EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: hitech95 on July 18, 2018, 01:53:51 pm
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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 (http://www.ti.com/lit/ds/symlink/txb0108.pdf)
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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
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The OE is active low, so tie it to VSS
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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.
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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