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ADDRESSING UP TO 4 MBYTES OF EXTERNAL MEMORY

3.3 HARDWARE IMPLEMENTATION

3.3.1 Example using a microcontroller with internal memory

In this example, using the ST90158 with 64K of internal ROM, the microcontroller’s internal

memory can be used as a Static Segment.

The hardware connections are shown in Figure 7..

Figure 7. Accessing almost 4M with the ST90158

Due to the fact we are using OTP or ROM devices, when the user accesses an address in the

internal segment, no external signals (AS, DS...) will be output by the ST9+ if the ETO bit in the

EMR1 register is correctly configured (please refer to the datasheet). This is the case for seg-

ments: 0, 21, 22 and 23h in the ST90158. Therefore any external memory that uses the same

segment number as the internal memory will not be accessible. To make it accessible, we

would need a dedicated logical unit.

P0[0..7]

P1[0..7]

AS

DS

R/W

VCC

74HC573

ST90158

EXTERNAL MEMORY

22 bit address

8 bit data

G

R/W (RAM and Flash only)

LE

74HC04

E

P2[0..7]

4Mbytes

64K

Seg 0

Seg 0 (Not

Seg 2

Seg 3

Seg 3F

A[0..7]

A[8..15]

A[16..21]

A[0..21]

Seg 1

available)