ht82k74e Holtek Semiconductor Inc., ht82k74e Datasheet - Page 14

• Manufacturer Part Number: ht82k74e
• Description: 27mhz Keyboard/ Mouse Tx 8-bit Mcu
• Manufacturer: Holtek Semiconductor Inc.

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, TBHP
These three special function registers are used to con-
trol operation of the look-up table which is stored in the
Program Memory. TBLP is the table pointer and indi-
cates the location where the table data is located. Its
value must be setup before any table read commands
are executed. Its value can be changed, for example us-
ing the INC or DEC instructions, allowing for easy ta-
ble 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 defined location. Once TBHP is enabled, the in-
struction TABRDC [m] reads the ROM data as defined
by TBLP and TBHP value.
Rev. 1.00
Status Register
14
Otherwise, the TBHP function selected via a configura-
tion option is disabled, the instruction TABRDC [m]
reads the ROM data as defined by TBLP and the current
program counter bits.
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 or HALT in-
struction. The PDF flag is affected only by executing the
power-up.
The Z, OV, AC and C flags generally reflect the status of
the latest operations.
HALT or CLR WDT instruction or during a system
C is set if an operation results in a carry during an ad-
dition operation or if a borrow does not take place dur-
ing a subtraction operation; otherwise C is cleared. C
is also affected by a rotate through carry instruction.
AC is set if an operation results in a carry out of the
low nibbles in addition, or no borrow from the high nib-
ble into the low nibble in subtraction; otherwise AC is
cleared.
Z is set if the result of an arithmetic or logical operation
is zero; otherwise Z is cleared.
OV is set if an operation results in a carry into the high-
est-order bit but not a carry out of the highest-order bit,
or vice versa; otherwise OV is cleared.
PDF is cleared by a system power-up or executing the
TO is cleared by a system power-up or executing the
WDT time-out.
CLR WDT instruction. PDF is set by executing the
HALT instruction.
CLR WDT or HALT instruction. TO is set by a
HT82K74E/HT82K74EE
December 15, 2009