treii28
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2v8 logic?

Tue Apr 25, 2017 2:28 am

I have one of these: https://www.sainsmart.com/sim900-gprs-g ... y-new.html and I'm trying to figure out if the '2v8 logic' is something that can be connected directly to gpio pins or if it's going to damage my pi and/or not be readable logic wise? If I need an interface between, what do I need?

SW

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FTrevorGowen
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Re: 2v8 logic?

Tue Apr 25, 2017 2:53 pm

Having followed your link and given that the module appears to use a "serial link" (ie.TxD, RxD) the reference to "2V8 logic" may be a "red herring". You probably need to determine what the voltage levels are on those pins - they could be 2V8, 3V3, or, most likely (given also that the module is powered by 5V) 5V which is not directly compatible with the Pi's (3V3) serial GPIO's.
Trev.
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corporatejoe
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Re: 2v8 logic?

Fri Jun 29, 2018 3:46 am

treii28 wrote:
Tue Apr 25, 2017 2:28 am
I have one of these: https://www.sainsmart.com/sim900-gprs-g ... y-new.html and I'm trying to figure out if the '2v8 logic' is something that can be connected directly to gpio pins or if it's going to damage my pi and/or not be readable logic wise? If I need an interface between, what do I need?

SW
Have you gotten any further with this project? I have a sim900 module that was dirt cheap when RadioShack was closing it's doors. http://wiki.seeedstudio.com/GPRS_Shield_v1.0/ It's been sitting for years and I'm just now looking at putting it to use with a PI zero. I'm assuming that the 3.3 logic from the PI will work fine with the 2.8 logic on the Sim900?

Mine has a slightly different board layout and it looks like I need to solder to access the 2.8 UART.

Joe-

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Re: 2v8 logic?

Fri Jun 29, 2018 9:57 am

It says that's for Arduino. Arduino uses 5V logic. I don't know what the 2.8 refers to, but you will need to determine exactly what the interface is doing before you can work out how to adapt it.
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smelly_mongoose
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Re: 2v8 logic?

Mon Dec 17, 2018 10:02 pm

As far as I understand, these types of breakout boards have two voltages. It is common that the microchip would operate at a lower voltage than the voltage of the breakoutboard, there are other integrated cirsuits (ICs) on that board and some of them might be using 5V. That is what there might be two voltages.

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mahjongg
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Re: 2v8 logic?

Mon Dec 17, 2018 11:22 pm

It says that it is "arduino compatible", and that means it is using TTL (5V) levels, meaning you have to resistor divide the 5V serial output to a level between 2 and 3.3V, so I would use a resistor divider with a (top) 2K2, and a (bottom) 3K3 resistor to divide the 5V to 3V.
the 3V3 from a GPIO output should generally be enough for the 5V compatible input, but to be sure you will need a datasheet, and see if ViH is above 3V or so. Otherwise you will need to add a level shifter, but in general that is not needed.

so with just the two resistors it should work.

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