Knowledge Base

There is no hardware exclusively devoted to the OBCB function. Basically, the OBCB comes “free” in the design of the SPLat board. On some custom boards, however, the OBCB may be associated with special purpose input conditioning circuitry. An example of that would be inputs designed to work off TTL levels rather than NPN sensors.

The idealized maximum count rate in current (March ’07) controllers is 2kHz.The actual maximum frequency will be limited by:

  • Asymmetries in the input waveforms (outside our control). A perfect square wave is best
  • Time jitter in the sample time (within our control but not perfect)
  • Asymmetric turn-on and turn-off delays at the inputs caused by input EMC filters.

We have specified a maximum of 1.8kHz for most controller models. This can only be achieved if the input waveform is a perfectly symmetrical square wave (equal ON and OFF times). Unless you have an oscilloscope and can check your input waveform, it can be very difficult to know how symmetrical your input is. For example, a proximity switch or a photo cell on an encoder disk or cog wheel could produce a quite asymmetrical waveform, depending on its mechanical and sensitivity adjustments. Any input state, be it an on or off state, that lasts less than 277µS is likely to be missed.

Warning: The OBCB is liable to miss counts while the processor is writing to any of the non-volatile memories (non-volatile memory writes block the processor for up to several milliseconds). If your program writes to non-volatile memory you must make sure it does not happen at a time when it has to count input pulses.

Please see the documentation for the SPLat controller you are using for information on which inputs are used.