XB200 GPIO ports LED output

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XB200 GPIO ports LED output

Post by schrambo » Tue Jan 05, 2016 3:11 am


I'm finally getting my act together and putting my bladeRF + XB200 transverter board and all cabling inside a project box.

Amongst having the TX, RX, ADC and DAC hooked up to external bulkhead SMA connectors to the front of my enclosue, if it was possible to rig up some LEDs for power, TX and RX activity and if any other sort of status activity that I'm not aware of.

(DISCLAIMER: I'm a basic bottom feeder noob when it comes to electronics: be gentle please ;))

I'm looking at page 7 of the XB200 schematics and I notice on J13 there is VCCIO_R on pin 5 and ground on 3 and 4.
I assume if I can determine the voltage between this, and using the appropriate resistor, this can be my power LED indicator. Correct?

Discussing thing with a work colleague who knows decades of electronics of knowledge more than I, he says that for anything else if I want TX and RX LED inidcations that things will then need to be programmed using the other GPIO pins. If this is so, I can only hope someone has already started work on this.

Has anyone worked on this kind of thing before?

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Re: XB200 GPIO ports LED output

Post by bglod » Tue Jan 12, 2016 11:52 am

Hi schrambo,

LEDs can be tricky because there are a number of factors at play, and they all kind of move around on you. This is a pretty decent write-up about the process of choosing LEDs and current limiting resistors, if you haven't already seen it. Note that they just use the typical values in the datasheet, which may make for some extremely bright LEDs depending on the chosen LED. If that turns out to be the case, increase the current limiting resistor to suit your needs. You can also use the relative luminous intensity vs. forward current charts in the LED datasheet to ballpark what resistor you need. Remember to account for the LED forward voltage drop (it's a function of the forward current) when choosing the value of the resistor.

The power LED is going to be the simplest because there is no control logic required. Your assumption is correct. Connecting VCCIO_R + resistor + LED will give you a power indicator. Note that VCCIO_R is only 1.8 V, so you will need to choose an LED with a forward voltage less than 1.8 V -- generally red LEDs only, and brightness may suffer. You'll have to tap into the 3.3 V or 5 V supply if you'd like to use a green (Vf ~2V) or blue (Vf ~3.7V) LED instead.

The TX/RX traffic indicators will be a little more involved. You have two options. If you stick with red LEDs, you can drive the LED directly from the GPIO in a similar fashion as the power LED -- the GPIO would be the power source. For other color LEDs, you'd need to add a transistor to the circuit. I've attached a quick schematic I drew up in LTSpice (if you don't have it, I recommend it to test circuits before building them). Q1 is a common 2N3904 NPN transistor. The value of R1 is chosen in a similar fashion as your power LED. The value of R2 isn't too critical, but it is required for current-limiting of the GPIO pin to prevent damage -- a value of 1k should work well here.

As far as I know, no work has been done with respect to a TX/RX packet indication. However, jynik may be able to offer support from the software side of things.
Electrical Engineer
Nuand, LLC.

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Re: XB200 GPIO ports LED output

Post by jynik » Tue Jan 12, 2016 12:17 pm

On the software side of things...
Of course, the above are from the host side -- you may want to take over control of these in the FPGA, depending on your application.

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