Robert E. Griffith wrote:
> To expand on what Alan said with an example, the quickest path to get
> up and running with the FTDI chip is this.  Take a working design
> that current uses a serial port.  Add a FTDI serial chip to the
> board.  The existing design's serial interface just connects to the
> FTDI chip.  Don't modify the firmware at all.  When you plug it into
> a PC, install the default drivers that FTDI distributes.  They will
> just add a serial port to the PC - say COM4.  Now take the existing
> software for the old design and configure it to use COM4. The old
> firmware and PC software both still work without modification.  They
> think they are still using a serial port.
>
> FTDI makes an evaluation cable that you can just plug into your serial
> product.  It gives you the same user experience.  If you like it, put
> the chip on the board.
>
> The Rabbit dev environment uses a serial connection to their core
> modules. They sell the FTDI eval cable as an option.  When using it,
> you can get 240 baud (or something like that), so it does go a little
> faster than a standard serial cable - probably because the single
> ended serial singles are so short.


FTDI specifies top serial speed at 150kbps, but this is not much related to
the FTDI chip possibility, since it can transfer about 1.5Mbps via USB.
The limitation for 150kbps here is related to the PC serial port itself,
that simply can not talk faster than this.  FTDI could say that their
serial chip can talk at 700kbps, but will be talking with no one, since no
PC serial port can do it, and most devices that "could" connect to a PC
serial port will not do it either.

So, suppose you have a microcontroller board, talking serial 38400bps with
a PC COM1: and you implement the FTDI 232 to the board, so now, your
microcontroller board output talks USB @1.5Mbps with the PC, but still
talking 38400bps with the microcontroller on board.

Now, suppose you made a very fast gadget that can talk @150kbps with the
COM1: (you didn't use 200kbps because the COM1: can not do more than
150kbps) and later you install the FTDI 232 chip on board. The board output
will talk USB @1.5Mbps, but the microcontroller still talking @150kbps with
the FTDI.

Other problem could be related to the Windows drivers itself, since your
Windows software probably will "try" to configure this virtual COMX: port
to talk at maximum speed of 150kbps, so, it wouldn't help if your
microcontroller at the external board can talk faster with the FTDI, the PC
application will not do it faster than 150kbps.  So, it doesn't matter if
your car has slick Formula-1 tires, you have just one horse to pull it.

By other side, the parallel chip, will allow you (probably - requires tests
and actually "read better" the documentation), to communicate with the FTDI
as fast as you can, and as fast as the chip can deliver data to you, based
on 1.5Mbps, it could "probably" give you a throughput of 1.5/11 =
136kBytes/second, 10 times faster than the 150kbps of the serial chip.  Of
course that if you still using the virtual serial COMX port at the windows
side, you will be limited to the 150kbps, but you are not tied to it.
Probably windows drivers for the paralel chip can allow you to a faster
speed at your PC program.

In one way or another, we need to think seriously about USB.  Most actual
notebooks are coming out without serial ports. If you think, it doesn't
make sense any more to use them.  All modems are part of the motherboard or
PCMCIA, all mouses are PS/2 interface or attached, why would you need a
serial port for? That thing is lazy for anything else. They just ripped it
off.

I even wonder how long USB will stay on board, FireWire is pressing it down
hard.

>
> If you take a little dev time, you can change your design and
> firmware to use the parallel interface FTDI chip instead of RS232
> serial.  I would think that the clocked parallel interface would not
> limit the through much - if at all. (don't really know, though)
>
> So Neil, I would say that if your purpose is to learn about USB, do
> not use the FDTI chip.  It hides all the details of USB from you
> which is good for updating an existing product quickly, but not for
> learning.  With a normal USB stack, you will determine how your
> device is identified on the bus, what services/interfaces it
> supports, and be able to choose how to send data - streaming or
> asynchronously (I can't remember the real terms for these two right
> now).  That's what USB is all about.
>
> As far as the PC software goes, you have a choice of having your
> device support one of the built-in interfaces or writing your own
> driver.  One of the built-in interfaces is the storage interface.
> That is why you can plug a USB memory/hard disc into a WinXP computer
> for the first time and it will work without installing any drivers.
> I don't know if there are any other common interfaces included in
> WinXP or other OS.
>
> --BobG
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Alan B. Pearce
> Sent: Monday, September 30, 2002 4:02 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: USB with a Flash PIC?
>
>> Where did you see this?  The website is fairly confusing, so I must
>> be missing it somewhere.
>
>> Wagner Lipnharski wrote:
>
>>> The FT8U245AM is really amazing @ $5.75 for single quantity.
>
> Try http://www.ftdichip.com/ although as Wouter (IIRC) said, their
> web shop is still under construction. Also note that they have B
> versions of both chips available now. Apparently these fix some
> problems in the A chips, and add some bits that make them nicer or
> easier to use.
>
> Also it seems to me that you are looking for more speed than these
> chips may give you. I suspect the FTDI chips are really designed to
> allow existing equipment designs to migrate to USB with minimal
> changes to the hardware, rather than be used for new whizz-bang
> applications. Their basic design philosophy seems to be to use
> existing standard drivers, but allow the OEM to write their own
> driver if desired. From this point of view you may find that chips
> from Cypress, or other manufacturers, may allow you faster USB data
> transfer speeds, but you may have to put more work in at the driver
> level.

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