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

After lurking for a while and finally making a few posts I feel a little more comfortable asking a question or three... I just recently started trying to use the SX chip in projects coming from the basic stamp. Currently on the board is a motorcycle control system for various functions, rider initiated just SX sequenced once initiated. Anyway the input in question is an active low input using the internal pullups to pull it high, there is a 5.1 volt zener diode reverse biased to ground and from there a 4.7k current limiter inline with the incoming wire. Ground signal comes in from wire, through 4.7k. Worked fantastic on the bench, once installed on a bike, (2004 Kawasaki ZX10) the output triggered continously. First thing I tried was a 2k pullup to 12V external to the board letting the zener pull it down. No luck still spontaneous actuation. After scratching my head a while wondering how anything external could still get past that I added a .1 mf cap to the input from the +5 supply and external to the 4.7k pullup. This seems to have solved the problem sitting in the garage and on a dyno. The whole point of this post is, I intend to send this thing down a drag strip with a very good friend piloting so I'm trying to be as careful as possible in development. Can anyone shed some light on why I needed a filter cap on an incoming active low line that was already pulled up and zenered out 9 ways to Sunday? Also, is there anything else I can expect as a common problem? The power supply for the SX is a L4940D2M5 with a .1mf cap, and a .1mf cap solder directly across the pins on the SX itself so I dont think I can get the filter cap any closer. The SX in question is an SX 18. This was chosen for the moment mainly for cost more than any other reason. Once I achieve something rock solid I plan on moving to a SMT version. All outputs are driven into FET's, irlz44's and irll2705s with 220 ohm protection resistors between the SX pins and the FET's. Sorry that there are no schematics with this but there arent any in existance. So far I've been working directly from CAD layout to putting parts down.

Chris

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