Level shifting with a TXB0101 between Arduino

Has anyone ever used the TXB0101 level shifting IC's to interface between an Arduino and Telit GE865? I serially connecting through a TXB0101 (single channel bidirectional logic level shifter) and a TXB0106 (six channel) and for some reason the communication from the Telit (2.8v) to the Arduino (5v) is extremely noisy...to the point that the data is garbled. The Arduino(5v) Tx to Telit (2.8v) is very clean but as soon as I connect the other signal it all goes to hell. I was originally using the Sparkfun logic level breakout pcbs which worked well but I wanted to try the other IC's because they have an OE that I would like have controll over to HiZ the serial port. Should I be using pull up resistors maybe? Any help would be appreciated.

Wayne

Have you got any decoupling on the chip. You should have a 0.1uF across both the supplies, that is VccA and VccB. http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Yup, I have them across both supplies. It's strange though that the signal is only noisy from the Telit to the Arduino and not the other way around...

Have you got OE wired high?

Yup. If I use the sparkfun logic level converter for Rx and Tx it works beautifully. If I take the Tx (referencing Arduino) and pipe it through the TXB (tried multiple chips) and the Rx on the sparkfun converter it works beautifully (all signals look perfectly clean on the scope). When I put the Tx back on the sparkfun board and move the Rx to the TXB the signal is extremely noisy (to the point it won’t read it) but the Tx signal still looks clean. I’m stumped…

Try a pull up on the noisy input side about 2K2.

Thanks Mike...I have a question about this for you (sorry my basic electronics is very rusty)...Is the pull resistor going to increase the current to the TXB? I just finished reading over the TXB and Telit data sheets and it looks like the Telit 2.8 volt logic only supplies about 1 mA and the TXB needs 2 mA...what do you think? Thanks again for all the help.

Wayne

it looks like the Telit 2.8 volt logic only supplies about 1 mA

yes "supplies" is what it outputs, this is a pull up on the input side I am suggesting.

You just threw me…I’ve found that pictures make everything so much easier…

Are you saying point A or point B? (keep in mind the noise is at both points). I promise this is the last question I’ll ask…thanks again…

Ard 5v TXB Telit 2.8V



<----- <------ *



(A) (B)

Point B up to the 2.8V.

the noise is at both points

yes it will be, because what ever is on the input will be on the output, it's a buffer.