XBee's and Logic Level Conversion

Wireless & IoT
1

Hello,

I have a pair XBee modules and BASIC Stamps that I need to conect for

a project I’m working on, but I must first address the logic level

issue; that is, the BASIC Stamp’s logic level of five volts is, naturally,

incompatible with the XBee’s three.

The SelmaWare AppBee-Mod uses the 74LVC244A “logic buffer” for logic level

conversion, but I can’t find a detailed schematic; just pictures. The

SparkFun logic level converter (http://www.sparkfun.com/commerce/produc … ts_id=8745),

however, uses only 10k resistors and the BSS138 "N-Channel Logic Level

Enhancement Mode Field Effect Transistor", a device that I am unfamiliar

with and was only able to ascertain the name from the datasheet.

The schematic leaves me a tad confused, however; the part description

refers to the “RXI”, “RXO”, “TXI”, and “TXO” pins – which seems simple

enough – but the schematic shows instead “_LV” and _HV" pins.

If someone would be kind enough to indicate which pins of the logic level

converter need be connected where, in terms of the names seen on the

schematic, I should be able to complete my circuit design.

2

velociostrich,

The last image for this board gives the most clear diagram for hookup. I was a bit lost, too, until finding it. The board does require power on the HV and LV pins, +5V and +3V in your case, and don’t forget the grounds.

Jon

3

Thanks, I think I get it now; the last two pictures on the product description page make it make sense.

4

Scratch, that–I understand what pins go to what on the schematic, but what it’s doing has me confused. It looks as though the RX side just uses a voltage divider to step down the voltage and uses the BSS138 to step up (?) the voltage for the TX side, but, if anything, I thought that I don’t have to “step up” the voltage for the XBee’s DOUT pin, but just have to cut down the BASIC Stamp’s output…?

So do I connect the BASIC Stamp to the TX half then the DIN pin for transmitting and to the RX half then to the DOUT pin for receiving or what?

I apologize in advance if I’m not being clear.

5

You’re correct about the voltage divider. It reduces the Stamp’s Dout voltage to protect the XBee’s Din circuit.

The other circuit basically switches HV (5 volts) using a LV (3 volt) signal. It isn’t necessary if the Stamp recognizes 3 volts as a high. Some 5 volt logic won’t have a problem, some will. It won’t hurt anything to try a direct connection from the XBee Dout to the BASIC Stamp Din. If it works, you’re good. If not, use the “step up” side of the translator.

6

Okay, thanks for the info.