Electronics > Beginners
Troubleshooting a not working level-shifter circuit
(1/1)
hitech95:
Hello,
as a title I created a small circuit to interface a SIM800 module with an Arduino. (See the one used on the datasheet, 2 bypass on both rails 100nF and a 10K pullup for OE)

Unfortunately communication does not work. I tried to replace the GSM module with an FTDI adapter and tried to read a serial output from the arduino. I tried to put a 50hz and 1khz square wave on side A and checked if on the opposite side I got something but i got nothing.

To note after the failed attempt with the 2.8V of the SIM800 IO I decide to do some test on a more standard voltage: 3.3V on side A and 5V on side B.

The pin has an intermediate value between the supply of the side from which I am analyzing and the other.

Thinking I had blown with the soldering temperature when I replaced the chip, but the second and third gave a similar problem.
In one case I obtained random characters as if there were a GND problem. (Not the case due to the power supply that share the same ground on both 5V and 3.3V rails)

I tried to apply a pullup resistor to one side to see if the other side pin was miming the behaviour. But that pin was stuck on "HI" even if on side B i was settingthe pin back to GND.

I used the same chip back in time when I was working on a serial connection between the RPI and a arduino. In that case i was having similar problems but those was due to some fake ics I got from ebay...  |O the one i'm using right now are from farnell/element14.

I don't have lots of equipment to test this out. (I don't have even have a proper scope)

If someone have an idea on what is going wrong I would appreciate that.
I'm still learning so it may be a dummy mistake!

Thanks,
hitech95

Datasheet:
http://www.ti.com/lit/ds/symlink/txb0108.pdf
hitech95:
I started wondering if there is a problem with the OE pin...
This IC is used in a lot of boards with configurations similar to the one i'm using....  |O
HB9EVI:
The OE is active low, so tie it to VSS
hitech95:
Section 8.3.4
"The TXB0108 has an OE input that is used to disablethe device by setting OE = low".

Actually by removing the pullup and putting VCCA directly on OE I have got a working state by toggling the pin manually.
But it is till having problems with signals. I get a odd voltage in between the supply and ground.
HB9EVI:
yep, sorry, I got it the wrong way. Watch out for the decoupling of the supply voltages, add to both VCCs 100n as close as possible to the supply pins
Navigation
Message Index
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod