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ST9 EXTERNAL MEMORY INTERFACE CONFIGURATION

1 SIGNAL DESCRIPTION

The External Memory Interface for the ST9 microcontroller exists in two models:

a) small model that allows you to address a maximum of 64 Kbytes of external memory and

b) large model which can address the full ST9 memory space (4 Mbytes).

1.1 SMALL MEMORY MODEL

This model is supported by x158, xF120 devices.

In this model, the ST9 is limited to a maximum of 64 Kbytes of directly addressable external

memory. It is due to the fact that you can only output 16-bit addresses, which corresponds to

a 64 Kbyte external memory space.The signals used in the small memory model interface are

the following:

– I/O Port 0 (8 pins): outputs the LSB of the address (A[0..7]) multiplexed with the 8 bits of data

(D[0..7]).

– I/O Port 1 (8 pins): outputs the MSB of the address (A[8..15]).

– Address Strobe ASn: is active during clock phase T1. An ASn rising edge indicates that the

memory address and control signals are valid.

– Data Strobe DSn: is active during clock phase T2. During an external memory read cycle,

the data on Port 0 must be valid before the rising edge of DSn. During an external memory

write cycle, the data on Port 0 are output on the falling edge of DSn and they are valid on the

rising edge of DSn.

– Read/Write RWn: identifies the type of memory cycle. RWn=1 identifies a Read cycle and

RWn=0 identifies a Write cycle.

If you want to access more than 64 Kbytes of memory with this interface, you will have to use

some I/O pins as chip select signals for your external memories and handle the management

of these pins by software.

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