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HTG1390

4-bit Microcontroller

Holtek Semiconductor 

Preliminary

HTG1390

System Architecture

Program counter – PC

This counter addresses the program ROM and

is arranged as an 12-bit binary counter from

PC0 to PC11 whose contents specify a maxi-

mum of 4096 addresses. The program counter

counts with an increment of 1 or 2 with each

execution of an instruction.

When executing the jump instruction (JMP,

JNZ, JC, JTMR,...), a subroutine call, initial

reset, internal interrupt, external interrupt or

returning from a subroutine, the program

counter is loaded with the corresponding in-

struction data as shown in the table.

Notes: P0~P11: Instruction code

@: PC11 keeps current value

S0~S11: Stack register bits

Program memory

• Location 8

Activating the PS or PP input pins of the

processor with the interrupts enabled during

Halt mode causes the program to jump to this

location.

Program memory – ROM

The program memory is the executable memory

and is arranged in a 4096×8 bit format. The

address is specified by the program counter

(PC). Four special locations are reserved as de-

scribed as follows.

• Location 0

Activating the processor RES pin causes the

first instruction to be fetched from location 0.

• Location 4

Contains the timer interrupt resulting from a

TIMER overflow. If the interrupts are enabled it

causes the program to jump to this subroutine.

• Locations n00H to nFFH

These are the 256 bytes of each page in pro-

gram memory. This area from n00H to nFFH

and F00H to FFFH can be used as a look–up

table. Instructions such as READ R4A, READ

MR0A, READF R4A, READF MR0A can read

the table and transfer the contents of the

table to ACC and R4 or to ACC and a data

memory address specified by the register pair

R1,R0. However as R1,R0 can only store 8

bits, these instructions cannot fully specify

the full 12 bit program memory address. For

this reason a jump instruction should be first

used to place the program counter in the right

page. The above instructions can then be used

to read the look up table data.

Mode

Program Counter

PC11 PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0

Initial reset

Internal interrupt

0

0 0 0 00000000

0

0 0 0 00000100

External interrupt

0

0 0 0 00001000

Jump, call instruction P11 P10 P9 P8 P7 P6 P5 P4 P3 P2 P1 P0

Conditional branch

@ P10 P9 P8 P7 P6 P5 P4 P3 P2 P1 P0

Return from subroutine S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0

Program memory

7

17th Nov ’98

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