DSHOT on the Wire

DSHOT is the digital FC-to-ESC protocol. Every PID loop the flight controller sends a fixed-length frame carrying a throttle value plus a checksum; in bidirectional mode the ESC answers on the same wire with its electrical RPM. This is the byte-level view — what actually travels down the signal wire. For the tuning side (RPM filter, notches) see DSHOT and RPM Filter.

The physical layer — a bit is a high-time

DSHOT is not a UART. Each bit occupies a fixed slot; whether it is a 1 or a 0 is decided by how long the line stays high inside that slot. A 1 holds high for twice as long as a 0.

Because every bit slot is the same width, a frame is always exactly 16 × bit_period, regardless of throttle — and the receiver can clock each bit off a single rising-then-falling edge.

T1H is the high time that counts as a 1; T0H the high time that counts as a 0. The number in the name is the raw bitrate.

Frame structure

Every frame is 16 bits, sent MSB first (leftmost bit first on the wire):

• 11-bit throttle — 2048 values. 0 = disarmed, 1–47 = special commands, 48–2047 = the 2000 usable throttle steps.
• 1-bit telemetry request — asks the ESC to send classic telemetry on the separate telemetry wire (unrelated to bidirectional DSHOT).
• 4-bit CRC — checksum over the preceding 12 bits.

The checksum

The CRC is computed over the 12-bit value (throttle shifted up by one, OR'd with the telemetry bit):

1crc = (value ^ (value >> 4) ^ (value >> 8)) & 0x0F

Worked example — throttle 1046 (half throttle), telemetry bit clear, so the 12-bit value is 100000101100:

1value  = 100000101100
2>> 4   = 000010000010
3^      = 100010101110
4>> 8   = 000000001000
5^      = 100010100110
6& 0x0F = 000000000110   → CRC = 0110

The 16 bits on the wire become 1000001011000110.

Special commands (throttle 1–47)

Values below 48 are commands, not throttle. Most are only acted on while the motor is stopped, and several must be repeated (typically 6×) so a single glitched frame can't trigger them.

This is how the Configurator's Reverse motor direction button works — it sends command 20 or 21 six times; the ESC persists it. There is no set variable for it.

Arming

The ESC will not accept real throttle until it has seen a run of disarm frames. Bluejay, for example, requires roughly 300 ms of 0 commands before it leaves the disarmed state — which is why a freshly powered quad ignores throttle for a moment.

Frame rate vs PID loop

The FC does not spam frames; it emits exactly one per PID loop iteration, locked to the loop rate. So the loop rate sets the required DSHOT speed, not the other way around:

• 8 kHz loop → a frame every 125 µs → DSHOT300 (53 µs/frame) is plenty.
• 32 kHz loop → a frame every 31.25 µs → you need DSHOT600 to fit the frame.

Running a faster DSHOT than your loop needs buys nothing on its own.

Bidirectional DSHOT

Bidirectional (a.k.a. inverted) DSHOT is what feeds the RPM filter. Two things change versus plain DSHOT:

1. The line is inverted — idle high, pulses low (1 = long low, 0 = short low). This is the ESC's cue to reply with eRPM. Requires DSHOT300 or faster.

2. The CRC is inverted — same math, complemented before masking:

1crc = (~(value ^ (value >> 4) ^ (value >> 8))) & 0x0F

After the FC finishes its frame it releases the line and listens; the ESC drives the same wire back. There is a fixed ~30 µs turnaround to switch line direction, DMA and timers — independent of DSHOT speed. Because a reply follows every command, the achievable frame rate is roughly halved.

sequenceDiagram
    participant FC as Flight Controller
    participant ESC
    FC->>ESC: throttle frame (inverted, inverted CRC)
    Note over FC,ESC: ~30 µs line turnaround
    ESC-->>FC: eRPM telemetry (GCR, same wire)
    Note over FC,ESC: repeats every PID loop

The eRPM telemetry frame

The reply is again 16 bits, but laid out differently:

The 12-bit payload is a tiny floating-point number: the 9-bit period base left-shifted by the 3-bit exponent gives the motor's electrical commutation period in microseconds. The CRC here uses the un-inverted formula and is sent un-inverted.

Convert period to RPM:

1eperiod_us = period_base << exponent
2eRPM       = 60,000,000 / eperiod_us
3RPM        = eRPM / pole_pairs        # 14-pole motor → 7 pairs

Example — eperiod = 500 µs → eRPM = 120,000 → on a 14-pole motor ≈ 17,140 RPM. Betaflight does exactly this per motor, then places the RPM-filter notches on RPM / 60 Hz and its harmonics.

Why the reply is GCR-encoded

The ESC does not send those 16 bits raw. Sending them as DSHOT-style pulses back proved jittery, so the reply is GCR (Group-Coded Recording) encoded — the same trick used by floppy/tape drives to guarantee frequent transitions for reliable clock recovery.

Step 1 — nibble mapping (16 → 20 bits). Each 4-bit nibble maps to a 5-bit code:

116-bit:  1000 0010 1100 0110
2nibbles:   x8   x2   xC   x6
3mapped:   1A   12   1E   16
420-bit:  11010 10010 11110 10110

Step 2 — transition encoding (20 → 21 bits). The 20-bit GCR is turned into a 21-bit level sequence that starts with 0: a GCR 1 toggles the output level, a GCR 0 keeps it. This halves the number of edges on the wire, cutting jitter further.

The 21 bits go out un-inverted at 5/4 × the DSHOT bitrate (so DSHOT600 → 750 kbit/s).

Decoding (on the FC) is one instruction — undo the transition encoding:

1gcr = value ^ (value >> 1)

then reverse the nibble table and pull out the period.

Extended DSHOT Telemetry (EDT)

The eRPM float has redundant encodings (the same period can be written with different exponent/mantissa pairs). Bluejay/BLHeli_32/AM32 exploit this: by always normalising eRPM one way, a frame shaped eee 0mmmmmmmm can never occur naturally — so when the FC does see one, it reads it as a typed telemetry packet instead:

1pppp mmmmmmmm      # 4-bit type, 8-bit value

Types include temperature (0x02), voltage (0x04, 0.25 V/step), current (0x06) and debug/state fields. This carries temperature, voltage and current back without any extra wire, interleaved occasionally so it doesn't disturb RPM filtering. Enable it with special commands 13/14.

What this means in practice

• Every throttle update is checksummed — noise on the wire is detected, not silently flown.
• Bidirectional DSHOT is a request/response on one wire — that 30 µs turnaround plus the reply is why it roughly halves frame rate, so high loop rates want DSHOT600.
• eRPM is a period, not an RPM — the FC inverts it and divides by pole pairs; get the pole count wrong and the RPM filter tracks the wrong frequency. See Betaflight Tuning Math for the notch placement and FPV Terminology for the acronyms.

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