ESP32_FC - RMT Edition

...some experiments with the ESP32 RMT generating DSHOT pulses for BLDC ESCs running BLHeli_S

The DShot Protocol

The DSHOT protocol consists of transmitting 16-bit packets to the ESCs: 11-bit throttle value, 1-bit to request telemetry and a 4-bit checksum. There are three major protocol speeds: DSHOT150, DSHOT300 and DSHOT600.

DShot -Timing:

DSHOT	Bitrate	TH1	TH0	Bit Time µs	Frame Time µs
150	150kbit/s	5.00	2.50	6.67	106.72
300	300kbit/s	2.50	1.25	3.33	53.28
600	600kbit/s	1.25	0.625	1.67	26.72
1200	1200kbit/s	0.625	0.313	0.83	13.28

Calculating the CRC

The checksum is calculated over the throttle value and the telemetry bit, so the “first” 12 bits our value in the following example:

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

Bidirectional DSHOT

Biderictional DSHOT is also known as inverted DSHOT, because the signal level is inverted, so 1 is low and a 0 is high. This is done in order to let the ESC know, that we are operating in bidirectional mode and that it should be sending back eRPM telemetry packages.

Calculating the Bidirectional CRC

The calculation of the checksum is basically the same, just before the last step the values are inverted:

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

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References:

• DSHOT - the missing Handbook

• DSHOT in the Dark