I now have access to a Rock64-4GB (Rock64_V2.0 board); I hope to put FreeBSD on it someday

[Ian Lepore]

On Sat, 2018-01-06 at 13:58 -0800, Rodney W. Grimes wrote:
> > 
> > On 2018-Jan-6, at 8:45 AM, Emmanuel Vadot <manu at
> > bidouilliste.com> wrote:
> > 
> > > 
> > > On 2018-01-06 17:43, Emmanuel Vadot wrote:
> > > > 
> > > > On 2018-01-05 15:45, Mark Linimon wrote:
> > > > > 
> > > > > On Fri, Jan 05, 2018 at 06:27:41AM -0800, Mark Millard wrote:
> > > > > > 
> > > > > > the early boot baudrate for the console is apparently
> > > > > > 1.5Mbit/s
> > > > > I've had to use minicom.  That's the only thing that I'm sure
> > > > > supports it.
> > > > > mcl
> > > > Works fine with tip(1) here using an FDTI usb<->serial, using a
> > > > PL2303XA doesn't work (but should based on the datasheet), and
> > > > my
> > > > CP2108 should work but I haven't tested yet.
> > > Also my PL2303XA adapter works on linux using minicom but only
> > > for RX, that might be a driver issue.
> > I've been using Serial on an old MacOS X laptop.
> > I've done more experiments. Here is what I've
> > observed. . .
> > 
> > For a CH340G, Serial did not allow 1500000
> > (built-in driver for USB ID 1a86:7523:0254)
> > but Serial is being updated to allow it. (I
> > have a preliminary release now, so I have
> > 1500000 support now.)
> I have to seriously laugh at the idea of doing
> TTL serial ports at 1.5MHz down unbalanced,
> unterminated single end wires.  Just not a
> reliable way to communicate.
> 

I've validated ftdi<->ftdi and ftdi<->imx6-uart at 3.3v ttl level over
24ga jumper wires about 20 inches long at 4mpbs.  Purposely not a good
signal environment, just bare jumpers strung across my desk.

I've also tested transfers between pairs of good-quality end-user type
usb-serial adapters connected with a null modem adapter at 2mbps; those
connections amounted to a couple meters of wire, but they were at rs232
line level.  I've never found adapters that can go faster than 2mpbs,
because of the cheap ttl<->rs232 chips they contain (most of them max
out around 1mbps).

> Hopefully your doing this at 5V, at least
> then you have a good noise margin, at 3.3V
> you lose another 33% of that.
> 
> Also most of these USB/232 adapters have no
> way to do flow control, so you better have
> a darn big fifo or your host usb stack better
> be darn fast at getting data off the chip.
> 

Modern ftdi chips have 2K or 4K fifo.  I would expect other modern ubs-
serial chips to have similarly big fifos, but even the older ones all
had 256 or 384 bytes.  And on a usb 2.0 bus it can empty out 512b every
125us.

-- Ian