CRSF-Enfinite Individual

Why

It might be more efficient to use key value pairs instead of compound sensors?

Frame Overview

The payload of a frame is a collection of Sensor Item objects, each having a pre-determined sensor ID (eType) and Value.

eType Encoded Format

eType both defines what value the sensor represents (e.g. ESC 0 Voltage) as well as the type of data that follows. The lower two bits of the eType describe format of the Value that follows ((eType << 2) | ValueType). eTypes are encoded as varint. There are two types of data currently supported by this standard:

Byte Lower 2 Bits	Type	Description
00	VARINT	Variable length integer. Encoded uint, int, bool, enum
01	LEN	Length of data chunk. Encoded string, byte arrays, subitems
10 or 11	None	Reserved for future use

A parser is able to skip eTypes it does not understand how to parse by examining the eType field.

• Examine eType field's lower two bits
  • 00 (VARINT) - Read another varint, discard the value. The next byte following this value begins a new eType.
  • 01 (LEN) - Read another varint, skip the number of bytes specified by the varint. Do not include the LEN varint itself in this size. The next byte following this value begins a new eType.

When specifying eTypes, the eType number is independent of the lower two bits that will be used to decribe the data. For example Battery Cell Voltages is eType 2, but would be seen on the wire as (2 << 2) | 01 = 9.

Strings

eTypes that contain string values use the LEN value to specify the size of the UTF-8 encoded string length that follows.

Field	Bytes	Value	Description
eType	0x25	37	Model Name eType (0x09 + type LEN)
LEN	0x0b	11	11 bytes follow
Model Name	0x47 0x65 0x6e 0x74 0x6c 0x65 0x20 0x4c 0x61 0x64 0x79	Gentle Lady	11 chars

VARINT Lists

A list of values of the same sensor type may be shortened by omitting the eType on subsequent values. Only lists of VARINT are supported with this shorthand. To encode this, use LEN for the item type instead of VARINT and follow it with the length of all the included encoded values. For example, a list of 4x cell voltages could be expressed normally as:

Field	Bytes	Value	Description
eType	0x08	8	Voltage Millivolts (type VARINT)
Cell Voltage 0	0xde 0x1c	3678	Cell 3.678V
eType	0x08	8	Voltage Millivolts (type VARINT)
Cell Voltage 1	0xdc 0x1c	3676	Cell 3.676V
eType	0x08	8	Voltage Millivolts (type VARINT)
Cell Voltage 2	0xd1 0x1c	3665	Cell 3.665V
eType	0x08	8	Voltage Millivolts (type VARINT)
Cell Voltage 3	0xf9 0x1c	3705	Cell 3.705V

Instead a list of VARINTs can be used to reduce encoded length (12 bytes vs 10 bytes).

Field	Bytes	Value	Description
Voltage Millivolts eType	0x05	5	Voltage Millivolts begin (type LEN)
Voltage Millivolts Value	0x08	8	Size of all the cell voltage values
Cell Voltage 0	0xde 0x1c	3678	Cell 3.678V
Cell Voltage 1	0xdc 0x1c	3676	Cell 3.676V
Cell Voltage 2	0xd1 0x1c	3665	Cell 3.665V
Cell Voltage 3	0xf9 0x1c	3705	Cell 3.705V

Compound Sensors

In order to logically group sensors without allocating an eType for each instance of an object (e.g. multiple ESCs) a sensor value is allowed to have subitems.

Example: Index (eType 0x00)

• Type: VARINT
• Description: An integer index to identify which instance this sensor is representing. Used in a subitem.

Eample: Voltage Millivolts (eType 0x01)

• Type: VARINT / LEN
• Decription: A battery's single cell voltage in millivolts (0.001V).

Example: Battery Voltage (eType 0x02)

• Type: LEN
• Description: Top level item for sending total battery or multiple cell voltages
• Subitems:
  • Index (0x00) - Optional. 0-based battery index of which instance this item represents. If not present in the subitem, battery 0 (first battery) is implied. Index must be the first value in the submessage.
  • Voltage Millivolts (0x01) - (Repeats, Optional) Cell voltage. Repeat for each cell present. Fill only the optional fields to the last available value. i.e. do not fill the entire report with zeroes if the values are not available.
• Notes
  • All cells must be present in a single report. Do not split cell voltages across messages.
  • "No battery present" should be indicated as no cell voltages.
  • No total voltage is transmitted, the total voltage should be the sum of the cell voltages.
  • Implementations may choose to report total battery voltage using this compound sensor by reporting the total voltage in Cell Voltage 0. However, in that case they must not also publish cell voltages.

Example payload (cell voltage):

Field	Bytes	Value	Description
eType	0x09	9	Battery voltage begin (type LEN)
LEN	0x0c	12	Length of subitems
Index eType	0x00	0	Index item begin (type VARINT)
Index Value	0x00	0	First battery instance
Voltage Millivolts eType	0x05	5	Voltage Millivolts begin (type LEN)
Voltage Millivolts Value	0x08	8	Size of all the cell voltage values
Cell Voltage 0	0xde 0x1c	3678	Cell 3.678V
Cell Voltage 1	0xdc 0x1c	3676	Cell 3.676V
Cell Voltage 2	0xd1 0x1c	3665	Cell 3.665V
Cell Voltage 3	0xf9 0x1c	3705	Cell 3.705V

Example payload (total voltage):

Field	Bytes	Value	Description
eType	0x09	9	Battery voltage begin (type LEN)
LEN	0x03	3	Length of subitems
Voltage Millivolts eType	0x04	4	Voltage Millivolts begin (type VARINT)
Total Voltage	0x84 0x73	14724	14.724V