Add hearbeat register

[bruno-f-cruz]

Summary

This PR adds a new register to the protocol called HEARTBEAT.

Motivation

We want to be able to periodically check the status of the device. Including synchronization, mode and error status.

Detailed Design

We will add a new register with the following specs:

Name: Heartbeat
Address: 18
Format: U16
Access: Read-only

For now, only 3 bits will be implemented:

• IS_STANDBY
• IS_SYNCHRONIZED

The other 14 bits will remain free for future status bits.

More information is available in the PR.

Design Meetings

See further discussion in the following issues:

• App Behavior following a Synchronization #11 , Implicit (error) state should be represented in a queriable register #32
• See Add support for errors in harp core #63 for a spin off discussion

[galenlynch]

+1 on this! This would be hugely useful, and head off a lot of data quality issues.

[glopesdev]

Should we fix this address now? Or do we need to wait for merging #39 and #38 beforehand?

In general I guess this is an interesting question regarding numbering of newly proposed common registers, should the address be part of the proposal, or should we number them incrementally as they get merged on a first-come, first-served basis?

[bruno-f-cruz]

I dont think there is any requirement for registers to be sequential right now. Or at least in the protocol. I think that in general we should honor the "reserved" register of the PR in case people implemented a proof-of-concept already. But maybe there is a stronger reason to follow the sequence?

[glopesdev]

I'm not sure what the exact issue is, but for some reason the visual diagram of the register does not render correctly on my browser as the two bits are overlapping.

[bruno-f-cruz]

It appears to be a rendering bug when the labels are too lengthy. Replacing by single character strings:

Not really sure how to go about it... Shortening the name of the register seems a rather questionable choice :P

[glopesdev]

How about grouping the reserved bits as we do for the other registers? e.g.:

---
displayMode: compact
---
gantt
    title R_HEARTBEAT (018)
    dateFormat X
    axisFormat %

    section Bit
    15-2      :reserved, 0, 1
    1         :bit1, 1, 2
    0         :bit0, 2, 3

    section Id
    -               :idr, 0, 1
    IS_ACTIVE       :id0, 1, 2
    IS_SYNCHRONIZED :id1, 2, 3

    section Default
    -      :dr, 0, 1
    -      :d7, 1, 2
    -      :d6, 2, 3

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