ht82j31a Holtek Semiconductor Inc., ht82j31a Datasheet - Page 6

• Manufacturer Part Number: ht82j31a
• Description: 16 Channel A/d Mcu With Spi Interface
• Manufacturer: Holtek Semiconductor Inc.

When executing instructions requiring jumps to
non-consecutive addresses such as a jump instruction,
a subroutine call, interrupt or reset, etc., the
microcontroller manages program control by loading the
required address into the Program Counter. For condi-
tional skip instructions, once the condition has been
met, the next instruction, which has already been
fetched during the present instruction execution, is dis-
carded and a dummy cycle takes its place while the cor-
rect instruction is obtained.
The lower byte of the Program Counter, known as the
Program Counter Low register or PCL, is available for
program control and is a readable and writeable regis-
ter. By transferring data directly into this register, a short
program jump can be executed directly, however, as
only this low byte is available for manipulation, the
jumps are limited to the present page of memory, that is
256 locations. When such program jumps are executed
it should also be noted that a dummy cycle will be in-
serted.
The lower byte of the Program Counter is fully accessi-
ble under program control. Manipulating the PCL might
cause program branching, so an extra cycle is needed
to pre-fetch. Further information on the PCL register can
be found in the Special Function Register section.
Stack
This is a special part of the memory which is used to
save the contents of the Program Counter only. The
stack can have either 6 levels depending upon which
device is selected and is neither part of the data nor part
of the program space, and is neither readable nor
writable. The activated level is indexed by the Stack
Note:
Rev. 1.00
Initial Reset
INT_A External Interrupt
INT_B External Interrupt
Timer/Event Counter
Overflow
Reserved
SPI_A Interrupt
SPI_B Interrupt
Skip
Loading PCL
Jump, Call Branch
Return from Subroutine
PC11~PC8: Current Program Counter bits
@7~@0: PCL bits
#11~#0: Instruction code address bits
S11~S0: Stack register bits
Mode
PC11 PC10 PC9
S11
b11
#11
0
0
0
0
0
0
0
b10
S10
#10
0
0
0
0
0
0
0
b9
S9
#9
0
0
0
0
0
0
0
Program Counter
PC8
b8
S8
#8
0
0
0
0
0
0
0
6
@7
b7
S7
#7
Program Counter Bits
0
0
0
0
0
0
0
Pointer, SP, and is neither readable nor writeable. At a
subroutine call or interrupt acknowledge signal, the con-
tents of the Program Counter are pushed onto the stack.
At the end of a subroutine or an interrupt routine, sig-
naled by a return instruction, RET or RETI, the Program
Counter is restored to its previous value from the stack.
After a device reset, the Stack Pointer will point to the
top of the stack.
If the stack is full and an enabled interrupt takes place,
the interrupt request flag will be recorded but the ac-
knowledge signal will be inhibited. When the Stack
Pointer is decremented, by RET or RETI, the interrupt
will be serviced. This feature prevents stack overflow al-
lowing the programmer to use the structure more easily.
However, when the stack is full, a CALL subroutine in-
struction can still be executed which will result in a stack
overflow. Precautions should be taken to avoid such
cases which might cause unpredictable program
branching.
Arithmetic and Logic Unit - ALU
The arithmetic-logic unit or ALU is a critical area of the
microcontroller that carries out arithmetic and logic op-
erations of the instruction set. Connected to the main
Program Counter + 2
@6
b6
S6
#6
0
0
0
0
0
0
0
@5
b5
S5
#5
0
0
0
0
0
0
0
@4
b4
S4
#4
0
0
1
0
0
1
1
@3
b3
#3
S3
0
0
1
1
1
0
0
@2
September 19, 2007
b2
#2
S2
0
1
0
0
1
0
1
HT82J31A
@1
b1
#1
S1
0
0
0
0
0
0
0
@0
b0
#0
S0
0
0
0
0
0
0
0