op of S ta ck

St ac k

Po in te r

Bo tt om o f St ac k

Stack Block Diagram

If the stack is full and an enabled interrupt takes place, the interrupt request flag will be recorded

but the acknowledge signal will be inhibited. When the Stack Pointer is decremented, by RET or

RETI, the interrupt will be serviced. This feature prevents stack overflow allowing the programmer

to use the structure more easily. However, when the stack is full, a CALL subroutine instruction

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.

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