Multiplexed 7-segment display voltage spikes

[MikeK]

I have two 4-digit common cathode 7-segment displays connected to an old Microchip PIC.  Since I couldn't measure LED current with a multimeter because of the multiplex speed, I hooked up the scope only to be surprised by these voltage spikes.  Is this typical for switching LEDs so fast?  Is there any concern?

1st image: between the 220 ohm resistor and the LED segment
2nd image: same, but zoomed in
3rd image: between the PIC pin and the 220 ohm resistor
4th image: same, but zoomed in

[uer166]

There's no spike: it looks like you enable the anode first, then after 1us you enable the cathode. During that time the pin is at 5V since the cathode isn't sinking current yet.

[MikeK]

Each update is:

1. Turn off all cathodes (set high).
2. Set new segment (anode).
3. Turn on the cathode for the new segment.

[MikeK]

Okay, I have to rethink and try other code.  That was a good (and fast) catch.

[MikeK]

Yeah, that was it.  Thanks.  I guess it's not a big deal either way, but I changed it to turn off the anode before setting the cathode.

[floobydust]

It's good you checked this because the overlap can add extra noise to the system power as well you get can visible ghosting or smearing between the digits.
 
It's because the segment and digit drivers take time to turn off. After doing that, you need to have a delay time of a few usec before turning them back on.
Switching speed between the seg/digit drivers varies depending on the circuit design but leave a little extra time.