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Author Topic: Strange effect of a level shifter between Atmega (3.3v) <=> SIM900 (2.8v)  (Read 1407 times)
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Hello,

I'am using a level shifter IC to convert 3.3v TX to 2.8RX of a SIM900 (GSM module) It works great.
But, I also want to use it on the PWR_PIN that switch on the SIM900. (because It have unused pins)

The SIM900 Datasheet said :


I have wired the level sifter like this :

TP1 is on the GSM. TP2 is on the ATMEGA.

The level sifter is http://www.ti.com/product/sn74avc4t245?qgpn=sn74avc4t245

Why if I pull TP2 to low, then TP1 dont goes low.
But, if I take a wire and set TP1 to GND, the SIM900 switch on, and then if I pull TP2 to low from the ATMEGA it shutdown properly the SIM900 (so the level shifter works like it should). Can you help me to undersand why ?

When I put TP1 to GND with a wire, the power current increase to 0.1 A (idle 0.05 A), why ?


Big thanks to you

J
« Last Edit: March 11, 2013, 12:56:24 pm by jujun » Logged

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Yes tp2 connects to the output of a ''245, If it is high you are potentially damaging the '245 by shorting tp2 to ground. If you NEED to pull PT2 down add a series resistor between the gate and TP2 470R should be within device limits

Bob
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I have made a mistake, it was TP1 not TP2 in this : When I put TP2 to GND with a wire, the power current increase to 0.1 A (idle 0.05 A), why ?

So I corrected my post.

Yes tp2 connects to the output of a ''245, If it is high you are potentially damaging the '245 by shorting tp2 to ground. If you NEED to pull PT2 down add a series resistor between the gate and TP2 470R should be within device limits

Bob
I think that TP2 is wired on the INPUT side of the level sifter, so It should be OK.
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what you fail to realize is that the '245 is a tranceiver... What is the state of the 2V8 OE?
Data sheet attached. The only way that you can draw that kind of current is when you have the wrong OE enabled.
The additional 50 mA increase in current when you pull down what you think is an input... is the sign that you aren't properly managing the data transceiver.

Bob

* 74HC_HCT245.pdf (50.29 KB - downloaded 11 times.)
« Last Edit: March 11, 2013, 03:07:47 pm by Docedison » Logged

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Good ! I understand what you said about the power increase. By the way it show that the port A is an output.

It seems that you are not taling about the same chip.

The one I use have reversed OE pin. ( /OE )



I have all /OE and DIR to GND, so it good for B to A for the two bus I think.

But I still cant understand why the serial link 1A2 1B2 works like a charm, and on the other hand, the simple power on on 2A2 2B2 only works when the SIM900 is already on and is going to shutdown.

Maybe it is just about the 2.8VVD_EXT from the SIM900 module that is not realy good when the SIM900 is off. But I still see 2.9 V on it.
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