HT48R50A datasheet(8/46 Pages) HOLTEK

8 / 46 page

Functional Description

HT48R50A-1

Rev. 1.10

July 2, 2001

Execution flow

The system clock for the microcontroller is de-

rived from either a crystal or an RC oscillator.

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 in-

struction cycle while decoding and execution

takes the next instruction cycle. However, the

pipelining scheme causes each instruction to ef-

fectively execute in a cycle. If an instruction

changes the program counter, two cycles are re-

quired to complete the instruction.

Program counter - PC

The program counter (PC) controls the se-

quence in which the instructions stored in the

program ROM are executed and its contents

specify a full range of program memory.

After accessing a program memory word to fetch

an instruction code, the contents of the program

counter are incremented by one. The program

counter then points to the memory word contain-

ing the next instruction code.

When executing a jump instruction, conditional

skip execution, loading PCL register, subrou-

tine call or return from subroutine, initial reset,

internal interrupt, external interrupt or return

from interrupts, the PC manipulates the pro-

gram transfer by loading the address corre-

sponding to each instruction.

The conditional skip is activated by instruc-

tions. Once the condition is met, the next in-

struction, fetched during the current

instruction execution, is discarded and a

dummy cycle replaces it to get the proper in-

struction. Otherwise proceed to the next in-

struction.

The lower byte of the program counter (PCL) is

a readable and writeable register (06H).

Moving data into the PCL performs a short

jump. The destination will be within the cur-

rent program ROM page.

When a control transfer takes place, an addi-

tional dummy cycle is required.

Program memory - ROM

The program memory is used to store the pro-

gram instructions which are to be executed. It

also contains data, table, and interrupt entries,

and is organized into 4096´15 bits, addressed

by the program counter and table pointer.

Certain locations in the program memory are

reserved for special usage:

· Location 000H

This area is reserved for program initializa-

tion. After chip reset, the program always be-

gins execution at location 000H.

F e tc h IN S T ( P C )

E x ec ut e I N S T ( P C - 1 )

F e tc h IN S T ( P C + 1 )

E x ec ut e I N S T ( P C )

F e tc h IN S T ( P C + 2 )

E x ec ut e I N S T ( P C + 1 )

PC + 1

PC + 2

S yst e m C l o ck

OS C 2 ( R C o nl y )

Execution flow

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