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xbee sx pro and intel edison: coupling issues?

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I am trying to couple an intel edison board with a sparkfun UART shield to the xbee sx pro

the UART shield includes a level shifter TXB0104. Using a scope I succesfully manage to generate an UART signal and sending continuosly a byte at 9600baud.
with the scope I measured: vmax=3.687v, vmin=0.2v

now, when I connect the din to the Tx of the uart, the scope measures vmax=2.75v, vmin=1.25v

this causes a garbage being sent across and the data is not properly tranmitted.

I tried to disable/enable the pullup resistors without success.

any advice/better way to connect the xbee?
-dan
asked Aug 25, 2016 in XBee Programmable Development by dan3901 New to the Community (1 point)

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1 Answer

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The board you are using was never designed to connect the New 1W XBee PRO SX module to it. The SX may simply draw too much current for that board to support it.

The SX wants 1A at 3.3VDC. Its VMAX is also 3.60V. You are out of spec at 3.68VDC.
answered Aug 25, 2016 by mvut Veteran of the Digi Community (11,298 points)
Wants 1A where?? In the DIN? No way!
The power to the sx is sourced by the digi development board where the sx is soldered.
The TX of the 3.3v shifted UART of the Edison connected to the DIN of the SX

What is the  current [mA] on the DIN pin and the xbee internal pullup values ?
That is 1 Amp of current on the VCC line not DI.

How you have this mounted is good to know.

What voltage level does the Edison board run at? is it 3V?

What test pins are you connecting too for your DI and DO?
SX:  the edison is powered by the vcc/gnd sourced from the header pin on the digi devboard.
The tx of the edison is levelshifted with the TXB0104 to 3.3v, and connected to the DIN of the sx pro
No other DIOs are used at this stage.

Would you advice to add an extra resistor to limit the pullup effect and reduce the drop on the dynamic range of the signal when connecting to the sx?
Or i should add an Nmosfet shifter?
What are the exact test points you are pulling VCC from to power your Edison board? Most of test points on the SX Development board are 3V connections. In which case you would not use a level shifter at all as your device would not have a 5V output  but a 3V output. That is unless the board has an opamp circuit for the VCC to force it to 5V.
the Edison I use is mounted on a breakout board that includes a level shifter 1.8->3v (TXB0104)
The HV Vcc of the shifter is connected to the Vcc of the XBEE  sx dev board

Now, it looks like the XBEE's DIN cannot be driven by the signal being sent by the TBX0104, which doesnt have enough drive strength

Given we cannot hack the boards,  could you suggest a coupling circuitry to be able to drive the xbee DIN?

many thanks
dan
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