Extended DSHOT Telemetry, as the name suggests, extends the telemetry data which is sent from ESC (via signal wire) to the flight-controller. For this to work, bi-directional DSHOT needs to be active.
In addition to the eRPM data which was the initial data being transmitted back to the flight-controller, EDT allows the encoding of further details in the telemetry frame.
- Bits are counted from the least significant bit (LSB) starting at 0
The frame structure of the telemetry frame is 16 bits long: eeem mmmm mmmm cccc
eee: the exponentmmmmmmmmm: the eRPM valuecccc: the checksum
To calculate the real eRPM value, it is shifted to the left by the exponent:
eRPM = m << e
or in other words:
eRPM = m * 2 ^ e
From this, one can easily see that the same eRPM values could be encoded in multiple different ways.
8 can be encoded in this four ways:
000 0 0000 1000
001 0 0000 0100
010 0 0000 0010
011 0 0000 0001
512 can be encoded in this seven ways:
001 1 0000 0000
010 0 1000 0000
011 0 0100 0000
100 0 0010 0000
101 0 0001 0000
110 0 0000 1000
111 0 0000 0100
This is true for most values, thus the idea was born to normalize eRPM values, so that no ambiguity is left and each value has only one representation, the other - now free - values can be used to encode different data.
The last three bits (the exponent) together with the 8th bit (the MSB of the value) allow to distinguish between eRPM and extended DSHOT frames. Those four bits are called the prefix.
To normalize an eRPM value, the goal is to set Bit 8 by decreasing the exponent and left shifting the value. An implementation of the normalization could look something like so:
-
- Check exponent
- 1.a. Exponent is 0: Done
- 1.b. Exponent > 0
- 2.a. Bit 8 is 1: Done
- 2.b. Bit 8 is 0
-
- Decrease exponent, left shift value by one
-
- Go to step 1
After applying the normalization, the following ranges are possible for eRPM values:
000 0 mmmm mmmm- [0, 1, 2, ... , 255]000 1 mmmm mmmm- [256, 257, 258, ..., 511]001 1 mmmm mmmm- [512, 514, 516, ... , 1022]010 1 mmmm mmmm- [1024, 1028, 1032, ..., 2044]011 1 mmmm mmmm- [2048, 2056, 2064, ..., 4088]100 1 mmmm mmmm- [4096, 4112, 4128, ..., 8176]101 1 mmmm mmmm- [8192, 8224, 8256, ..., 16352]110 1 mmmm mmmm- [16384, 16448, 16512, ..., 32704]111 1 mmmm mmmm- [32768, 32896, 33024, ..., 65408]
If the last four bits are 0 OR the 8th bit is 1, it is a eRPM frame, the other ranges are EDT frames.
This is where EDT versions come into play if not explicitly stated, the values are available in all versions
001 0 mmmm mmmm- Temperature frame in degree Celsius, just like Blheli_32 and KISS [0, 1, ..., 255]010 0 mmmm mmmm- Voltage frame with a step size of 0,25V [0, 0.25 ..., 63,75]011 0 mmmm mmmm- Current frame with a step size of 1A [0, 1, ..., 255]100 0 mmmm mmmm- Debug frame 1 not associated with any specific value, can be used to debug ESC firmware101 0 mmmm mmmm- Debug frame 2 not associated with any specific value, can be used to debug ESC firmware110 0 mmmm mmmm- Stress level frame [0, 1, ..., 255] (since v2.0.0)111 0 mmmm mmmm- Status frame: Bit[7] = alert event, Bit[6] = warning event, Bit[5] = error event, Bit[3-0] - Max. stress level [0-15] (since v2.0.0). At this moment Bit[4] is not being used.
The 4 bits checksum is calculated the same way no matter if eRPM or EDT frame. Value in this example are the 12 data bits.
crc = (value ^ (value >> 4) ^ (value >> 8)) & 0x0F;
To enable EDT the ESC has to receive the DIGITAL_CMD_EXTENDED_TELEMETRY_ENABLE command (DSHOT command 13) at least 6 times in succession. It then answers with a single version frame:
111 0 vvvv vvvv
How the received data is then interpreted is up to the receiving device - especially how the DEBUG frames are interpreted.
To disable EDT , the ESC has to recieve the DIGITAL_CMD_EXTENDED_TELEMETRY_DISABLE command (DSHOT command 14) at least 6 times in succession. It then answers with a single disabled frame:
1110 1111 1111
The only mandatory frame is the status frame. All available data is sent if EDT has been enabled - it is not necessary to enable specific frames on the receiving end.
By default eRPM telemetry is sent back to the FC, and every ESC firmware has to decide the best frame scheduling based on their configurations, use cases or preferences.
The following is an example schedule for transmitting EDT frames:
eRPM: is sent if there is no other telemetry frameStatus: 1000 ms for example, at ms cycle 0; as a response of extended telemetry enable/disable commands; next frame when an event happensTemperature: 1000 ms for example, at ms cycle 333Voltage: 1000 ms for example, at ms cycle 666
A bi-directional response frame (EDT frame) is to be sent 30μs after a DSHOT frame has been released. This interval does not depend on DSHOT speed.
- TBD: To be defined
- prefix: The first four bits of the telemetry frame
- FC: Flight controller or receiver of the telemetry frame* Stress level: Indicates the effort the ESC has to make to keep commutation going. In Blheli_S/Bluejay for example it is related to desync events. When it reaches the maximum indicates that the ESC cannot make reliable commutation. Zero is the ideal value, when ESC commutation is perfect.
- ESC alert event - Notification made for statistical/debug purposes. In Bluejay, for example this is the demag event. Later all this events can be inspected in the Blackbox Viewer or any other tool.
- ESC warning event - Notifies suspecting or unusual behaviour. In Bluejay, for example this is the desync event. Later all this events can be inspected in the Blackbox Viewer or any other tool.
- ESC error event - Notifies a serious problem. In Bluejay, for example this is the stall event. Later all this events can be inspected in the Blackbox Viewer or any other tool.
- v2.1.1 - Clarified Bit index direction
- v2.1.0 - Added section about timing
- v2.0.2 - Fixed typo in stress level frame
- v2.0.1 - Improved wording, fixed typos
- v2.0.0 - Updated status frame to add demag, desync and stall events, and max demag metric. Replaced debug3 frame by stress level frame.
- v1.0.0 - Initial version
