HT46R01B/02B/01N/02N
HT48R01B/02B/01N/02N
Program Counter
neither readable nor writeable. At a subroutine call or in-
terrupt acknowledge signal, the contents of the Program
Counter are pushed onto the stack. At the end of a sub-
routine or an interrupt routine, signaled by a return in-
struction, 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.
During program execution, the Program Counter is used
to keep track of the address of the next instruction to be
executed. It is automatically incremented by one each
time an instruction is executed except for instructions,
such as ²JMP² or ²CALL² that demand a jump to a
non-consecutive Program Memory address. Note that
the Program Counter width varies with the Program
Memory capacity depending upon which device is se-
lected. However, it must be noted that only the lower 8
bits, known as the Program Counter Low Register, are
directly addressable by user.
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.
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.
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Program Counter
Device
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Program Counter
High Byte
PCL
Register
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HT46R01B/01N
HT48R01B/01N
PC9~PC8
PCL7~PCL0
PCL7~PCL0
Arithmetic and Logic Unit - ALU
HT46R02B/02N
HT48R02B/02N
PC10~PC8
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
microcontroller data bus, the ALU receives related in-
struction codes and performs the required arithmetic or
logical operations after which the result will be placed in
the specified register. As these ALU calculation or oper-
ations may result in carry, borrow or other status
changes, the status register will be correspondingly up-
dated to reflect these changes. The ALU supports the
following functions:
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.
·
·
·
Arithmetic operations: ADD, ADDM, ADC, ADCM,
SUB, SUBM, SBC, SBCM, DAA
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.
Logic operations: AND, OR, XOR, ANDM, ORM,
XORM, CPL, CPLA
Rotation RRA, RR, RRCA, RRC, RLA, RL, RLCA,
RLC
·
·
Increment and Decrement INCA, INC, DECA, DEC
Stack
Branch decision, JMP, SZ, SZA, SNZ, SIZ, SDZ,
SIZA, SDZA, CALL, RET, RETI
This is a special part of the memory which is used to
save the contents of the Program Counter only. The
stack is neither part of the Data or Program Memory
space, and is neither readable nor writeable. The acti-
vated level is indexed by the Stack Pointer, SP, and is
Rev.1.10
12
February 12, 2010
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
