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HT83F20P Datasheet(PDF) 8 Page - Holtek Semiconductor Inc |
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HT83F20P Datasheet(HTML) 8 Page - Holtek Semiconductor Inc |
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8 / 61 page  HT83FXX Rev. 1.00 8 May 12, 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. 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 has 4 levels and is neither part of the data nor part of the program space, and is neither readable nor writable. 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 con- tents of the Program Counter are pushed onto the stack. At the end of a subroutine or an interrupt routine, sig- naled by a return instruction, ²RET² or ²RETI², the Pro- gram 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. Mode Program Counter *10 *9 *8 *7 *6 *5 *4 *3 *2 *1 *0 Initial Reset 00000000000 Timer Base Overflow 00000000100 Timer Counter 0 Overflow 00000001000 Timer Counter 1 Overflow 00000001100 SIM Interrupt 00000010100 Skip Program Counter + 2 Loading PCL *10 *9 *8 @7 @6 @5 @4 @3 @2 @1 @0 Jump, Call Branch #10 #9 #8 #7 #6 #5 #4 #3 #2 #1 #0 Return from Subroutine S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0 Program Counter Note: *10~*0: Program counter bits S10~S0: Stack register bits #10~#0: Instruction code bits @7~@0: PCL bits 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 4 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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