Hi All,
I've posted this over in the Adafruit support forums on Saturday but not had any reply yet so thought I might ask here as well if that is ok....
I've just soldered up my quad-alphanumeric with backpack and strapped it to the breadboard for testing. I've also added a 5V DC input for the first time to the breadboard. To check the DC input was all good I hooked up a simple resistor/LED between DC 5V in and ground and all was good.
Powered off I put the backpack on the breadboard and connected everything up as in the guide. Vcc is connected to the +ve rail of the breadboard which is connected to +ve of the DC in. Vic is connected to the 3.3V out of the Teensy 3.2 I am using. Teensy GND is connected to the ground rail, as is DC -ve and GND from the backpack. The LED mentioned earlier is also still connected up.
With the 5V DC disconnected I connected the Teensy to USB and to my surprise the LED lit up weakly.
I checked connections and all was as it should be and there is no direct connection between Teensy and LED other than GND which is - I believe - as it should be.
A bit of lead disconnecting lead me to see that if I unplug the Vic connection then the LED goes out. I buzzed out the backpack to make sure I hadn't soldered the Vic and Vcc together and there is no continuity between them. However, when I plug USB into the Teensy (still with 5V DC disconnected) the LED lights and then there IS continuity between Vic and Vcc...
Doing it the other way and with DC in connected and USB disconnected there is no continuity between Vic and Vcc.
So, my question is...... is this as it should be? I really don't want to be putting 5V back through USB or the Teensy....
Ah ok - that makes sense; although having trouble working out how to fix it.... (not helped by most Google searches just bringing up articles on 48V phantom power)
I have just found that for the Teensy 3.2 there should be 4.2k resistors to 3.3V for both SDA and SCL so will get those added; would those be the fix for the phantom powering issue as well?
So the point is, all parts of the system must be powered by a common power supply. The 5 V powering the LED backpack must connect to the 5 V (I believe it is "Vin") of the Teensy. You power it either via the USB port or an alternative 5 V supply. If you disconnect power to one part, you must disconnect all other connections (except ground) to that part.
Since you have not given a link to the backpack, I do not know what all the respective connections are.
OK, gotcha - that makes sense. The final 'product' would have a single power supply but I hadn't got round to splitting the Vin from Vusb on the Teensy so thought I would try with two; but seems that is where the problem comes from which makes sense - I shall get that trace cut and power it all from the single 5V external supply.
Apologies, I was meant to link to it but clearly didn't. It is this one:
It has a Vin for the LED power and a Vic for the logic level. As these are different voltages for Teensy (5v to drive LEDs, 3.3V logic) I had in my head I could use two separate physical supplies whereas what it actually needs is a single power supply but still taking 3.3V from the Teensy output. This will then mean power is applied at the same time to both the Teensy and Backpack.