Accumulator - ACC
The Accumulator is central to the operation of any microcontroller and is closely related with opera
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tions 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 calcula
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tion 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 tempo
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rary 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 Accumula
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tor 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 ac
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cessible to programmers by locating it within the Special Purpose area of the Data Memory. By ma
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nipulating 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.
Look-up Table Registers - TBLP, TBHP, TBLH
These three special function registers are used to control operation of the look-up table which is
stored in the Program Memory. TBLP is the table low byte pointer and indicates 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 using the ²INC² or ²DEC² instructions, allowing for easy ta
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ble data pointing and reading. TBHP is the table high byte pointer but is only available in the
HT48RU80/HT48CU80 devices. 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.
Watchdog Timer Register - WDTS
The Watchdog feature of the microcontroller provides an automatic reset function giving the
microcontroller a means of protection against spurious jumps to incorrect Program Memory ad-
dresses. To implement this, a timer is provided within the microcontroller which will issue a reset
command when its value overflows. To provide variable Watchdog Timer reset times, the Watch
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dog Timer clock source can be divided by various division ratios, the value of which is set using the
WDTS register. By writing directly to this register, the appropriate division ratio for the Watchdog
Timer clock source can be setup. Note that only the lower 3 bits are used to set division ratios be
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tween 1 and 128, the remaining 5 bits of the 8-bit register can be used by programmers for other
purposes.
Chapter 1 Hardware Structure
33
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