Re: FTDI USB and the PIC


Apr 28, 2005

 


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#9165 Apr 28, 2005

I've successfully used a bootloader in combination with an FTDI 232BM

USB chip for simple serial communication to the PC, but I have a few

questions about how best to power it.



The application in question is battery powered, so I want to conserve

as much as possible. Can I power the PIC off the battery, and use the

power rail coming from the USB port with a ferrite bead to run the

FTDI chip? Most examples show the entire circuit either powered by

the battery, or by the USB port, which isn't quite what I need.



If it is not possible to have the USB port power the FTDI chip while

the battery powers the PIC, what about powering the FTDI chip right

off one or two of the PIC ports? I believe the normal current draw of

an FTDI 232BM is less than 25 mA at all times.



I understand you can put the FTDI chip to "sleep", but I would rather

not have it draw any current since it will rarely be used.



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#9171 Apr 28, 2005

I don't know this for certain but I suspect the issue comes down to

having a voltage from the PIC landing on the FTDI chip when it is de-

energized. Many times the specs will limit an input voltage to some

low value over Vcc but don't really discuss what happens when Vcc is

0 or undefined.





--- In piclist@yahoogroups.com, "tadaosystems" flipflops67@h...>

wrote: > I've successfully used a bootloader in combination with an FTDI

232BM > USB chip for simple serial communication to the PC, but I have a

few > questions about how best to power it.

>

> The application in question is battery powered, so I want to

conserve > as much as possible. Can I power the PIC off the battery, and use

the > power rail coming from the USB port with a ferrite bead to run the

> FTDI chip? Most examples show the entire circuit either powered

by > the battery, or by the USB port, which isn't quite what I need.

>

> If it is not possible to have the USB port power the FTDI chip

while > the battery powers the PIC, what about powering the FTDI chip

right > off one or two of the PIC ports? I believe the normal current

draw of > an FTDI 232BM is less than 25 mA at all times.

>

> I understand you can put the FTDI chip to "sleep", but I would

rather > not have it draw any current since it will rarely be used.


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