HT85F2260 Datasheet, PDF (29/226 Pages) Holtek Semiconductor Inc – Standard 8051 8-Bit Flash MCU

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.
P ro g ra m C o u n te r
T o p o f S ta c k
S ta c k
P o in te r
S ta c k L e v e l 1
S ta c k L e v e l 2
S ta c k L e v e l 3
P ro g ra m
M e m o ry
B o tto m o f S ta c k S ta c k L e v e l 2 5 6
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.
Rev. 1.00
29 of 226
May 15, 2013