RS232 over USB - Page 1
Forum section: electronics / beginners

author: rubenvann
author_rank: Newbie
timestamp: October 22, 2016, 05:24:00 pm
content: Hi there,I'm posting this in beginners because I have never interfaced with pheripherals over GPIO, and this is probably a dumb question.I want to set up communication between my Mimas v2 board and my laptop. I have a small FTDI board laying around, which is similar to this one:http://www.bitstoc.com/image/cache/data/Products/USB%20to%20UART%20Converter%20FTDI%20FT232%20Breakout%20Board%201%20a-500x500.JPGWhen I plug it in it shows up as a COM port and (I hope) I'll be able to use it from, for example, a C program, to read and write to.However, I'm a bit hesitant to connect the board to the GPIO pins on my Mimas board. My Mimas GPIO work on 3.3V, the board works on 5V. There is a jumper on the FTDI board which has the markings 3V3 and 5V. However, the voltage drop from the VCC to GND pins on the FTDI board is 5V, even when I set the jumper to 3.3V.Is this normal? My first impression was that I should connect the VCC on the FTDI board to the VCC on the GPIO board (false! This will cause a short circuit since the one is driven to 3.3V and the other to 5V). I thought the VCC and GND on the GPIO were inputs to distinguish high and low values, but appearently they are outputs. I'm not sure what to do with the VCC and GND with the FTDI board...Should I just set the jumper to 3.3V and connect the TX and RX pins (and ignore the VCC and GND of the FTDI board) or can this cause damage to the board?

author: JS
author_rank: Frequent Contributor
timestamp: October 22, 2016, 07:54:10 pm
content: I don't know the board, for sure isn't usb to 232, it seems to be usb to TTL. 3.3V is probably the logic level output, to be able to communicate directly to a 3.3V device. The VCC is probably directly the input USB supply which you could use if you find handy, arduinos for example have a regulator on board, so you could hook that 5V to Vin in the arduino board and the board would regulate down to 3.3V. If you don't have that regulator on the board better feed the board from somewhere else, a 3.3V PS preferably so you don't blow your µC.JS

author: Ian.M
author_rank: Super Contributor
timestamp: October 22, 2016, 10:25:56 pm
content: The 5V capable FTDI USB UART chips have a separate VCCIO pin that sets the logic '1' level for the I/O buffers of their serial port pins, so its quite common to find they have 5V on their VCC pin when configured for a 3.3V interface. Before hooking it up, scope its TXD output to confirm the logic levels or identify the VCCIO pin of the actual chip and probe it to confirm the jumper has actually set VCCIO to 3.3V, and isn't just for a regulated power output from the board.

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2016, 09:07:21 am
content: That doesn't tell you *ANYTHING* other than something is being fed 3.3V. However logic level serial with RS232 framing idles high, so check the voltage on the TXD pin of the header without sending any data. (i.e open a terminal program using the port to initialise it, but don't type anything) If its 3.3V with nothing else connected to it you know VCCIO must be 3.3V and you are good to go.

author: LukeW
author_rank: Frequent Contributor
timestamp: October 26, 2016, 12:46:20 am
content: Remember that the internal 3.3V regulator in the FTDI chip can only supply a small amount of current - so if the jumper is set to 3.3V then you can't draw any significant external current off the VCCIO header.(This is why many modules like this often provide 5V on the "Vcc out" pin even if they're set up for 3.3V IO levels, because they don't want to add an extra regulator.)

author: ebclr
author_rank: Super Contributor
timestamp: October 26, 2016, 12:53:56 am
content: Note that your ftdi board have a yellow jumper, this jumper set to operate at 3.3V or 5V chose the one that matches your desireAlso probably you have a fake Ftdi and need special instructions to install the driver