ST6387 datasheet(12/84 Pages) STMICROELECTRONICS

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ST6365, ST6375, ST6385 ST6367, ST6377, ST6387

MEMORY SPACES (Cont’d)

Data ROM Addressing. All the read-only data are

physically implemented in the ROM in which the

Program Space is also implemented. The ROM

therefore contains the program to be executed and

also the constants and the look-up tables needed

for the program. The locations of Data Space in

which the different constants and look-up tables

are addressed by the ST6 Core can be considered

as being a 64-byte window through which it is pos-

sible to access to the read-only data stored in the

ROM. This window is located from the 40h ad-

dress to the 7Fh address in the Data space and al-

lows the direct reading of the bytes from the 000h

address to the 03Fh address in the ROM. All the

bytes of the ROM can be used to store either in-

structions or read-only data. Indeed, the window

can be moved by step of 64 bytes along the ROM

in writing the appropriate code in the Write-only

Data ROM Window register (DRWR, location

C9h). The effective address of the byte to be read

as a data in the ROM is obtained by the concate-

nation of the 6 less significant bits of the address in

the Data Space (as less significant bits) and the

content of the DRWR (as most significant bits). So

when addressing location 40h of data space, and

0 is loaded in the DRWR, the physical addressed

location in ROM is 00h.

Note: The data ROM Window can not address

window above the 16K byte range.

Data ROM Window Register (DRWR)

Address: C9h - Write only

Reset Value: XXh

DRWR7-DRWR0. These are the Data Rom Win-

dow bits that correspond to the upper bits of data

ROM program space. This register is undefined af-

ter reset.

Caution: This register contains at least one write

only bit. Single bit instructions (SET, RES, INC

and DEC) should not be used.

Note: Care is required when handling the DRWR

as it is write only. For this reason, it is not allowed

to change the DRWR contents while executing in-

terrupts drivers, as the driver cannot save and

than restore its previous content. If it is impossible

to avoid the writing of this register in interrupts

drivers, an image of this register must be saved in

a RAM location, and each time the program writes

the DRWR it writes also the image register. The

image register must be written first, so if an inter-

rupt occurs between the two instructions the

DRWR register is not affected.

Figure 7. Data ROM Window Memory Addressing

DRWR

DATA ROM

WINDOW REGISTER

CONTENTS

DATA SPACE ADDRESS

40h-7Fh

IN INSTRUCTION

PROGRAM SPACE ADDRESS

765432

543

210

543

210

READ

VR01573B

DATA SPACE ADDRESS

59h

Example:

(DWR)

DWR=28h

ROM

ADDRESS:A19h

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