INTRODUCTION
This application note presents the different ST9 resources for configuring and initializing its external memory interface.

The ST9 has a single 4 Mbyte memory space segmented in 64 segments of 64 Kbytes, plus an independent register file space. The memory space contains internal memories (internal ROM and RAM with predefined addresses) and you can map your external memories (at the addresses in any segments not used for internal memories). Please refer to the MMU chapter of the ST9 datasheet for more information on the way this memory space is addressed.

The ST9 external memory access cycle is composed of 2 clock phases (cf. Figure 1):
– Phase T1: the memory address is output through the ST9 EMI (External Memory Interface).
– Phase T2: if the memory access is a Read cycle, the data signals are sampled by the ST9.
   If the memory access is a Write cycle, the ST9 outputs data to be written in external memory.

The different signals provided and used by the EMI are described in Section 1 of this application note, and their configuration is explained in Section 2.2. A software example of the ST9 EMI configuration is given in Section 5.