In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Kramer,

to be honest, I have the feeling that you are bringing up a very theoretical question here. Let me analyze/comment this in simple steps:

1. Did you ever run into a situation when programming an SX28 with all the program memory used for program code? When the answer is "no" - don't worry for now, and keep going to hit the limit. In case the answer is "yes" - switch to an SX48 with twice as much program memory space. This will allow for much more code before hitting the limit again. 

2. There is no way to directly expand any SXes program memory by adding external components. Due to the Harward architecture, there is no way to directly read instruction code from external memory and have it executed by the internal processor engine, i.e. you can't directly convert data (i.e. values read in via input ports) into instructions.

3. The only way to do it would be "BASIC-Stampish", i.e. by running an interpreter engine on the SX, reading and executing tokenized code from an external EEPROM. (I have developed such a system for a PLC-alike SX system, called µControl).

4. A minimum of SX I/O pins would be required when you use serial external memory devices, like I²C or SPI EEPROMS, or FRAMs with the drawback of reduced access speed. Using external memory devices with parallel data and address busses will "eat up" almost all of your available SX I/O pins, so you would have to add external address latches/multiplexers, for example.

5. Always keep in mind that the SX is perfect for many different applications, but not for each and every one. So you will always have to make a decision if it makes sense using an SX for a special application, or not. Since I learned about the SX, I've been using it for so many different purposes - complicated ones, but also for really trivial things. Instead of soldering together a bunch of "low-level" digital components, like gates, flip-flops, or shift registers, I just take an SX, and let it do the required task in software. This is so much fun, and saves you a lot of time! So, with the SX you may have a solution on hand for many applications within a minimum of time, but (like with any kind of device) there are limits as well, you should be aware of.

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