Go Down

Topic: FT232RL to Atmega UART - is this correct? (Read 4 times) previous topic - next topic

Graynomad

Yep.

Also be aware that your Tx pin may actually power up the FTDI chip depending on how it is constructed. IE if the FT232 has ESD protection diodes a logic high on the FT chip's Rx can try to power up the chip.

I don't know if this is the case with this chip, but I've just designed it into something in a similar way to what you are doing and I've added some circuitry to stop that.

The other thing to do is make sure your processor's Tx pin is tristate when the USB is not plugged in, if you can know that.

_______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

jtw11

Quote
Also be aware that your Tx pin may actually power up the FTDI chip depending on how it is constructed.


That's not a problem - I'm not sleeping the '232 - just leaving it powered up the whole time.

Quote
The other thing to do is make sure your processor's Tx pin is tristate when the USB is not plugged in, if you can know that.


Why is this? Do both RX & TX go high when USB is unplugged? I know previously I've seen 1k series resistors on the RX and TX lines, but I believe the 232RL has internal series resistors?

Graynomad

Quote
both RX & TX go high when USB is unplugged?

It's the TX of the processors that is a problem. It will happily power the 232 IF it has ESD diodes (I don't know if it does or not). On the Arduinos I assume the 1k resistors pretty much solves the problem.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

CrossRoads

I always thought the 1Ks were to limit the FT232 to 5mA of drive current and to let the Arduino or external devices overdrive that if the pins were used for other purposes.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Graynomad

Yes I think that is the case, I suppose a nice side affect may be that when the USB is not plugged in it helps with the above issue, if indeed it is an issue with this particular chip, I know it is with most.

_____
Rob
Rob Gray aka the GRAYnomad www.robgray.com

Go Up