HT66F016 datasheet(25/116 Pages) HOLTEK

25 / 116 page

HT66F016/HT66F017/HT68F016/HT68F017

Rev. 1.00

May 14, 2012

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.

· 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

²CLR WDT² instruction. PDF is set by executing the

²HALT² instruction.

· TO is cleared by a system power-up or executing the

²CLR WDT² or ²HALT² instruction. TO is set by a

WDT time-out.

In addition, on entering an interrupt sequence or execut-

ing a subroutine call, the status register will not be

pushed onto the stack automatically. If the contents of

the status registers are important and if the subroutine

can corrupt the status register, precautions must be

taken to correctly save it.

· STATUS Register

Bit

76543210

Name

¾¾

PDF

R/W

¾¾

R/W

POR

¾¾

xxxx

²x² unknown

Bit 7, 6

Unimplemented, read as

²0²

Bit 5

TO: Watchdog Time-Out flag

0: After power up or executing the

²CLR WDT² or ²HALT² instruction

1: A watchdog time-out occurred.

Bit 4

PDF: Power down flag

0: After power up or executing the

²CLR WDT² instruction

1: By executing the

²HALT² instruction

Bit 3

OV: Overflow flag

0: no overflow

1: an operation results in a carry into the highest-order bit but not a carry out of the

highest-order bit or vice versa.

Bit 2

Z: Zero flag

0: The result of an arithmetic or logical operation is not zero

1: The result of an arithmetic or logical operation is zero

Bit 1

AC: Auxiliary flag

0: no auxiliary carry

1: 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

Bit 0

C: Carry flag

0: no carry-out

1: an operation results in a carry during an addition operation or if a borrow does not take place

during a subtraction operation

C is also affected by a rotate through carry instruction.

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