Thread: Discussion Hubsan X4 protocol analysis
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Old Dec 24, 2012, 06:55 PM
PhracturedBlue is offline
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PPM is basically an analog protocol. It defines a specific set of actions to be taken (servo movement) and the receiver will virtually never come up with exactly the same input as the transmitter outputs. SPI is just an interface. It defines a digital way to communicate any binary data between a master and slave device. They aren't really comparable.

All of the modules I've been working with are simply RF transceivers, they have no knowledge of the meaning of the data they are transmitting, and so can't take PPM as an input.

A 'univeral' module that worked only with PPM would need some secondary input (either a dial, buttons, or some other 'trick' like power-up with inputs set in a specific configuration) to communicate additional information. It may be a necessity, but it isn't an ideal situation.

Personally, I think PPM is a terrible protocol. In the days of analog transmitters and primitive microcontrollers, it made sense, but these days, an all digital system would make more sense. But we've got what we've got, so we need to make do.

And yes I know the PPM signals are digital, but the information is transmitted as a pulse-width which must be sampled and is thus dependent on the sampling rate and so will have inherent sampling error.
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