gnuk with GD32F103

[NIIBE Yutaka]

Karsten Müller <email at kmkcl.de> wrote:
> * PCB is based on FST-01, without SPI-Chip (and other pin for USB-EN -
> like nitrokey_start)

FST-01G and FST-01SZ don't use USB-EN any more.  I removed the pin
because it is not needed.  I know a design with USB-EN, like Maple-Mini,
and that is a reason why FST-01 has that.

The intended use of USB-EN in FST-01 was for a device to ask host to do
re-enumeration.  For this purpose, simply driving USB signals (D+ and
D-) to GND is actually OK.  For strict conformance to the USB
specification, it's better supporting high impedance state of D+.  But,
(for me), the fixed pull-up of D+ can be acceptable, as long as it
works.

In short, I suggest removing USB-EN.  Please have a look at FST-01SZ
schematics, it pulls-up D+ line to +3V3 directly (no USB-EN control at
all).

> * I also test with and without USB-EN pin (once in software - comment
> out define, once in hardware by desoldering resistor)

The resistor should be there to pull up D+ line to +3V3.  Having USB-EN
means, it is configurable (pull-up to +3V3 or not).  "Removing" should
be being fixed to pull up to +3V3, not removing the registor itself.

In chopstx/board/board-fst-01.h, hardware pin of USB-EN pin (PA10) is
designated to the pin which usb_cable_config function controls.  In
chopstx/board/board-fst-01g.h and chopstx/board/board-fst-01sz.h, there
is nothing.

So, if you use FST-01SZ configuration for a hardware with USB-EN, USB
functionality is simply not available.  To enable USB functionality, you
need to put a resistor (1K5) to pull up to +3V3.
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