ST6369 datasheet(14/71 Pages) STMICROELECTRONICS

14 / 71 page

D7-D2. These bits are not used but have to be writ-

ten to “0”.

PRPR1-PRPR0. These are the program ROM

banking bits and the value loaded selects the cor-

responding page to be addressed in the lower part

of 4K program address space as specified in Table

3. This register is undefined on reset.

MEMORY SPACES (Continued)

Note. Only the lower part of address space has

been bankswitched because interrupt vectors and

common subroutines should be available all the

time. The reason of this structure is due to the fact

that it is not possible to jump from a dynamic page

to another, unless jumping back to the static page,

changing contents of PRPR, and, then, jumping to

a different dynamic page.

Care is required when handling the PRPR as it is

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

change the PRPR contents while executing inter-

rupts drivers, as the driver cannot save and than

restore its previous content. Anyway, this opera-

tion may be necessary if the sum of common rou-

tines and interrupt drivers will take more than 2K

bytes; in this case could be necessary to divide the

interrupt driver in a (minor) part in the static page

(start and end), and in the second (major) part in

one dynamic page. If it is impossible to avoid the

writing of this register in interrupts drivers, an im-

age of this register must be saved in a RAM loca-

tion, and each time the program writes the PRPR it

writes also the image register. The image register

must be written first, so if an interrupt occurs be-

tween the two instructions the PRPR is not af-

fected.

PRPR

Program ROM Page Register

(CAH, Write Only)

D7 D6 D5 D4 D3 D2 D1 D0

PRPR0

PRPR1

UNUSED

Figure 11. Program ROM Page Register

PRPR1

PRPR0

PC11

Memory

Page

Static Page (Page 1)

Page 0

Page 1

(Static Page)

Page 2

Page 3

Table 3. Program ROM Page Register Coding

ROM Page

Device Address

Description

PAGE 0

0000H-007FH

0080H-07FFH

Reserved

User ROM

PAGE 1

“STATIC”

0800H-0F9FH

0FA0H-0FEFH

0FF0H-0FF7H

0FF8H-0FFBH

0FFCH-0FFDH

0FFEH-0FFFH

User ROM

Reserved

Interrupt Vectors

Reserved

NMI Vector

Reset Vector

PAGE 2

0000H-000FH

0010H-07FFH

Reserved

User ROM

PAGE 3

0000H-000FH

0010H-07FFH

Reserved

User ROM

Table 4. ST6369 Program ROM Map

ST6369

10/67

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