HT82K68 Holtek Semiconductor Inc, HT82K68 Datasheet - Page 7

• Manufacturer Part Number: HT82K68
• Description: Multimedia Keyboard Encoder OTP
• Manufacturer: Holtek Semiconductor Inc

Functional Description
Execution flow
The HT82K68E system clock is derived from ei-
ther a crystal or an RC oscillator. The system
cl ock is inter nal l y di vi ded in to 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
effectively execute within one cycle. If an in-
struction changes the program counter, two cy-
cles are required to complete the instruction.
Program counter - PC
The 12-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 4096 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-
Preliminary
Execution flow
7
nal interrupt or return from subroutine, the PC
manipulates the program transfer by loading
the address corresponding to each instruction.
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 locations.
Once a control transfer takes place, an addi-
tional dummy cycle is required.
Program memory - PROM
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 with 3072 ´ 16 bits, addressed
by the program counter and table pointer.
Certain locations in the program memory are
reserved for special usage:
·
Location 000
This area is reserved for the initialization
program. After chip reset, the program al-
ways begins execution at location 000H.
HT82K68E
August 8, 2000