HT49C10 datasheet(8/47 Pages) HOLTEK

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HT49C10

September 28, 1999

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

O S C2 ( RC o n l y )

Execution flow

Functional Description

Execution flow

The system clock is derived from either a crys-

tal or an RC oscillator. It 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 one instruction

cycle while decoding and execution takes the

next instruction cycle. The pipelining scheme

causes each instruction to effectively execute in

a cycle. If an instruction changes the value of

the program counter, two cycles are required to

complete the instruction.

Program counter - PC

The 10-bit program counter (PC) controls the

sequence in which the instructions stored in the

program ROM are executed. The contents of

the PC can specify a maximum of 1024 ad-

dresses.

After accessing a program memory word to fetch

an instruction code, the value of the PC is incre-

mented by one. The PC then points to the mem-

ory word containing the next instruction code.

When executing a jump instruction, conditional

skip execution, loading a PCL register, a sub-

routine call, an initial reset, an internal inter-

rupt, an external interrupt, or returning from a

subroutine, the PC manipulates 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 execu-

tion, is discarded and a dummy cycle replaces it

to get a proper instruction; otherwise proceed

with the next instruction.

The lower byte of the PC (PCL) is a readable

and writeable register (06H). Moving data into

the PCL performs a short jump. The destina-

tion is within 256 locations.

When a control transfer takes place, an addi-

tional dummy cycle is required.

Program memory - ROM

The program memory (ROM) is used to store

the program instructions which are to be exe-

cuted. It also contains data, table, and inter-

rupt entries, and is organized into 1024´14 bits

which are addressed by the PC and table

pointer.

Certain locations in the ROM are reserved for

special usage:

· Location 000H

Location 000H is reserved for program initial-

ization. After chip reset, the program always

begins execution at this location.

· Location 004H

Location 004H is reserved for the external in-

terrupt service program. If the INT0 input

pin is activated, and the interrupt is enabled,

and the stack is not full, the program begins

execution at location 004H.

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