Hibike is a lightweight communications protocol designed for the passing of sensor data for the PiE Robotics Kit, a.k.a Frank or "Kit Minimum" to some.
We make a few starting assumptions concerning the endpoints of communication: the device controlling a sensor is a Smart Device (SD), and a Beaglebone Black (BBB) is used as the central control board of each robot and thus is in charge of communicating with each Smart Device. These two communicate with each other via serial: the Beaglebone running pySerial and the Smart Device by using the built-in serial library. As each Smart Device communicates with the Beaglebone on its own separate port, we conveniently have no need to worry about any race conditions or other problems arising from concurrency.
Refer to Section 5 for an outline of the general behavior of the Hibike protocol.
All messages have the relatively simple structure of Message ID, Payload, and Checksum as depicted below. A more complete description of each field is given below the diagram.
+------------+------------------+---------------------+------------+
| Message ID | Payload Length | Payload | Checksum |
| (8 bits) | (8 bits) | (length varies) | (8 bits) |
+------------+------------------+---------------------+------------+
Message ID - an 8-bit ID specifying the type of message being sent or received. More information about each message type is specified in the following sections.
Payload Length - an 8-bit unsigned integer specifying the number of bytes in the payload
Payload - Varies wildly depending on the type of message being sent. This will, of course, be described in more detail in Section 4.
Checksum - An 8-bit checksum placed at the very end of every message. Really, any checksum scheme appending 8-bits to the end of the message will do, but an exceedingly simple one recommended exactly for its simplicity is making the checksum the XOR of every other byte in the message.
Each Smart Device will be assigned an 88-bit UID with the following data.
+-------------------+--------------+-----------------------------+
| Device Type | Year | ID |
| (16 bits) | (8 bits) | (64 bits) |
+-------------------+--------------+-----------------------------+
Device Type - 16-bit ID specifying the type device the Smart Device controller is attached to. Device types are enumerated in Section 4
Year - 8-bit ID corresponding to the competition year that the Smart Device was manufactured for. The 2015-2016 season will correspond to 0x00
ID - Randomly generated 64-bit number that will uniquely identify each Smart Device within a specific device type and year. With 64-bit IDs, the probability of a hash collision with 1000 of 1 type of device per year is roughly 0.05%
Message ID Enumeration:
+---------+--------------------------+
| ID | Message Type |
+------------------------------------+
| 0x00 | Subscription Request |
+------------------------------------+
| 0x01 | Subscription Response |
+------------------------------------+
| 0x02 | Data Update |
+------------------------------------+
| 0x03 | Device Update |
+------------------------------------+
| 0x04 | Device Status |
+------------------------------------+
| 0x05 | Device Response |
+------------------------------------+
| 0x06 | Ping |
+------------------------------------+
| 0x08 | Description Request |
+------------------------------------+
| 0x09 | Description Response |
+------------------------------------+
| 0xFF | Error |
+------------------------------------+
Device Type Enumeration:
+---------+----------------+
| ID | Sensor |
+--------------------------+
| 0x00 | LimitSwitch |
+--------------------------+
| 0x01 | LineFollower |
+--------------------------+
| 0x02 | Potentiometer |
+--------------------------+
| 0x03 | Encoder |
+--------------------------+
| 0x04 | BatteryBuzzer |
+--------------------------+
| 0x05 | TeamFlag |
+--------------------------+
| 0x06 | Grizzly |
+--------------------------+
| 0x07 | ServoControl |
+--------------------------+
| 0x08 | LinearActuator |
+--------------------------+
| 0x09 | ColorSensor |
+--------------------------+
| 0x10 | DistanceSensor |
+--------------------------+
| 0x11 | MetalDetector |
+--------------------------+
| 0xFFFF | ExampleDevice |
+---------+----------------+
Note: These assignments are totally random as of now. We need to figure out exactly what devices we are supporting. Note: As of now, Grizzlies are not supported by Hibike (pyGrizzly should be used instead) But they should be in the near future, to preserve the idea of treating every peripheral as a SmartDevice.
