Powering the XBee 3 in my PCB design

Question

Hello and good day,


I am almost finished with my pcb design which has an XBee 3 on it. I am planning on routing 3.3V to the Vcc pin (pin 1 on the XBee3 TH, pin 2 on XBee3 MMT). The documentation says I will need 8.2pF, 10uF and 0.1uF capacitor close to the Vcc of the XBee3. So, when I turn on my 3.3v supply with the XBee3 Vcc attached and ground connected, the XBee3 doesn't power on (like it normally does when powered via usb type-b to usb on computer usb port).
My setup is correct though, no?


thanks

Answer 1

Hello
Firstly, isn't the pin number 1 Vcc pin on XBee 3?
How do you observe the XBee 3 not powered on? Is it connected a serial port or something?
What type of capacitors did you use?
Lastly, can you provide the related part of the schematic?

Comments

Yes, it is pin 1 on the XBee3 TH, I edited my post for this correction.

I observe it not powered on because when I plug the XBee on the breakout board into my computer via the usb type b to usb type a on the computer, the xbee's breakoutboard's rx LED, tx LED and user LED each light up. But when I bring 3.3v from my 12v to 3.3v voltage regulator into pin 1 Vcc pin of the XBee3 on the breakout board, the LED's do not light up. Secondly I have micropython firmware uploaded onto the device and it runs on start-up (PS=1).. this firmware sends a message to the coordinator device.
When I power up the xbee via the computer usb (and see visual confirmation) the firmware runs and I see the message being handled in my python program...

However, when I attempt to turn on the XBee by sending 3.3v into Vcc, again the LEDs don't turn on, and I don't see the message handling at auto-start like I expect.

What type of capacitors?:
 near the Vcc pin I used a 10uf, .1uF, and 33nF (The hardware documentation says 8.2pF which should arrive in the mail soon but I don't have any right now, could that be the cause by itself?...)

I don't think this forum allows me to post pictures of the schematic or else I would do so.

https://www.allaboutcircuits.com/projects/build-9-linear-voltage-regulators-from-2.5v-to-15v-that-use-the-same-pcb/

I am using ^this^ exact 3.3v setup for the voltage regulator on my breadboard (takes 12V and converts to 3.3v). then I send that 3.3v directly into the Vcc pin of the XBee3 (adding in the 10uF, .1uF, and 33nF close to the Vcc pin - each going from this 3.3v/Vcc node to ground).

So, hopefully its just the 8.2pF that I replaced with the 33nF that is the issue hopefully, seeing how that is 33nF is 4000 times higher than their recommended 8.2pF lol

---

ok, so I found a 7.5 pF capacitor. I replaced the 33nF with this 7.5 Cap. So here is where I am at:

if I turn on the XB with the usb to computer cord, it turns on and all is good. then if I power up the 3.3v voltage regulator and send this 3.3v into Vcc and then unplug the USB... the XBee stays on and all is good!

so the issue lies with the fact that the XB won't turn on with my 3.3V vreg supply into Vcc.. but it will keep it on after I power up the xbee with the help of the computer supply.

---

further detail, when I run 3.3v to Vcc as the standalone power supply.. the xb's rx and tx LEDs do turn on after about 30 seconds.. but I don't receive a message so the firmware auto-start (PS=1) doesn't get triggered...

---

Can you try it with no capacitor and with only 1 capacitor (0.1 uF)? I believe the 3.3 V regulator output or capacitors cause a ramp in voltage (current) that extend the rise time (from 0 V to 3.3 V).