AN1069/0700

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AN1069

APPLICATION NOTE

ADDRESSING UP TO 4 MBYTES OF MEMORY FROM A ST9+

WITH A 16-BIT EXTERNAL BUS

By Microcontroller Division Applications

INTRODUCTION

This application note is to help developers of ST9+ applications that need to address external

memory. It refers to ST9+ microcontrollers which have only a 16-bit external address bus

(such as the ST90158). The information in this application note does not apply to ST9+ micro-

controllers that have a 22-bit external address bus.

1 ST9 EXTERNAL MEMORY MANAGEMENT

Through the Memory Management Unit (MMU), ST9+ devices can internally address up to 4M

bytes of linear memory using a 22-bit address bus (refer to the datasheet or ST9+ user guide

for more information on the MMU).

Nevertheless, most general purpose ST9+ microcontrollers do not output this 22-bit address

bus, they only output the 16 Least Significant Bits (LSBs) of the 22-bit address. This 16-bit ad-

of external memory directly.

Figure 1. ST9+ Memory Access Overview

CORE

22-BIT ADDRESS

8-BIT DATA

ST9+

EXTERNAL MEMORY

INTERFACE

P0[0..7]

P1[0..7]

AS, RW, DS,

16-BIT ADDRESS

WITH 8-BIT DATA

DS2 (ST92F120

INTERNAL MEMORY

only)

MULTIPLEXED

1