HT46R46/C46/R47/C47/R48A/C48A/R49
Rev. 1.41
46
December 30, 2008
Instruction Set
Introduction
C e n t r a l t o t h e s u c c e s s f u l o p e r a t i o n o f a n y
microcontroller is its instruction set, which is a set of pro-
gram instruction codes that directs the microcontroller to
perform certain operations. In the case of Holtek
microcontrollers, a comprehensive and flexible set of
over 60 instructions is provided to enable programmers
to implement their application with the minimum of pro-
gramming overheads.
For easier understanding of the various instruction
codes, they have been subdivided into several func-
tional groupings.
Instruction Timing
Most instructions are implemented within one instruc-
tion cycle. The exceptions to this are branch, call, or ta-
ble read instructions where two instruction cycles are
required. One instruction cycle is equal to 4 system
clock cycles, therefore in the case of an 8MHz system
oscillator, most instructions would be implemented
within 0.5
m
s and branch or call instructions would be im-
plemented within 1
m
s. Although instructions which re-
quire one more cycle to implement are generally limited
to the JMP, CALL, RET, RETI and table read instruc-
tions, it is important to realize that any other instructions
which involve manipulation of the Program Counter Low
register or PCL will also take one more cycle to imple-
ment. As instructions which change the contents of the
PCL will imply a direct jump to that new address, one
more cycle will be required. Examples of such instruc-
tions would be
²
CLR PCL
²
or
²
MOV PCL, A
²
. For the
case of skip instructions, it must be noted that if the re-
sult of the comparison involves a skip operation then
this will also take one more cycle, if no skip is involved
then only one cycle is required.
Moving and Transferring Data
The transfer of data within the microcontroller program
is one of the most frequently used operations. Making
use of three kinds of MOV instructions, data can be
transferred from registers to the Accumulator and
vice-versa as well as being able to move specific imme-
diate data directly into the Accumulator. One of the most
important data transfer applications is to receive data
from the input ports and transfer data to the output ports.
Arithmetic Operations
The ability to perform certain arithmetic operations and
data manipulation is a necessary feature of most
microcontroller applications. Within the Holtek
microcontroller instruction set are a range of add and
subtract instruction mnemonics to enable the necessary
arithmetic to be carried out. Care must be taken to
ensure correct handling of carry and borrow data when
results exceed 255 for addition and less than 0 for sub-
traction. The increment and decrement instructions
INC, INCA, DEC and DECA provide a simple means of
increasing or decreasing by a value of one of the values
in the destination specified.
Logical and Rotate Operations
The standard logical operations such as AND, OR, XOR
and CPL all have their own instruction within the Holtek
microcontroller instruction set. As with the case of most
instructions involving data manipulation, data must pass
through the Accumulator which may involve additional
programming steps. In all logical data operations, the
zero flag may be set if the result of the operation is zero.
Another form of logical data manipulation comes from
the rotate instructions such as RR, RL, RRC and RLC
which provide a simple means of rotating one bit right or
left. Different rotate instructions exist depending on pro-
gram requirements. Rotate instructions are useful for
serial port programming applications where data can be
rotated from an internal register into the Carry bit from
where it can be examined and the necessary serial bit
set high or low. Another application where rotate data
operations are used is to implement multiplication and
division calculations.
Branches and Control Transfer
Program branching takes the form of either jumps to
specified locations using the JMP instruction or to a sub-
routine using the CALL instruction. They differ in the
sense that in the case of a subroutine call, the program
must return to the instruction immediately when the sub-
routine has been carried out. This is done by placing a
return instruction RET in the subroutine which will cause
the program to jump back to the address right after the
CALL instruction. In the case of a JMP instruction, the
program simply jumps to the desired location. There is
no requirement to jump back to the original jumping off
point as in the case of the CALL instruction. One special
and extremely useful set of branch instructions are the
conditional branches. Here a decision is first made re-
garding the condition of a certain data memory or indi-
vidual bits. Depending upon the conditions, the program
will continue with the next instruction or skip over it and
jump to the following instruction. These instructions are
the key to decision making and branching within the pro-
gram perhaps determined by the condition of certain in-
put switches or by the condition of internal data bits.
