DShot protocol

Summary¶

DShot is the digital protocol that carries throttle commands from the flight controller to the ESC. It replaced analog PWM by encoding the throttle value as a digital packet with a CRC checksum — corrupted packets are discarded rather than executed, eliminating the motor spikes that electrical noise caused in analog systems. Bidirectional DShot (BiDi) extends the protocol: the ESC uses the same wire to return the motor's eRPM every PID cycle. This eRPM feedback is what makes the RPM filter possible. libdrone uses DShot600 (600 kbit/s) with bidirectional DShot enabled on all four ESC outputs.

Concept¶

Why analog PWM was insufficient¶

The original ESC protocol — inherited from RC servo control — was a 50 Hz analog pulse: a pulse between 1000 and 2000 µs wide, repeated 50 times per second. Width encoded the throttle command.

The problems with this for a fast PID loop: 1. Noise susceptibility: a voltage spike on the signal wire could be read as a different pulse width, causing a sudden unintended motor command. 2. Resolution: ~1000 discrete levels across the throttle range. 3. Update rate: 50 Hz meant 20 ms between commands — 160× slower than an 8 kHz PID loop. Each new throttle command was delayed by up to 20 ms.

Later protocols (Oneshot, Multishot) raised the update rate but kept the analog format and its noise susceptibility. DShot solved both problems by making the protocol fully digital.

DShot packet structure¶

Each DShot packet is 16 bits:

[11 bits throttle] [1 bit telemetry request] [4 bits CRC]

• 11-bit throttle: 2048 discrete levels (0 = disarmed, 48–2047 = throttle range). More than double the resolution of analog PWM.
• Telemetry request bit: when set, requests extended telemetry from the ESC on the next packet (temperature, voltage, current).
• 4-bit CRC: computed from the preceding 12 bits. If the receiver computes a different CRC, the packet is discarded. Noise-corrupted packets are never executed.

At DShot600 (600 kbit/s), transmitting 16 bits takes ~26.7 µs — comfortably within the 125 µs window at 8 kHz loop rate.

Bidirectional DShot¶

BiDi DShot adds an eRPM response to every packet exchange on the same wire. After the FC finishes transmitting, it switches its UART to receive mode. The ESC responds with a 21-bit eRPM packet in the gap before the next command. The entire exchange fits within one PID loop cycle.

FC transmits: [DShot600 packet — 26.7 µs] Gap: [signal line quiet — ~5 µs] ESC responds: [eRPM packet — ~21 µs] Remaining: [~72 µs idle — other tasks run here]

The eRPM value is what Betaflight's RPM filter uses to track motor vibration frequencies in real time. Without BiDi DShot, the RPM filter cannot function.

``` Mechanical RPM = eRPM ÷ pole_pairs

For BrotherHobby 2507 (7 pole pairs): Mechanical RPM = eRPM ÷ 7 ```

DShot variant comparison¶

DShot600 is the recommended choice for H7-class flight controllers. On noisier electrical environments or longer wire runs, DShot300 provides more margin against signal corruption.

Reference¶

libdrone DShot configuration¶

Verifying BiDi DShot is working¶

In Betaflight Motors tab, with props removed: 1. Advance master throttle slider to ~20%. 2. All four motors should display non-zero eRPM values. 3. eRPM values should increase proportionally as slider advances. 4. If any motor shows 0 eRPM: check AM32 Configurator that BiDi DShot is enabled on that motor output; check DShot signal wire continuity.

DShot commands (special functions)¶

Beyond throttle, DShot encodes special commands in the 0–47 throttle range:

These are sent by the FC during ESC configuration, not during normal flight. They require the motor to be at idle (throttle = 0) and the FC to send the command multiple times for confirmation.

Procedure¶

Enabling DShot600 and BiDi DShot in Betaflight¶

1. In Betaflight Configurator → Configuration tab:
2. Motor Protocol: DShot600
3. Enable Bidirectional DSHOT: checked
4. In ESC Configurator (AM32 Configurator or BLHeli Suite):
5. For each motor output: enable Bidirectional DSHOT
6. Save and reboot both FC and ESC (power cycle).
7. Verify: in Betaflight Motors tab, all four motors report eRPM when spun up.
8. In Betaflight Filtering tab: confirm RPM filter shows as active.

Rationale¶

Why BiDi DShot is specified rather than optional¶

BiDi DShot is the prerequisite for the RPM filter, which is the prerequisite for effective PID tuning. Without BiDi DShot, the RPM filter falls back to static notch filters which do not track motor frequency with throttle. The result: significantly more gyro noise reaching the D term, requiring lower D gain, resulting in more prop wash and poorer disturbance rejection. BiDi DShot is not a nice-to-have on libdrone — it is a hard dependency of the tuning baseline.

Connections¶

requires: - electronic-speed-controllers related: - brushless-motors - rpm-filter leads_to: - rpm-filter