FTDI FT245R power requirement

Jan 12, 2009 5 Replies
M

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



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P

That would not surprise me.

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.

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.

Paul Carpenter | paul@pcserviceselectronics.co.uk PC Services Timing Diagram Font GNU H8 - compiler & Renesas H8/H8S/H8 Tiny For those web sites you hate
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R

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
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D

Un bel giorno Mark Borgerson digitò:

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

emboliaschizoide.splinder.com
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M

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

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D

Un bel giorno Mark Borgerson digitò:

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
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