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RPC Generic Data Operations Server Example

RPC, or remote procedure calls, is a way for code running on two different CPUs (in this case, a PC and an EVB connected via USB) to call procedures between each other almost as if the procedure was running locally. It does so by communicating over a transport layer - in our case, we use serial-over-USB over second USB port on the EVB.

The rpc-server.cc example shows how to use ns-rpc when the EVB is the server and the PC is the client. In this configuration, the PC calls remote procedures and the EVB executes those procedures . This example is designed to work with generic_data.py (part of ns-rpc), a python script running on the PC that can be run as server or client.

sequenceDiagram
    participant PC
    participant EVB
    note over EVB: server
    note over PC: client
    PC->>+EVB: ns_rpc_data_sendBlockToEVB(&outBlock);
    Note left of PC: generic_data.py mocks up<br/>a dataBlock, sends to EVB
    Note right of EVB: The EVB prints the<br> block on SWO interface
    EVB->>-PC: ns_rpc_data_success
    PC->>+EVB: ns_rpc_data_fetchBlockFromEVB(&returnBock)
    Note right of EVB: Mocks up a <br/> dataBlock and returns
    Note left of PC: generic_data.py prints <br/> the returned dataBlock
    EVB->>-PC: ns_rpc_data_success
    PC->>+EVB: ns_rpc_data_computeOnEVB(&outBlock, &resultBlock)
    Note left of PC: Mocks up a <br/> dataBlock and sends<br/> to EVB
    Note right of EVB: Multiplies the dataBlock<br/> by 2 and returns
    Note left of PC: generic_data.py prints <br/> the resulting dataBlock
    EVB->>-PC: ns_rpc_data_success

We demonstrate the following EVB->PC procedure calls:

  • ns_rpc_data_sendBlockToEVB(&outBlock): Passes a generic dataBlock to the EVB server. dataBlock_t is defined as part of the GenericDataOperations RPC interface. The example server simply prints the this block.
  • ns_rpc_data_fetchBlockFromEVB(&inBlock): Requests a generic dataBlock from the EVB server. dataBlock_t is defined as part of the GenericDataOperations RPC interface. The example server mocks up a small dataBlock and returns it to the client, which prints it on the CLI.
  • ns_rpc_data_computeOnEVB(&computeBlock, &resultBlock): Passes a dataBlock to the EVB, and gets a different dataBlock in return. The two do not have to be the same shape or data type. This RPC is intended for remote computation such as calculating MFCC remotely, or testing the EVB's inference results versus the remote results. The example mocks a simple dataBlock and sends it to the EVB, which multiplies it by 2 and returns it, whereupon the PC prints the the result on the CLI.

It's a client/server system needing some careful staging, described below.

Installation and Setup

  1. You'll need to install some Python-based PC-side software, following the instructions here..

NOTE for Windows, see our Windows eRPC application note

You'll also need to connect the sensor (an MPU6050) to the EVB (instructions here).

Running the RPC-server example

Running RPC requires a bit of staging. The PC-side server cant start until the USB TTY interface shows up as a device, and that doesn't happen until we start servicing TinyUSB, and even then it doesn't happen immediately.

To address this, rpc-client loop swaiting for a button press, servicing USB. This gives the user a chance to start the server then pressing the button to let the EVB it is ready to start RPCing.

  1. Compile and flash the NeuralSPOT rpc-server example
  2. Connect the second USB cable to your laptop - you'll now have 2 USB connections between the EVB and the laptop
  3. Run the rpc-server EVB code (it should be running if you flashed, unless you have something like GDB interrupting execution)
  4. Monitor the EVB SWO printout - you should see "Start the PC-side client, then press Button 0 to get started"
  5. At this point, the second USB connection should come alive. It will mount as a USB TTY device. On a Mac, it'll look something like /dev/tty.usbmodem1234561, on a windows PC it'll be COMx or similar. If the device doesn't pop up, there is a problem. It won't show up until "Press Button" shows up, so make sure you got that far.
  6. Start the laptop-side RPC client . 
    $> cd .../neuralSPOT/tools
$> python -m generic_data -t /dev/tty.usbmodem1234561 -o myaudio.wav -m client
Client started - press enter send remote procedure calls to EVB
    If all went well, you can press Button 0, which will start the server. At this point you can press 'enter' on the PC, which sends the 3 example RPC calls to the EVB.