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Author Topic: Hi, new to arduino. Question on TTL-232R-3V3  (Read 520 times)
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Hi,
I have an arduino Pro Mini 3V3 on order, but I already have a TTL-232R-3V3 from a previous project. However I haven't used it
for some time and have reinstalled Windows XP Pro on my PC since then. I have let windows install the driver it found on the net as I
do not have the driver CD for the cable. I have noticed that pin 3, (VCC) out of the cable is measuring 4.8volts when I am expecting it to be
3.3volts, however other pins are at the correct 3V3 level. Is that expected operation, only in the start-up example for the Pro Mini, here :
http://arduino.cc/en/Guide/ArduinoProMini
it is showing VCC from the cable supplying power to the Pro Mini.
Is it that ~5V is supplied to the Pro Mini, which has its own 3V3 regulator on-board?

Thanks,
Texy
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That is normal. Pin 3 of the cable supplies +5V even for the "+3.3V cable".

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OK. Thanks for the clarification, although the circuit diagram for the Pro Mini seems to suggest that VCC (from the FTDI cable) goes straight to VCC on the Pro Mini board :
http://arduino.cc/en/uploads/Main/Arduino-Pro-Mini-schematic.pdf

Perhaps that circuit only applies to the +5V version, not the 3V3 version ?
Certainly on offical specifications of the Pro Mini, VCC is stated as being +5 or 3.3V :
http://arduino.cc/en/Main/ArduinoBoardProMini
 smiley-confuse

Texy
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Searching finds the same question asked a while ago :
http://arduino.cc/forum/index.php/topic,22999.0.html

Still confused - I,d expect the external VCC from the FDTI to be wired to the 'raw' input, not VCC on the board.

Texy
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Does the FTDI have a solder jumper to cut or install to select 3.3V or 5V out?
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It doesn't matter if you power the mini with 3.3V or 5V on the Vcc pin (with said cable). The ATmega chip can deal with it.

It does matter if you have other devices connected to Vcc as well. If they are strictly 3.3V, you will blow them up. With that in mind, it would have been wise to route the Vcc pin through the regulator as well. Ultra low drop regulators would only take away maybe 0.1V or less at small loads.

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Does the FTDI have a solder jumper to cut or install to select 3.3V or 5V out?
No it's the usual sealed USB connector type.
Texy
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It does matter if you have other devices connected to Vcc as well. If they are strictly 3.3V, you will blow them up. With that in mind, it would have been wise to route the Vcc pin through the regulator as well. Ultra low drop regulators would only take away maybe 0.1V or less at small loads.


Exactly my issue - I chose the 3v3 type because the arduino needs to connect to a 3v3 device. I will need to be careful whilst I am using the USB serial link.
Thanks people.
Texy
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