BS85B12-3 HOLTEK [Holtek Semiconductor Inc], BS85B12-3 Datasheet - Page 32

• Manufacturer Part Number: BS85B12-3
• Description: Touch Key Flash Type 8-Bit MCU with LCD/LED Driver
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

Rev. 1.00
Accumulator - ACC
Program Counter Low Register - PCL
Look-up Table Registers - TBLP, TBHP, TBLH
Status Register - STATUS
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.
To provide additional program control functions, the low byte of the Program Counter is made
accessible to programmers by locating it within the Special Purpose area of the Data Memory. By
manipulating this register, direct jumps to other program locations are easily implemented. Loading a
value directly into this PCL register will cause a jump to the specified Program Memory location,
however, as the register is only 8-bit wide, only jumps within the current Program Memory page are
permitted. When such operations are used, note that a dummy cycle will be inserted.
These three special function registers are used to control operation of the look-up table which is stored
in the Program Memory. TBLP and TBHP are the table pointer and indicates the location where the
table data is located. Their value must be setup before any table read commands are executed. Their
value can be changed, for example using the INC or 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 defined location.
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 management 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, operations related to the status register may give different results 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 instruction. The PDF flag is affected only by executing the
The Z, OV, AC and C flags generally reflect the status of the latest operations.
HALT or CLR WDT instruction or during a system power-up.
C 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.
AC 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.
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 highest-order bit but not a carry out of the
highest-order bit, or vice versa; otherwise OV is cleared.
Touch Key Flash MCU with LCD/LED Driver
26
BS85B12-3/BS85C20-3
February 1, 2011