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HT46R46/C46/R47/C47/R48A/C48A/R49

Rev. 1.41

December 30, 2008

Accumulator

ACC

The Accumulator is central to the operation of any
microcontroller and is closely related with operations
carried out by the ALU. The Accumulator is the place
where all intermediate results from the ALU are stored.
Without the Accumulator it would be necessary to write
the result of each calculation or logical operation such
as addition, subtraction, shift, etc., to the Data Memory
resulting in higher programming and timing overheads.
Data transfer operations usually involve the temporary
storage function of the Accumulator; for example, when
transferring data between one user defined register and
another, it is necessary to do this by passing the data
through the Accumulator as no direct transfer between
two registers is permitted.

Program Counter Low Register

PCL

To provide additional program control functions, the low
byte of the Program Counter is made accessible to pro-
grammers by locating it within the Special Purpose area
of the Data Memory. By manipulating this register, direct
jumps to other program locations are easily imple-
mented. Loading a value directly into this PCL register
will cause a jump to the specified Program Memory lo-
cation, however, as the register is only 8-bit wide, only
jumps within the current Program Memory page are per-
mitted. When such operations are used, note that a
dummy cycle will be inserted.

Look-up Table Registers

TBLP, TBLH

These two special function registers are used to control
operation of the look-up table which is stored in the Pro-
gram Memory. TBLP is the table pointer and indicates
the location where the table data is located. Its value
must be setup before any table read commands are ex-
ecuted. Its value can be changed, for example using the

INC

DEC

instructions, allowing for easy table data

pointing and reading. TBLH is the location where the
high order byte of the table data is stored after a table
read data instruction has been executed. Note that the
lower order table data byte is transferred to a user de-
fined location.

Status Register

STATUS

This 8-bit register contains the zero flag (Z), carry flag
(C), auxiliary carry flag (AC), overflow flag (OV), power
down flag (PDF), and watchdog time-out flag (TO).
These arithmetic/logical operation and system manage-
ment flags are used to record the status and operation of
the microcontroller.

With the exception of the TO and PDF flags, bits in the
status register can be altered by instructions like most
other registers. Any data written into the status register
will not change the TO or PDF flag. In addition, opera-
tions related to the status register may give different re-
sults due to the different instruction operations. The TO
flag can be affected only by a system power-up, a WDT

time-out or by executing the

CLR WDT

HALT

in-

struction. The PDF flag is affected only by executing the

HALT

CLR WDT

instruction or during a system

power-up.

The Z, OV, AC and C flags generally reflect the status of
the latest operations.

is set if an operation results in a carry during an

addition operation or if a borrow does not take place
during a subtraction operation; otherwise C is
cleared. C is also affected by a rotate through carry
instruction.

is set if an operation results in a carry out of the

low nibbles in addition, or no borrow from the high
nibble into the low nibble in subtraction; otherwise
AC is cleared.

is set if the result of an arithmetic or logical opera-

tion is zero; otherwise Z is cleared.

is set if an operation results in a carry into the

highest-order bit but not a carry out of the high-
est-order bit, or vice versa; otherwise OV is cleared.

PDF

is cleared by a system power-up or executing

the

CLR WDT

instruction. PDF is set by executing

the