HT47C20 Holtek Semiconductor Inc, HT47C20 Datasheet - Page 9

• Manufacturer Part Number: HT47C20
• Description: 8-Bit Microcontroller
• Manufacturer: Holtek Semiconductor Inc

Functional Description
Execution flow
The system clock for the HT47C20 is derived
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 one
instruction cycle while decoding and execution
takes the next instruction cycle. However, the
pipelining scheme causes each instruction to ef-
fectively execute in one cycle. If an instruction
changes the program counter, two cycles are re-
quired to complete the instruction.
Program counter - PC
The 11-bit program counter (PC) controls the
sequence in which the instructions stored in the
program ROM are executed and its contents
specify a maximum of 2048 addresses.
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 containing the next instruction code.
When executing a jump instruction, conditional
skip execution, loading PCL register, subrou-
tine call, initial reset, internal interrupt, exter-
nal interrupt or return from subroutine, the PC
manipulates the program transfer by loading
the address corresponding to each instruction.
Execution flow
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The conditional skip is activated by instruction.
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 the proper instruction. Otherwise pro-
ceed with the next instruction.
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 256 loca-
tions.
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 2048´16 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 the initialization
program. After chip reset, the program al-
ways begins execution at location 000H.
Location 004H
This area is reserved for the external inter-
rupt service program. If the INT input pin is
activated, and the interrupt is enabled and
January 18, 2000
HT47C20