FTDI FT245R power requirement

I've used the FT245R for USB data upload  in a number of data logger 
designs.  I have generally used the logger 3.3V supply to power
the chips.   However, I just ran across the following note in
the most recent FT245R data sheet:

"Notes:
1. The minimum operating voltage VCC must be +4.0V when using the 
internal clock generator."


This not was not present in earlier incarnations of the data sheet
and I assumed that the chip really did operate over the specified
3.3 to 5.25V  VCC range.  In fact,  I have had no problems that I
can track back to using just 3.3V for VCC.

I sent an email on this subject to FTDI support last week and
their response was:

"If you were having issues with VCC = 3V3 it would manifest itself as 
3V3 being less than 3V3 and the device would not enumerate. The 4.0V 
allows for some headroom on the VCC supply."


I haven't observed any problems with enumeration at  VCC = 3.3V. 
Has anyone else  seen problems or gotten a better explanation
from FTDI?

Note that the same power restrictions also apply to the FT232R
serial-to-USB converter.

Most of the other FTDI chips specify a VCC of 4.35 volts or
higher and use external crystals or ceramic resonators.  
I picked the FT245R because it was specified for 3.3V operation
and didn't require the large ceramic resonator.  Now I'm
wondering if I face either reliability issues or having to
redesign my PC boards.


Mark Borgerson

In article <MPG.23d509274162cc9989699@news.motzarella.org>, 
mborgerson@comcast.net says...
> I've used the FT245R for USB data upload  in a number of data logger 
> designs.  I have generally used the logger 3.3V supply to power
> the chips.   However, I just ran across the following note in
> the most recent FT245R data sheet:
> 
> "Notes:
> 1. The minimum operating voltage VCC must be +4.0V when using the 
> internal clock generator."
> 
> This not was not present in earlier incarnations of the data sheet

That would not surprise me.

> and I assumed that the chip really did operate over the specified
> 3.3 to 5.25V  VCC range.  In fact,  I have had no problems that I
> can track back to using just 3.3V for VCC.
> 
> I sent an email on this subject to FTDI support last week and
> their response was:
> 
> "If you were having issues with VCC = 3V3 it would manifest itself as 
> 3V3 being less than 3V3 and the device would not enumerate. The 4.0V 
> allows for some headroom on the VCC supply."
> 
> 
> I haven't observed any problems with enumeration at  VCC = 3.3V. 
> Has anyone else  seen problems or gotten a better explanation
> from FTDI?

No to both, but I suspect they have found that the internal clock
generator, can hic-cup or stall if 3V3 actually goes to 3V3 -5%, or
a dip in rail or noise. May also be related to operation at ends of
temperature range over period of time.

> Note that the same power restrictions also apply to the FT232R
> serial-to-USB converter.
> 
> Most of the other FTDI chips specify a VCC of 4.35 volts or
> higher and use external crystals or ceramic resonators.  
> I picked the FT245R because it was specified for 3.3V operation
> and didn't require the large ceramic resonator.  Now I'm
> wondering if I face either reliability issues or having to
> redesign my PC boards.

I suspect it is more to do with temperature range and tolerances
on bottom end of Vcc range. Suspect the device was not fully 
parameterised, which considering past problems with datasheets 
would not surprise me.

-- 
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On Jan 12, 6:07=A0pm, Mark Borgerson <mborger...@comcast.net> wrote:
> I've used the FT245R for USB data upload =A0in a number of data logger
> designs. =A0I have generally used the logger 3.3V supply to power
> the chips. =A0 However, I just ran across the following note in
> the most recent FT245R data sheet:
>
> "Notes:
> 1. The minimum operating voltage VCC must be +4.0V when using the
> internal clock generator."
>
> This not was not present in earlier incarnations of the data sheet
> and I assumed that the chip really did operate over the specified
> 3.3 to 5.25V =A0VCC range. =A0In fact, =A0I have had no problems that I
> can track back to using just 3.3V for VCC.
>
> I sent an email on this subject to FTDI support last week and
> their response was:
>
> "If you were having issues with VCC =3D 3V3 it would manifest itself as
> 3V3 being less than 3V3 and the device would not enumerate. The 4.0V
> allows for some headroom on the VCC supply."
>
A thought (hack) comes to mind:
1. the 3v3 pin needs a 100nF cap even if the output is not used, so
maybe it is direct access to the output of their LDO
2. The LDO is most likely an open drain device
3. Your poswer supply is MAX 3v3
So, join VIO, VDD and 3V3 all together. Viola! 3v3 will not be less
than 3v3.
Rocky

Un bel giorno Mark Borgerson digit�:

> I've used the FT245R for USB data upload  in a number of data logger 
> designs.  I have generally used the logger 3.3V supply to power
> the chips.

Is there any specific reason you don't use the USB power? Noise issues?

-- 
emboliaschizoide.splinder.com

In article <1r5gwyf6ji4ky$.1qxt9215avcae.dlg@40tude.net>, antonio12358
@hotmail.com says...
> Un bel giorno Mark Borgerson digit=F2:
>=20
> > I've used the FT245R for USB data upload  in a number of data logger=20
> > designs.  I have generally used the logger 3.3V supply to power
> > the chips.
>=20
> Is there any specific reason you don't use the USB power? Noise issues?
>=20
>=20
The 8 data bits of the FT245 are connected directly to the logger
cpu data bus.  The USB connection is normally unplugged while
the logger collects data.  If the FT245 loses power, it will
drag down the CPU data bus.

Mark Borgerson

Un bel giorno Mark Borgerson digit�:

>>> I've used the FT245R for USB data upload  in a number of data logger 
>>> designs.  I have generally used the logger 3.3V supply to power
>>> the chips.
>> 
>> Is there any specific reason you don't use the USB power? Noise issues?
>> 
>> 
> The 8 data bits of the FT245 are connected directly to the logger
> cpu data bus.  The USB connection is normally unplugged while
> the logger collects data.  If the FT245 loses power, it will
> drag down the CPU data bus.

If you connect USBP to VCC, and connect your onboard +3.3V to VCCIO, when
you disconnect the USB cable the USB core will be unpowered, but the I/O
buffer section will be still powered, and probably it will keep the outputs
hi-Z.

I know that this is explicitly forbidden by the datasheet [*], but it seems
the lesser of two evils. It's worth a try.


[*] Well, actually they say that "[...] in bus powered designs a
regulator which is supplied by the +5V on the USB bus should be used".
"Should", noy "shall" or "must". :)

-- 
emboliaschizoide.splinder.com