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HTG1390

4-bit Microcontroller

Holtek Semiconductor 

Preliminary

HTG1390

Note that the page number n must be greater

than zero as some locations in page 0 are re-

served for specific usage as mentioned. This

area may function as normal program memory

as required.

The program memory mapping is shown in the

diagram.

In the execution of an instruction the program

counter is added before the execution phase, so

careful manipulation of READ MR0A and

READ R4A is needed in the page margin.

There are two areas in the data memory, the

temporary data area and the display data area.

Access to the temporary data area is from 00H

to 7FH. Locations E0H to FAH represent the

display data area. The locations between the

temporary and display data areas are unde-

fined and cannot be used.

When data is written into the display data area

it is automatically read by the LCD driver

which then generates the corresponding LCD

driving signals.

Stack register

The stack register is a group of registers used to

save the contents of the program counter (PC)

and is arranged in 13 bits×1 level. One bit is

used to store the carry flag. An interrupt will

force the contents of the PC and the carry flag

onto the stack register. A subroutine call will

also cause the PC contents to be pushed onto

the stack; however the carry flag will not be

stored. At the end of a subroutine or an inter-

rupt (indicated by a return instruction RET or

RETI), the contents of the stack register are

returned to the PC.

Executing “RETI” instruction will restore the

carry flag from stack register, but “RET” doesn’t.

Working registers – R0,R1,R2,R3,R4

There are 5 working registers (R0,R1,R2,R3,

R4) usually used to store the frequently ac-

cessed intermediate results. Using the instruc-

tions INC Rn and DEC Rn the working

registers can increment (+1) or decrement (–1).

The JNZ Rn (n=0,1,4) instruction makes effi-

cient use of the working registers as a program

loop counter. Also the register pairs R0,R1 and

R2,R3 are used as a data memory pointer when

the memory transfer instruction is executed.

Data memory – RAM

The static data memory (RAM) is arranged in

256×4 bit format and is used to store data. All of

the data memory locations are indirectly ad-

dressable through the register pair R1,R0 or

R3,R2; for example MOV A,[R3R2] or MOV

[R3R2],A.

Data memory

Accumulator – ACC

The accumulator is the most important data

register in the processor. It is one of the sources

of input to the ALU and the destination of the

results of the operations performed in the ALU.

Data to and from the I/O ports and memory also

passes through the accumulator.

Arithmetic and logic unit – ALU

This circuit performs the following arithmetic

and logical operations ...

• Add with or without carry

• Subtract with or without carry

• AND, OR, Exclusive-OR

• Rotate right, left through carry

• BCD decimal adjust for addition

• Increment, decrement

• Data transfers

• Branch decisions

The ALU not only outputs the results of data

operations, but also sets the status of the carry

flag (CF) in some instructions.

8

17th Nov ’98

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