AN1069 Datasheet, PDF (1/14 Pages) STMicroelectronics – This application note is to help developers

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-
dress is output on two 8-bit ports: P0 and P1. This allows the user to address 64 Kbytes (2¹⁶)
of external memory directly.
Figure 1. ST9+ Memory Access Overview
CORE
8-BIT DATA
22-BIT ADDRESS
ST9+
INTERNAL MEMORY
EXTERNAL MEMORY
INTERFACE
16-BIT ADDRESS
MULTIPLEXED
WITH 8-BIT DATA
P0[0..7]
P1[0..7]
AS, RW, DS,
DS2 (ST92F120
only)
AN1069/0700
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