PPM

How PPM Works

What is Pulse Position Modulation? Definition, Basics, pulse position modulation and demodulation, Applications of PPM - Electronics Coach

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.

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 length/period = HIGH time + LOW time.

Channel Value Pulse

HIGH time: 300us
LOW time: 700us + 970us * channel value (0%-100%)

Reset Pulse

HIGH time: 300us
LOW time: frame total length - sum of all channel value pulse lengths

RC Sender Interface

A generic interface for sending RC (Radio Control) signal

/* Set the value of a specific channel (0.0-1.0). Returns 1 if success, 0 if failed. */
virtual uint8_t setChannelValue(uint8_t channel, float value) = 0;

/* Initialize the RC output. */
virtual void init() = 0;

/* A callback that gets called after each RC value is sent. */
virtual void interrupt_callback(TIM_HandleTypeDef* timer) = 0;

PPM Output Driver

The driver for outputting PPM. Implements the RC sender interface.

/* Constructor */
PPMChannelOut(TIM_HandleTypeDef* timer, uint16_t timer_channel, uint8_t num_channels);

/* Implementation of the RcSender Interface. See LOS_D_RcSender.hpp */
uint8_t setChannelValue(uint8_t channel, float value);

/* Implementation of the RcSender Interface. See LOS_D_RcSender.hpp
  * After initialization, we will be constantly sending PPM signals.
  */
void init();

/* Implementation of the RcSender Interface. See LOS_D_RcSender.hpp */
void interrupt_callback(TIM_HandleTypeDef* timer);

/* Set the number of channels. Returns 1 if success, 0 if failed. */
uint8_t setNumChannels(uint8_t num_channels);