Error ID Enumeration:
+---------+---------------+
| Status | Meaning |
+-------------------------+
| 0xFB | Invalid |
| | Message Type |
+-------------------------+
| 0xFC | Malformed |
| | Message |
+-------------------------+
| 0xFD | Invalid UID |
+-------------------------+
| 0xFE | Checksum Error|
+-------------------------+
| 0xFF | Generic Error |
+-------------------------+
Note: These assignments are also fairly random and may not all even be needed.
-
Sub Request: BBB requests data to be returned at a given interval. The SD will respond with a Sub Response packet. Payload format:
+---------------+ | Delay | | (16 bits) | +---------------+Direction: BBB --> SD
-
Sub Response: SD sends (essentially) an ACK packet with the UID and delay state Payload format:
+--------------------------+--------------------+ | UID | Delay | | (88 bits) | (16 bits) | +--------------------------+--------------------+Direction: BBB <-- SD
-
Data Update: SD sends its state values based on the given delay (refresh rate). BBB does not send an ACK packet back. Payload format:
+------------------------+ | Reading | | (variable) | +------------------------+Direction: BBB <-- SD
-
Device Update: BBB writes a value to the SD state. SD responds with a Device Response packet. Payload format:
+---------------+--------------------+ | Param | Value | | (8 bits) | (32 bits) | +---------------+--------------------+Direction: BBB --> SD
-
Device Status: BBB polls a value from the SD state. SD responds with a Device Response packet. Payload format:
+---------------+ | Param | | (8 bits) | +---------------+Direction: BBB --> SD
-
Device Response: SD returns the (possibly updated) value of the param changed/polled earlier. Payload format:
+---------------+--------------------+ | Param | Value | | (8 bits) | (32 bits) | +---------------+--------------------+Direction: BBB <-- SD
-
Ping: BBB pings SD for enumeration purposes. The SD will respond with a Sub Response packet. Payload format:
+---------------+ | Empty | | (0 bits) | +---------------+Direction: BBB --> SD
-
DescriptionRequest: BBB requests a descriptor string from the SD. The SD will respond with a sequence of DescriptionResponse packets. Payload format:
+---------------+ | Empty | | (0 bits) | +---------------+Direction: BBB --> SD
-
DescriptionResponse: SD returns a sequence of packets containing indexes and substrings. The final packet's substring is terminated with a 0 byte. The BBB assembles the substrings into a full device descriptor.
Payload format:
+---------------+-----------------------+ | Index | Descriptor Substring | | (8 bits) | (Variable) | +---------------+-----------------------+Direction: SD --> BBB
-
Error Packet: Not planned as of yet. May only be useful for debugging. See "Behavior" section for a higher-level description of how error-handling will work. Payload format:
+----------------+ | Error Code | | (8 bits) | +----------------+Direction: BBB ??? SD
Setup
- The BBB must be able to determine what sensors are connected on startup. This means Hibike supports any combination of devices but not hot-plugging.
- The BBB will send Subscription Request Packets to every detectable serial port with a delay value of 0, and determines the type of sensor based on the Subscription Response payload sent back
- The BBB must then allocate bandwidth (refresh rate) based on the type and number of each device connected
- Each supported device will have an associated data file that lists its reading length, parameters enumerations, etc...
Sensor Communication (Reading values)
- After setup the BBB sends the approrpiate Subscription Request Packet is sent to each device.
- The SD will then return values at regular intervals specified by the Subscription Request (delay field)
- Hibike also allows for the BBB to poll certain fields from the state of a SD, using a Device Status Packet.
- The SD will respond by returning a Device Response Packet with the value of the field specified.
Actuator Communication (Writing values)
- For devices that will have configurable states (like the Grizzly), the BBB can write data to a specfied parameter of the SD with a Device Update Packet
- The SD will then return a Sensor Response Packet with the newly written value of the specified param in the payload.
Error handling
- Still kind of up in the air, but in general, only the BBB will have error handling behavior
- Every packet sent from the BBB will have a timout for recieving the appopriate ACK packet back. If the packet does not come within the time limit, or the recieved packet is invalid, the BBB will try to resend the packet.
- If a SD recieves an invalid packet, it will simply not respond. This can change if need be.