使用protobuf和STM32F7设计协议

Question

I'm trying to do the same protocol that is defined and described here in that wiki https://wiki.trezor.io/Developers_guide-Message_Workflows

My toolset is Protobuf for embedded systems: Nanopb. STM32F7 using Serial port.

I'm trying to communicate right now between PC and STM32F7, and usually the communication will be done between two STM32F7 boards.

My questions:

1. What kind of protocol that is sufficient for a request and answer like the one in trezor ?

2. I googled and I found I have to use something like HLDC, is it necessary for that purpose or it's an overhead ?

3. Coding and design issue: I will fire a serial interrupt that always polling for the data that is communicated between two boards, now there will a very big state machine to decode each message type and do the event based on the message type. Is there an alternative design ?

Answer 1

Firstly, AFAIK, nanopb doesn't support the full range of possibilities in the Protobuf schema language. So you'll need a schema that works for nanopb, and hopefully that'll be good enough for the needs. However it can be very annoying as (so far as I know) the very useful oneof doesn't work.

Secondly, protobuf wireformat is not self delimiting. So you'll be squirting data down the serial cable, but it's not possible to reliably (if at all) tell where one message ends and another starts. So you'll need to transmit some sort of inter-message sync pattern of bytes, chosen to be unlikely to be encountered in a message. You'll have to read in the bytes inbetween sync patterns, place them in a buffer, and parse from that.

Thirdly, if you're sending a variety of different messages and you can't use oneof , then you'll need some other way of identifying what type of message has arrived so that you can parse it to the right type of object. That "way" could simply be a fixed sequence of message types, or a byte whose value identifies the message type, or a field that does the same thing in all the messages. oneof is attractive (though not available to you) because it can be used as a carrier for a variety of different message types; you simply parse the received data using the oneof 's parser.