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ST9 EXTERNAL MEMORY INTERFACE CONFIGURATION

2.1 PORT CONFIGURATION

The different ports used to interface with the external memory must be configured as Alternate

Function Output Push-Pull. This concerns Ports 0,1 and Port9

This configuration is done by writing the following values in the three I/O port configuration

registers:

– PxC2 = 00h

– PxC1 = FFh

– PxC0 = FFh

(where x is the number of the corresponding I/O port).

After Reset, if your microcontroller is a Romless version, this configuration will be the default

one. In this case, the microcontroller must directly address external memory just after the

Reset, as there is no internal memory.

For other microcontroller versions (not Romless), the Reset configuration for the I/O ports is

usually ‘Bidirectional Open-Drain Weak Pull-up’ (refer to your device datasheet for special I/O

Ports Reset configuration). In this case, you have to initialize the three I/O port configuration

registers by software as shown above, before being able to address external memory.

Note: By default, the memory control signals (/AS, /DS, /RW) don’t need any configuration,

because they are not mapped on I/O port pins. So these signals are output as soon as

the microcontroller starts accessing external memory.

However, in some microcontrollers (like the ST90158), you have to configure some of

these external memory interface signals. For example, to configure the RWn signal for

the ST90158 microcontroller, you must select ‘Alternate Function Push-Pull’ for the

P6.5 pin.