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HT48RA0A

8-Bit Remote Type MCU

Holtek Semiconductor 

HT48RA0A

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 operation but

also changes the contents of the status register.

Oscillator Configuration

There are two oscillator circuits in the HT48RA0A.

O SC1

O SC1

O SC2

C r y s ta l O s c illa to r

fS Y S /4

(N M O S o p e n

d r a in o u tp u t)

System oscillator

O SC2

R C O s c illa to r

Status Register - STATUS

This 8-bit status 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 sequence.

With the exception of the TO and PD flags, bits in the

status register can be altered by instructions like most

other register. Any data written 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, chip 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 executing the subroutine call, the status

register will not be automatically pushed onto the stack.

If the contents of the status are important and if the sub-

routine can corrupt the status register, precautions must

be taken to save it properly.

Both are designed for system clocks; the RC oscillator

and the Crystal oscillator, which are determined by code

options. No matter what oscillator type is selected, the

signal provides the system clock. The HALT mode stops

the system oscillator and ignores the external signal to

conserve power.

If an RC oscillator is used, an external resistor between

OSC1 and VSS in needed and the resistance must

range from 51kW to 1MW. The system clock, divided by

4, is available on OSC2, which can be used to synchro-

nize external logic. The RC oscillator provides the most

cost effective solution. However, the frequency of the

oscillation may vary with VDD, temperature and the chip

itself due to process variations. It is, therefore, not suit-

able for timing sensitive operations where accurate os-

cillator frequency is desired.

If the Crystal oscillator is used, a crystal across OSC1

and OSC2 is needed to provide the feedback and phase

shift for the oscillator. No other external components are

needed. Instead of a crystal, the resonator can also be

connected between OSC1 and OSC2 to get a frequency

reference, but two external capacitors in OSC1 and

OSC2 are required.

Labels

C

AC

Z

OV

PD

TO

¾

Bits

Function

C is set if the operation results in a carry during an addition operation or if a borrow does not

0 take place during a subtraction operation; otherwise C is cleared. C is also affected by a rotate

through carry instruction.

1

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.

2 Z is set if the result of an arithmetic or logic operation is zero; otherwise Z is cleared.

3

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.

4

PD is cleared when either a system power-up or executing the CLR WDT instruction. PD is set

by executing the HALT instruction.

5

TO is cleared by a system power-up or executing the CLR WDT or HALT instruction. TO is set

by a WDT time-out.

6~7 Unused bit, read as ²0²

Status register

Rev. 1.70

6

July 16, 2003

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