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Defining XML element: State units that should be used #216

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Defining XML element: State units that should be used #216

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19 changes: 14 additions & 5 deletions en/guide/define_xml_element.md
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Expand Up @@ -165,11 +165,20 @@ The main rules for messages are:
- There may be no more than 64 fields.
- The `<wip/>` tag may be added to messages that are still being tested.
- Fields:
- must have unique `name`s within a message.
- *should* have a description.
- *should* use the `units` attribute rather than including units in the description.
Each field should only have **one** or no units.
- *should* use the `enum` attribute where possible results are finite/well understood.
- `name`: required
- must be unique within a message.
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Do we have any other requirements on names? Ie do we prefer them to be snake cased (underscores). Do we prefer full names or do we allow truncation - eg. bat_status, vs battery_status etc?

- `type`: required
- There are a fixed set of types: `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t`, `int64_t`, `float`, `double`
- You can also declare an array for each type, e.g.: `uint8_t[3]`, `int32_t`
- Use the smallest data type that will contain the field data with sufficient resolution.
- `description` optional, but *should* be provided.
- The description should not include units information.
- `units`: optional, for indicating units
- SI units should be used where possible (units are constrained by the XML DTD)
> **Note** Use natural units like radians or quaternions where possible as these allow greater precision and fewer issues for implementers than units like degrees.
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@LorenzMeier I couldn't find a good place to capture the discussion about not just automatically using human readable units. This note is at least something that makes sure the point is made

- *should* only have **one** or no units (avoid defining fields where the units can change depending on other fields or conditions).
- `enum`: optional.
- *should* be used where possible results are finite/well understood and for bitmasks.

> **Warning** You cannot rely on generators to fully test for compliance with the above rules.
The *mavgen* code generator tests for duplicate message ids, duplicate field names and messages with more than 64 fields.
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