Documentation Index
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The PPM (Pulse Position Modulation) application reads servo-style pulse-width signals produced by RC receivers and standard ESC throttle inputs. It decodes the pulse width into a normalised level and applies it to the motor using the configured control type.
Signal specification
Default pulse-width range as defined in appconf_default.h:
| Parameter | Default value | Description |
|---|
pulse_start | 1.0 ms | Minimum pulse width (full reverse or idle) |
pulse_end | 2.0 ms | Maximum pulse width (full throttle) |
pulse_center | 1.5 ms | Center/neutral pulse width |
hyst | 0.15 | Deadband around center (normalised 0–1) |
ppm_config struct fields pulse_start, pulse_end, and pulse_center.
Most RC receivers and flight controllers output a standard 1–2 ms pulse at 50 Hz. VESC can read this directly on the PPM input pin.
Control types
The ppm_control_type enum determines how the decoded pulse level controls the motor:
typedef enum {
PPM_CTRL_TYPE_NONE = 0,
PPM_CTRL_TYPE_CURRENT,
PPM_CTRL_TYPE_CURRENT_NOREV,
PPM_CTRL_TYPE_CURRENT_NOREV_BRAKE,
PPM_CTRL_TYPE_DUTY,
PPM_CTRL_TYPE_DUTY_NOREV,
PPM_CTRL_TYPE_PID,
PPM_CTRL_TYPE_PID_NOREV,
PPM_CTRL_TYPE_CURRENT_BRAKE_REV_HYST,
PPM_CTRL_TYPE_CURRENT_SMART_REV,
PPM_CTRL_TYPE_PID_POSITION_180,
PPM_CTRL_TYPE_PID_POSITION_360,
} ppm_control_type;
| Control type | Description |
|---|
CURRENT | Bidirectional current control. Below center = reverse. |
CURRENT_NOREV | Current control, forward only. |
CURRENT_NOREV_BRAKE | Forward current with braking below neutral. |
DUTY | Direct duty-cycle control, bidirectional. |
DUTY_NOREV | Duty-cycle, forward only. |
PID | Speed PID, bidirectional. |
PID_NOREV | Speed PID, forward only. |
CURRENT_SMART_REV | Current control with timed smart-reverse. |
PID_POSITION_180 | Position PID over ±180°. |
PID_POSITION_360 | Position PID over 0–360°. |
Configuration parameters
The full ppm_config struct:
typedef struct {
ppm_control_type ctrl_type;
float pid_max_erpm;
float hyst;
float pulse_start;
float pulse_end;
float pulse_center;
bool median_filter;
SAFE_START_MODE safe_start;
float throttle_exp;
float throttle_exp_brake;
thr_exp_mode throttle_exp_mode;
float ramp_time_pos;
float ramp_time_neg;
bool multi_esc;
bool tc;
float tc_max_diff;
float max_erpm_for_dir;
float smart_rev_max_duty;
float smart_rev_ramp_time;
} ppm_config;
Key parameters
| Field | Default | Description |
|---|
median_filter | true | Applies a 3-sample median filter to reduce glitches. |
safe_start | SAFE_START_REGULAR | Requires throttle to return to neutral before output begins. |
ramp_time_pos | 0.4 s | Time to ramp from 0 to full positive output. |
ramp_time_neg | 0.2 s | Time to ramp from 0 to full negative output. |
multi_esc | true | Forwards the command to other VESCs on the CAN bus. |
tc | false | Enables traction control between CAN-connected ESCs. |
tc_max_diff | 3000 ERPM | Maximum allowed ERPM difference between ESCs for traction control. |
smart_rev_max_duty | 0.07 | Maximum duty cycle during smart reverse. |
smart_rev_ramp_time | 3.0 s | Time the stick must be held in reverse before smart-reverse activates. |
Runtime API
void app_ppm_start(void);
void app_ppm_stop(void);
void app_ppm_configure(ppm_config *conf);
float app_ppm_get_decoded_level(void);
void app_ppm_detach(bool detach);
void app_ppm_override(float val);
app_ppm_get_decoded_level returns the current decoded input in the range −1.0 to 1.0. app_ppm_detach and app_ppm_override let other firmware components take temporary control of the output without stopping the PPM thread.
Safe start
With safe_start enabled, the motor will not respond until the PPM signal returns to the neutral (center) position after power-on or a timeout. This prevents unintentional movement when power is applied with the throttle open.
MIN_PULSES_WITHOUT_POWER (50) valid pulses at neutral before allowing output. This constant is defined at the top of app_ppm.c.