ht82k68a Holtek Semiconductor Inc., ht82k68a Datasheet - Page 6

• Manufacturer Part Number: ht82k68a
• Description: Ht82k68a -- Multimedia Keyboard Encoder 8-bit Mask Mcu
• Manufacturer: Holtek Semiconductor Inc.

Functional Description
Execution Flow
The HT82K68A system clock is derived from either a
crystal or an RC oscillator. The system clock is internally
divided into four non-overlapping clocks. One instruc-
tion 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 de-
coding and execution takes the next instruction cycle.
However, the pipelining scheme causes each instruc-
tion to effectively execute within one cycle. If an instruc-
tion changes the program counter, two cycles 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 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.
Note: *11~*0: Program counter bits
Rev. 1.70
Initial Reset
External Interrupt
Timer Counter Overflow
Skip
Loading PCL
Jump, Call Branch
Return from Subroutine
#11~#0: Instruction code bits
Mode
#11
S11
11
0
0
0
11
S10
#10
10
10
0
0
0
S9
#9
0
0
0
9
9
Execution Flow
S8
#8
0
0
0
8
8
6
@7
S7
#7
0
0
0
7
Program Counter+2
When executing a jump instruction, conditional skip ex-
ecution, loading PCL register, subroutine call, initial re-
set, internal interrupt, external 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 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.
Once a control transfer takes place, an additional
dummy cycle is required.
Program Memory - ROM
The program memory is used to store the program in-
structions 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 ta-
ble pointer.
S11~S0: Stack register bits
@7~@0: PCL bits
Program Counter
@6
#6
S6
0
0
0
6
@5
S5
#5
0
0
0
5
@4
#4
S4
0
0
0
4
@3
S3
#3
0
0
1
3
@2
S2
#2
December 26, 2005
0
1
0
2
HT82K68A
@1
S1
#1
0
0
0
1
@0
S0
#0
0
0
0
0