UART Serial RX problems

Projects General Guidance
1

Hello everyone,

I am having trouble sending a signal from my FPGA to my Arduino Uno.

I didn't have an FTDI cable on hand so I grabbed an Arduino instead to use as the middle man. The FPGA sends data to the Arduino Uno which the Arduino Uno then sends through USB to a terminal such as Putty or the Arduino serial terminal window.

I was having problems getting the Arduino to display the correct data so I just put in a test character, for example hex 66. The Arduino was not displaying the correct value at all, and occasionally the value would change.

I connected my oscilloscope to the FPGA and confirmed that the correct data is being sent, and it is not being sent backwards or anything either.

I wasn't exactly sure what UART/serial protocol the Arduino likes, (1 or 2 stop bits, parity, ect.) and I couldn't find a good answer from looking online.

Again here is my configuration:

1 Start bit (0) I assume the arduino is looking for a low start bit

8 Data bits

0 Parity bits

1 Stop bit (1) I assume the arduino wants a high stop bit

Baud rate: 115,200

I originally had the baud rate at 460,800 but that was causing the Arduino to get really bizarre results. I assumed it couldn't go that fast so I turned it down to 115,200 which the datasheet and forums said was okay, and now I am here. The data again is quite precise, (not randomly changing values or getting weird values) but the data is always wrong.

What could the problem be?

2

Are you connecting the FPGA TX to Arduino RX (pin 0)?

3

Yes.

4

That won't work. You will need a separate port for the FPGA. There are software serial libraries to use to set up a second serial port on a Uno. Software serial can not support the speed of the hardware serial port. An alternative is to use an Arduino with more than one hardware serial port (Mega).

5

What do you mean? I think I am just using the hardware serial port. The FPGA sends data to the arduino and the arduino sends that data to the screen. The Arduino doesn't communicate back to the FPGA.

6

Yeah I am using only the hardware serial port. I have no libraries in the project of any kind.

7

The PC (USB) TX connects to pin 0 (RX) on the Arduino through the USB to TTL converter chip. When you also connect the FPGA TX to Uno RX you have 2 outputs connected together. What happens if one output is low and the other is high?

8

The PC is not connected to any TX. The computer doesn't send anything to the Arduino.

9

Can you provide a schematic of your setup? Do you have a USB connection between the PC and Uno?

10

Its pretty simple, I just have the USB plugged into the Uno, and I have the FPGA TX connected to the Arduino RX.

11

So the output of the FPGA is connected to the output of the MCU (= input of the FTDI chip).
Leo..

12

Yeah that sounds like what I wanted when I was doing this! Now the question is if that is what is actually happening.

Is there anywhere I can look up the protocol arduino uses? Like the number of start and stop bits, parity, ect?

13

BPAArduino:
Yeah that sounds like what I wanted when I was doing this! Now the question is if that is what is actually happening.

Is there anywhere I can look up the protocol arduino uses? Like the number of start and stop bits, parity, ect?

Just pop out the 28pin Arduino chip,

Plug the FPGA-TX into pin 1 TX, for Transmit from FPGA to PC
Plug the FPGA-RX into pin 0 RX, to Send from PC to FPGA.
Connect a shared GND

Also, note, this contraption is INVERSE TTL of RS232 Levels. So inactive is HIGH (TTL) which corresponds to -12V RS232.

Configure your PC software to use the Serial port that Window's Identifies as ARDUINO UNO

Configure your PC software with the FPGA's Baud rate (the USB s/w with configure the Arduino board)

All done.

Chuck.

14

That is one of the most creative solutions yet! I hadn't thought of that at all.

Only one problem, the computer and/or any terminal software I use does recognize that the 'Arduino Uno' is plugged in but they fail to open a connection with it. What may be causing this?

15

BPAArduino:
That is one of the most creative solutions yet! I hadn't thought of that at all.

Only one problem, the computer and/or any terminal software I use does recognize that the 'Arduino Uno' is plugged in but they fail to open a connection with it. What may be causing this?

Turn off handshaking. Windows defaults to hardware.

Chuck.