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W742C814

4-BIT MICROCONTROLLER

Winbond 

W742C814

6. FUNCTIONAL DESCRIPTION

6.1 Program Counter (PC)

Organized as an 14-bit binary counter (PC0 to PC13), the program counter generates the addresses

of the 4096 × 16 on-chip ROM containing the program instruction words. Before the jump or

subroutine call instructions are to be executed, the destination ROM page must be determined firstly.

The confirmation of the ROM page can be done by executing the MOV ROMPR, #I or MOV ROMPR,

R instruction. When the interrupt or initial reset conditions are to be executed, the corresponding

address will be loaded into the program counter directly. The format used is shown below.

Table 1. Vector address and interrupt priority

ITEM

Initial Reset

INT 0 (Divider0)

INT 1 (Timer 0)

INT 2 (Port RC)

INT 3 (Divider1)

INT 4 (Timer 1)

JP Instruction

Subroutine Call

ADDRESS

0000H

0004H

0008H

000CH

0014H

0020H

XXXXH

XXXXH

INTERRUPT PRIORITY

-

1st

2nd

3rd

4th

5th

-

-

6.2 Stack Register (STACK)

The stack register is organized as 42 bits x 8 levels (first-in, last-out). When either a call subroutine or

an interrupt is executed, the program counter will be pushed onto the stack register automatically. At

the end of a call subroutine or an interrupt service subroutine, the RTN instruction must be executed

to pop the contents of the stack register into the program counter. (Refer to Table 8) When the stack

register is pushed over the eight levels, the contents of the first level will be lost. In other words, the

stack register is always sixteen levels deep.

6.3 Program Memory (ROM)

The read-only memory (ROM) is used to store program codes; and the look-up table is arranged as

16384 x 4 bits. The program ROM is divided into two pages; the size of each page is 2048 x 16 bits.

So the total ROM size is 4096 x 16 bits. Before the jump or subroutine call instructions are to be

executed, the destination ROM page must be determined firstly. The ROM page can be selected by

executing the MOV ROMPR, #I or MOV ROMPR, R instruction. But the branch decision instructions

(e.g. JB0, SKB0, JZ, JC, ...) must jump to the same ROM page which the branch decision instructions

are in. The whole ROM can store both instruction codes and the look-up table. Each look-up table

element is composed of 4 bits, so the look-up table can be addressed up to 16384 elements.

Instruction MOVC R is used to read the look-up table content and transfer table data to the RAM. But

before reading the addressed look-up table content, the content of the look-up table pointer (TAB)

must be determined firstly. The address of the look-up table element is allocated by the content of

Publication Release Date: February 12, 2003

-7-

Revision A2

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