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https://electronicscoach.com/pulse-position-modulation.html
CRSF PPM Overview
CRSF information is communicated with frames that consist of PPM pulses that represent channel values and a reset pulse. Below is an example of a frame for 8 channels.
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The PPM pulses are unique in that the HIGH time is always the same, but the period changes depending on the channel value.
CRSF PPM Math
N = number of channels
Frame total length = N * 1670us + 3300us
PPM pulse period = HIGH time + LOW time.
Channel Value Pulse
HIGH time: 300us
LOW time: 400us + 970us * channel value (0%-100%)
Reset Pulse
HIGH time: 300us
LOW time: 3000us + N * 1670us - sum length of channel value pulses
RC Sender Interface
A generic interface for sending RC (Radio Control) signal
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