I mentioned before that the complexity in the decoder sketches is largely in getting the timing correct for 17 arbitrary functions. But there is also the timing for decoding the actual DCC command packets received from the DCC bus/track. This is almost entirely done by Alex Shepherd's terrific NmraDcc library. This referes to collecting the bits off the DCC bus, examining them based on the initialization of the decoder including the address, and then routing them to the appropriate routine for actionable activities -- like moving a servo or switching a LED. But in oorder to allow the library to do all of its activity it must be called regularly so it can look for updates -- like did it get a valid command packet. If you do not call it often enough, via NmraDcc.process() it may miss a DCC command packet because it was not looking at the bus from the beginning of the command. So if the decoder sketch hogs the processor for too long, it doesn't give the library enough time to do its job. Speed command packets are resent periodically by base stations. Function and accessory commands are often repeated once or twice and then stopped, but there is no official standard for how often command packets are resent over a long period of time. Resending DCC commands at all was implemented to account for a decoder misreading a command while moving over dirty track or rail gaps. Thus, there is no specific time for how often you must call NmraDcc.process() other than "as often as possible." You might also note that if you added a bunch more functions to the Pro Mini processor via an extender chip, you might very well increase the likelihood of missing a command packet, since you would be tending to more internal functions.

You don't call notifyDccFunc, it calls the decoder sketch whenever any DCC function command is received and validated for the addressed mobile decoder. Remember, that this is called for every DCC function command received, including the duplicates sent by the command station. So to turn on let's say F7, you might actually get 2 command packets from the base station in a short time to turn on F7, and it could be more or less depending on the base station and track/bus conditions, as well as how busy your decoder sketch is.

 

'Hope this makes things a bit clearer. Never be afriad to ask a question. I have told my students and sons this for years: it's how we all learn. Also please direct future questions to the current entry in this series: SMA20 New Low Cost 17 Channel DCC Decoders & Dual Motor,LED, & Servo Control Updated: 6 Fntns/Pin & New Featureshttp://model-railroad-hobbyist.com/node/24316which also has the most up to date info and library for these decoders, and it makes it a bit easier for me too! 

Have fun! 

Best regards,

Geoff