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HT95R24 Datasheet(PDF) 7 Page - Holtek Semiconductor Inc |
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HT95R24 Datasheet(HTML) 7 Page - Holtek Semiconductor Inc |
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7 / 53 page  HT95R24 Rev. 1.00 7 February 17, 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 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0 Initial Reset 0000000000000 External Interrupt 0000000000100 Timer/Event Counter 0 Overflow 0000000001000 Timer/Event Counter 1 Overflow 0000000001100 Peripheral Interrupt 0000000010000 RTC Interrupt 0000000010100 Multi-Function Interrupt 0000000011000 Skip Program Counter + 2 (Within current bank) Loading PCL PC12 PC11 PC10 PC9 PC8 @7 @6 @5 @4 @3 @2 @1 @0 Jump, Call Branch #12 #11 #10 #9 #8 #7 #6 #5 #4 #3 #2 #1 #0 Return from Subroutine S12 S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0 Program Counter Note: PC12~PC8: Current Program Counter bits @7~@0: PCL bits #12~#0: Instruction code address bits S12~S0: Stack register bits |
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