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ADDRESSING UP TO 4 MBYTES OF EXTERNAL MEMORY

3.3.2 Example Using a ROMless microcontroller

If we use a ROMLESS ST90158 device (which has no internal ROM), we cannot use internal

memory as a Static Segment, therefore we have to create an external Static Segment. An ex-

ample of the hardware implementation is given in Figure 9.. The external memory capability is

divided into 2 parts, the Static segments and the Dynamic segments. In this example, A15 is

used to select either Static or Dynamic segments. As a consequence of this, the Static and

Dynamic Segments are 32K bytes in size so we can access only 32k of external memory in

each segment. If you compare the address of the external memory described in Figure 9. and

the address of the ST9 described in Figure 10., you can see the difference starts from the 16th

bit of the address.

Like in ST90158 OTP devices, Segments 20, 21, 22 of external memory cannot be accessed.

The total accessible memory is:

2M (64 Kbytes x 64 segments)

-3 x 32K (internal segment)

+32K (static page)

+1K (internal RAM)

=1920K bytes

Figure 10. shows the memory map with segments containing 32K of memory.