Unicorn HAT HD working but don't forget to use GROUND!


#1

Hi all, just got the Unicorn HAT HD and wow the lights! After confirming the display worked on my RaspPi 2 I decided to try it detached from the board via some jumper wires. The Unicorn HAT HD pinout (pinout.xyz/pinout/unicorn_hat_hd) says it only needs 4 GPIO pins plus 5v and 3v3. So I carefully followed this and plugged in the wires as laid out. But wait, all I got was a line of red leds on the second row from the top.

After a bit of looking around I saw an old post here about the original Unicorn saying it wouldn’t work until the GND was plugged in, so I did that, turned on the Pi and ran the rainbow.py test and all burst into life!

Am I missing something or should it say that you need to use GND in the pinout diagram, anyway, I have posted this in the hope that it helps other hearties!

In this picture the colours are as follows:
Orange = 3v3
Green =5v
Brown = Pin 6 GND (the missing link!)
Blue = BCM 10 (MOSI)
Grey = BCM 9 (MISO)
Yellow = BCM 11 (SCLK)
Purple = BCM 8 (Chip Select)


#2

It’s pretty much a given that a HAT must use at least one ground connection, otherwise there’s no circuit (excepting any leakage current that might go through the IO pins).

Pinout.xyz communicates which ground pins to use by showing them as small, dark-blue pips down the GPIO header. Quite often we test (or use the schematics and board layouts to determine) exactly which grounds you can get away with using. The ground plane is usually interconnected on our boards, so sometimes as few as one works- other times the board might be too densely routed and need more.

I appreciate the little dark blue pips are not crystal clear, but once you know what they are, you’ll be set. We shied away from doing anything more obvious, since it makes the page look too cluttered.


#3

Hi, ah that makes sense now, thanks for the clarification. I can go forward reading the pinout correctly now. I couldn’t resist showing the final result, a bear with a scrollable temperature readout from a waterproof probe!