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ADDRESSING UP TO 4 MBYTES OF EXTERNAL MEMORY

3 ACCESSING UP TO 2 OR 4 MBYTES WITH ANY ST9+

3.1 PRINCIPLE

As previously mentioned in Section 1, the ST9 handles 22-bit addresses internally but outputs

only the 16 LSBs of the address through the external memory interface. In this section we will

see how, by handling it in software, it is possible to access almost 2M with a ROMLESS device

or almost to 4M bytes with an OTP, ROM, or Flash device.

To access more than 64K of external memory, you need to output the 6 Most Significant Bits

(MSBs) of the address. To do this, we have chosen to use the 6 LSBs of Port 2 (refer to Figure

5.).

Port 2 will be used by software to select the Dynamic Segment you want to access.

External memory segments and Internal memory segments are two different addressable

areas. For simplicity, both types of segment are numbered in the same way.

Figure 5. ST9+ Schematic

CORE

22-bit address

8-bit data bus

ST9+ internal memory

External Memory

Interface

P0[0..7]

P1[0..7]

AS, R/W, DS,

16-bit address multiplexed

with 8 bit data bus

DS2 (ST92F120 only)

P2[0..5]

I/O port logic