I find I am confused by USB

[jerryk]

Hello,

I'm trying to implement a custom USB type for a ham radio project; I need two serial ports and an audio port. At first glance, it seems simple: Just edit boards.txt to put in the option, and edit usb_desc.h to define the new type, pulling in pieces from other types that have the ports I want.

The first snag is endpoints. Apparently, due to timing constraints having to do with 480Mbit/s support, the Teensy 4.1 can only have 8 endpoints. Endpoints 0
and 1 are reserved. That leaves 6 endpoints. Each serial port requires two endpoints - one for data, the other for control. An audio port requires 3 endpoints. That's
7 endpoints, and we only have 6.

Why does a serial port need two endpoints? Can it work with only one?

There is also "seremu", which is a serial port that only requires one endpoint. I guess it's not as good as "Serial". But it seems good enough. Remote control of a ham radio transceiver is not exactly high-impact serial.

OK, I made an entry to usb_desc.h with one real serial port ( 2 endpoints ), one seremu port ( 1 endpoint ) and an audio port ( 3 endpoints ). 6 endpoints total.

Whups, it doesn't compile. Here's where things get interesting:
There is a seremu class, but it is never instantiated. Where would it be instantiated?

Well, it looks like it would be instantiated in usb_inst.cpp. But it isn't. In fact - usb_inst.cpp does not seem to be actually compiled. I stuck syntax errors in it, and they
are never found. Why is that?

 

[kd5rxt-mark]

@jerryk: Without seeing specific examples and/or information showing where you placed your syntax errors, there are a few very likely possibilities:

1) make sure you are modifying the correct usb_inst.cpp file
a) there are separate core files for T3 & T4

2) make sure you are modifying the correct area in the correct usb_inst.cpp
a) there are several compiler/pre-processor directives in that file
i) #if F_CPU >= 20000000 (checking if the processor speed exceeds 20MHz, plus the corresponding #else[/ICODE (towards the bottom of the file) for that [ICODE]#if)
ii) I placed int abc (without a properly trailing semicolon) inside of the section (see below for specific section) that is within the #if F_CPU >= 20000000 section, and not only was it compiled, the compiler complained as expected

#ifdef SEREMU_INTERFACE
usb_seremu_class Serial;
#endif

3) Your USB TYPE includes some kind of SERIAL device . . . SEREMU is used when there are no other SERIAL ports in use

Double-check your mods to make sure that you aren't modifying the wrong place or the wrong file, but it's more than likely that #3 above is what's actually coming into play.

Hope that helps . . .

Mark J Culross
KD5RXT

P.S. For the syntax error / compiler test, I selected T4.0, USB TYPE Flight Sim Controls, and edited the file in C:\Program Files (x86)\Arduino\hardware\teensy\avr\cores\teensy4

 

[jmarsh]

The first snag is endpoints. Apparently, due to timing constraints having to do with 480Mbit/s support, the Teensy 4.1 can only have 8 endpoints.

This isn't true, and also note that input and output endpoints may be completely independent despite possibly sharing the same endpoint number.

 

[jerryk]

This isn't true, and also note that input and output endpoints may be completely independent despite possibly sharing the same endpoint number.

If it isn't true that the Teensy is limited to 8, then how many endpoints can it have?

 

[jmarsh]

"8 bidirectional endpoints" = 8 input endpoints, 8 output endpoints = 16 total.