USB: what are the advantages (or disadvantages) or using HID over serial-over-USB?

Question

Disclaimer: I'm a novice at electronics and even more so to working with USB, please bear with me if I misunderstand some of the essentials behind how USB works. Any correction welcome!

The scenario: with a group of friends we are building a robotic sailing boat. We are managing all the on-board sensors through a µC (AVR) but the sailing AI is done on an embedded linux system. The electronics is connected to the the AI hardware via USB, and I am looking for advice on what protocol to use for the transmission.

We already tried both the serial-over-USB and using HID, and both worked good enough™ in our short, near-coast tests but before settling for one or the other, I'd like to know if there is some non-trivial but nevertheless important differences between the two that I missed to consider. For our project the most important characteristics are:

• Reliability: our robot is due to sail autonomously for a few days. Is either of the two protocol inherently "safer" / with better error handling / self-recovery functionality?
• Throughput: although in normal operating situations our throughput could well be under 1Kb/s, under certain conditions we will need to harvest data nearly real-time. Serial is limited to 115Kb/s, does HID have a speed limit other than the 1500Kb/s of the USB 1.0 protocol?

...but as I said: I'm pretty new to electronics/USB, so if you feel I am missing a key parameter, I'll be glad to hear.

Thanks in advance for your time and expertise!

Answer 1

To determine what solution will be best for throughput, you need to find the bottleneck and improve that until it's no longer the bottleneck until you can't remove anything anymore.

1. Your AVR has a fixed maximum frequency, aim to make its (optimized) processing routines the limiting factor. If your communications protocol can handle this speed, then there's nothing more you can do.
2. USB at 1.5Mbps represents moving 1 byte every 160 clocks. That's quite a lot of time. to do processing, and if you're triple-buffering your data or sending out full blocks you'll want this to be even faster. At 12 Mbps, you get 13 clock cycles per byte, which is much more difficult to achieve, and would be a reasonable goal.
3. The interface between USB (and even USB itself) will carry some overhead. Sending ASCII data over an asynchronous serial interface is a big overhead. A parallel FIFO USB interface chip will be much faster. A micro with built-in USB peripheral will be better yet.

To get more reliability, you need to stick with what works and what you know best. Relax the specs. Here, a tried-and-true serial interface is a fine choice.

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How does "serial over USB" work in your application?

If it's a software driver on the embedded Linux device, and a USB <-> parallel IC (or software serial port on a USB-enabled AVR, which sounds likely since you've tried HID mode), then you'll have no problems with higher bit rates.

However, true USB <-> serial ICs do exist and are popular (the FTDI232R on older Arduinos comes to mind...) which turn USB into logic-level serial RS232 communication. You don't need or want this layer for your application. It will reduce your throughput and add an additional asynchronous delay section into your application.

Answer 2

I did a project that required a USB interface (using PIC micro) a couple of years ago, I looked at both but HID is superior I feel:

HID has automatic detection and buffering of packets by windoze as opposed to keep having to 'reconnect' the serial port on each plug-in (and to a port number that is dependant upon the usb port used each time (LAME)) --programming VB for HID is simples!

Max throughput is lower for HID, i think its 64bytes (max hid buffer size) both ways every 10ms for USB 1.1 or 1ms for USB 2.0.

Answer 3

1) As for reliability I would prefer lower-level protocols like hardware serial with manual error handling. USB device might just disappear if controller does not like something, and if you won't reinitialize it - you won't see it again. So you need to learn to reinit USB.

2) As for speed - serial-over-USB is not limited to 115Kb/s in general, in bust be seeing limitation in one of our implementations. In general I would expect 'serial-over-USB' to be more efficient than HID, as HID should have more 'decoration' burden. Even faster is 'parralel-over-USB' which is used to pump at speeds close to USB throughput.

Answer 4

It's not the serial profile that's limited to 115200bps - in fact, it's rarely even the serial port nowadays. If you use something like an FT245 you can reach much higher speeds than that. The problem with standard asynchronous (RS-232 style) transfer is that you need to have gaps occasionally to have a chance to synchronise on bytes (or alternatively, send 0xff from time to time).

As for the difference between HID and USB serial, serial is bulk while HID is control and interrupt. That's fundamental for USB, and means HID has (potentially) lower latency but serial has higher capacity. There's not much more to it than that, neither is more reliable, and you'll want a way to reset your circuitry when things go wrong.

... by the way, this sounds familiar. Hi Mac! Pardon my absence from Abbenay yesterday.