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I am adapting a design using a PIC32MX (which is actually working) to put an extra add-on. The add-on is a fingerprint reader which communicates via UART. Regarding that I have the USB pins exposed in my design, I found a way to connect a ftdi 3.3V (https://www.sparkfun.com/products/9716) between UART and USB. I am planning to connect my VCC ( 3.3V in my design , which I believe it can give me more than 200 mA) to the VUSB red wire cable. The other pins are straight forward.

So my questions from now are:

  • The purpose of the VBUS pin on the PIC32mx is to recognize if there's any device plugged or not, right? Is there any problem if I leave it not connected?

  • Do I need to connect the VBUSON pin to somewhere? I am already using for something else (the pin is shared/not dedicated).

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  • \$\begingroup\$ Note: your ftdi 3.3V is pointing to "SparkFun FTDI Basic Breakout - 5V" \$\endgroup\$ – user1586 Dec 28 '15 at 19:41
  • \$\begingroup\$ This Sparkfun ftdi operates either at 3.3v or 5v. All you need to do is solder a small jumper between two pads and cut the default connection (if 3.3V wanted) \$\endgroup\$ – scuba Dec 28 '15 at 23:07
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Q1: No. The VBUS pin is used when your PIC32MX is itself acting as a device and it tells the PIC32's USB module when it has been connected to a USB host (like a PC) by detecting the nominal 5V which should be present.

Q2: As the manual tells us, the VBUSON pin is "USB Host and OTG bus power control output". In other words, this pin turns the Vbus 5V output from your board on/off when your PIC32 is acting as a USB host.

To address a question which you haven't asked ... your PIC32 must be operating as a USB host in order for your FTDI USB UART to be of any use. This means that it must be configured correctly and running a USB host firmware stack.

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  • \$\begingroup\$ It tells on the datasheet, the VBUS pin is an input so How can it act as 5V output? The best way to supply the enough power to my fingerprint reader is to give 3.3V direct from the regulator, right? or should I place a MOSFET from the VBUSON pin? Thanks. \$\endgroup\$ – scuba Dec 28 '15 at 18:59
  • \$\begingroup\$ Yes, the VBUS pin on the PIC is an input, but when your board is acting as a USB host it must supply 5V out on the USB connector's Vbus pin. Your FTDI USB UART is no doubt expecting to see 5V on its USB connector coming from your PIC and I doubt that 3.3V will make it happy. You'll need a 3.3->5V boost converter if you don't already have 5V available and the VBUSON pin from the PIC can turn this on/off if necessary. Having your PIC32 operate as a host is not quite as 'simple' as having it operate as a device. \$\endgroup\$ – brhans Dec 28 '15 at 19:23
  • \$\begingroup\$ IIRC, FTDI's most recent parts can operate truly at only 3.3V -- I don't think you need 5V at all, if the right SKU has been chosen. I think their internal VBUS comparator operates expecting a divided value, so you don't need to supply 5V, but this comment is all from memory. \$\endgroup\$ – Krunal Desai Dec 28 '15 at 19:53
  • \$\begingroup\$ What you guys recomend for a faster implementation? Does the VBUS pin supplies enough power for my fingerprint? It's mentioned it needs about 130ma to operate. \$\endgroup\$ – scuba Dec 28 '15 at 23:02
  • \$\begingroup\$ Find a way to get at some UART pins on your PIC ... \$\endgroup\$ – brhans Dec 28 '15 at 23:06

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