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HT48R54A Datasheet(PDF) 6 Page - Holtek Semiconductor Inc |
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HT48R54A Datasheet(HTML) 6 Page - Holtek Semiconductor Inc |
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6 / 39 page  HT48R54A Rev. 1.20 6 April 28, 2010 Functional Description Execution Flow The system clock for the microcontroller is derived from a crystal oscillator or an RC oscillator or a RTC oscilla- tor. The system clock is internally divided into four non-overlapping clocks. One instruction cycle consists of four system clock cycles. Instruction fetching and execution are pipelined in such a way that a fetch takes an instruction cycle while de- coding and execution takes the next instruction cycle. However, the pipelining scheme causes each instruc- tion to be effectively executed in a cycle. If an instruction changes the program counter, two cycles are required to complete the instruction. Program Counter - PC The program counter (PC) controls the sequence in which the instructions stored in the program ROM are executed and its contents specify a full range of pro- gram memory. After accessing a program memory word to fetch an in- struction code, the contents of the program counter are incremented by one. The program counter then points to the memory word containing the next instruction code. When executing a jump instruction, conditional skip ex- ecution, loading PCL register, subroutine call or return from subroutine, initial reset, internal interrupt, external interrupt or return from interrupt, the PC manipulates the program transfer by loading the address corresponding to each instruction. The conditional skip is activated by instructions. Once the condition is met, the next instruction, fetched during the current instruction execution, is discarded and a dummy cycle replaces it to get the proper instruction. Otherwise proceed with the next instruction. The lower byte of the program counter (PCL) is a read- able and writeable register (06H). Moving data into the PCL performs a short jump. The destination will be within 256 locations. When a control transfer takes place, an additional dummy cycle is required. Mode Program Counter *11 *10 *9 *8 *7 *6 *5 *4 *3 *2 *1 *0 Initial Reset 000000000000 External Interrupt 000000000100 Timer/Event Counter 0 Overflow 000000001000 Timer/Event Counter 1 Overflow 000000001100 Skip Program Counter+2 Loading PCL *11 *10 *9 *8 @7 @6 @5 @4 @3 @2 @1 @0 Jump, Call Branch #11 #10 #9 #8 #7 #6 #5 #4 #3 #2 #1 #0 Return from Subroutine S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0 Program Counter Note: *11~*0: Program counter bits S11~S0: Stack register bits #11~#0: Instruction code bits @7~@0: PCL bits T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 F e t c h I N S T ( P C ) E x e c u t e I N S T ( P C - 1 ) F e t c h I N S T ( P C + 1 ) E x e c u t e I N S T ( P C ) F e t c h I N S T ( P C + 2 ) E x e c u t e I N S T ( P C + 1 ) P C P C + 1 P C + 2 S y s t e m C l o c k O S C 2 ( R C o n l y ) P C Execution Flow |
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