HT95R25 datasheet(13/51 Pages) HOLTEK

13 / 51 page

HT95R25

Rev. 1.00

March 6, 2009

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

²INC² or ²DEC² 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.

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 addresses. 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 Watchdog

Timer clock source can be divided by various division ra-

tios, the value of which is set using the WDTS register.

By writing directly to this register, the appropriate divi-

sion ratio for the Watchdog Timer clock source can be

setup. Note that only the lower 3 bits are used to set divi-

sion ratios between 1 and 128.

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

²CLR WDT² or ²HALT² in-

struction. The PDF flag is affected only by executing the

²HALT² or ²CLR WDT² instruction or during a system

power-up.

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

the latest operations.

· 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.

T O

P D F

O V

A C

S T A T U S R e g i s t e r

A r i t h m e t i c / L o g i c O p e r a t i o n F l a g s

C a r r y f l a g

A u x i l i a r y c a r r y f l a g

Z e r o f l a g

O v e r f l o w f l a g

S y s t e m M a n a g e m e n t F l a g s

P o w e r d o w n f l a g

W a t c h d o g t i m e - o u t f l a g

N o t i m p l e m e n t e d , r e a d a s " 0 "

b 7

b 0

Status Register

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