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ADDRESSING UP TO 4 MBYTES OF EXTERNAL MEMORY

In the first part of this application note we will see how to use the existing interface to address

64 Kbytes or 128 Kbytes (ST92F120 only) of external memory without any additional software

overhead. In the second part we will see how to address almost 2 Mbytes with a ROMLESS or

almost 4 Mbytes with an OTP/EPROM/FLASH device. This last solution uses the P2 I/O port

to output the 22-bit address.

2 ACCESSING UP TO 64 KBYTES OF EXTERNAL MEMORY WITHOUT

SOFTWARE OVERHEAD

The ST9+ outputs a 16-bit address multiplexed with the 8-bit data. So, to access 64 Kbytes of

memory, we only need to make the correct hardware connections between the memory de-

vices and the microcontroller. These connections are described in Figure 2..

Figure 2. Accessing 64 Kbytes of external memory

The R/W signal is only needed for RAM and Flash or mixed memory components. The pull-up

resistor is needed only when using ROMLESS devices.

Examples of memory mapping are given in Figure 3. and Figure 4..

P0

P1

AS

DS

R/W

VCC

74HC573

ST9+

OTP

ROM

ROMLESS

FLASH

EXTERNAL MEMORY

16-bit address

8-bit data

G

ROM M27C1001

RAM M48Z64Y

Flash M29F010b

64 K

LE

74HC04

E

2