HT47C20 datasheet(13/56 Pages) HOLTEK

13 / 56 page

HT47C20

January 18, 2000

Arithmetic and logic unit - ALU

This circuit performs 8-bit arithmetic and logic

operation. The ALU provides the following

functions:

· Arithmetic operations (ADD, ADC, SUB,

SBC, DAA)

· Logic operations (AND, OR, XOR, CPL)

· Rotation (RL, RR, RLC, RRC)

· Increment and Decrement (INC, DEC)

· Branch decision (SZ, SNZ, SIZ, SDZ ....)

The ALU not only saves the results of a data op-

eration but can change the status register.

Status register - STATUS

This 8-bit register (0AH) contains the zero flag (Z),

carry flag (C), auxiliary carry flag (AC),

overflow flag (OV), power down flag (PD) and

watchdog time-out flag (TO). It also records the

status information and controls the operation se-

quence.

With the exception of the TO and PD flags, bits

in the status register can be altered by instruc-

tions like most other registers. Any data writ-

ten into the status register will not change the

TO or PD flags. In addition it should be noted

that operations related to the status register

may give different results from those intended.

The TO and PD flags can only be changed by

the watchdog timer overflow, system power-up,

clearing the watchdog timer and executing the

HALT instruction.

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

status of the latest operations.

In addition, on entering the interrupt sequence

or executing the subroutine call, the status reg-

ister will not be pushed onto the stack automat-

ically. If the contents of the status are

important and if the subroutine can corrupt the

status register, precautions must be taken to

save it properly.

Interrupts

The HT47C20 provides an external interrupt,

an internal timer/event counter interrupt, an

internal time base interrupt, and an internal

real time clock interrupt. The interrupt control

rupt control bits to set the enable/disable and

interrupt request flags.

Labels

Bits

Function

C is set if the operation results in a carry during an addition operation or if a bor-

row 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 the 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 logic operation is zero; otherwise Z is

cleared.

OV is set if the 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.

PD is cleared when either a system power-up or executing the CLR WDT in-

struction. PD is set by executing the HALT instruction.

TO is cleared by a system power-up or executing the CLR WDT or HALT in-

struction. TO is set by a WDT time-out.

Undefined, read as ²0²

STATUS register

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