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HTG12G0

Microcontroller

Holtek Semiconductor 

HTG12G0

Functional Description

Program counter – PC

The 12-bit program counter is controlled by PA3

which can change the ROM bank of the program

memory. There are two program memory banks

which are selected by PA3, each bank is 4KB

ROM. The instruction“OUT PA, A” is used to

change the value of PA3. Then, low or high 4K

ROM is selected accordingly. All instructions

are not effective on the crossing bank, unless

the value of PA3 is changed in advance.

The 12-bit program counter (PC) controls the

sequence in which the instructions stored in the

program ROM are executed and its contents

specify a max. of 4096 address.

After accessing a memory word to fetch an in-

struction code, the contents of the program

counter are incremented by one or two, then the

program counter will point to the memory word

containing the next instruction code.

When executing the jump instruction (JMP,

JNZ, JC, JTMR...), subroutine call, internal in-

terrupt, RTC interrupt or return from subrou-

tine, the PC manipulates the program transfer

by loading the address corresponding to each

instruction.

Program memory – ROM

The program memory is used to store program

instruction which is to be executed. It is organ-

ized into 8192×8 bits and addressed by the pro-

gram counter and PA3.

Certain locations in bank 0 of the program

memory are reserved for specific usage:

• Location 0004H

This area is reserved for TIMER interrupt

service program. A timer interrupt results

from TIMER overflow, if interrupt is enabled,

the CPU begins execution at location 0004H.

Mode

Program Counter

PA3 PC11 PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0

Initial

reset

1

0

0 0000000

0 00

Internal

interrupt

PA3 0

0 0000000

1 00

External

interrupt

PA3 0

0 0000001

0 00

Jump, call

instruction

PA3 PC11 PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0

Conditional

branch

PA3

@

PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0

Return from

subroutine

PA3

S11

S10

S9

S8

S7

S6

S5

S4

S3

S2

S1 S0

Notes: PC11~PC0: Instruction code bits

S11~S0: Stack register bits

@: PC11 keeps current value

PA3: Bank value bits

8

14th May ’99

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