Electronic Circuit Help Understanding PCB Schematic?

mreiner@beehaw.org to Technology@beehaw.org – 4 points –

Hello!

I've ventured far too deep into the custom ergonomic columnar-staggered mechanical keyboard rabbit-hole, and I think I've finally found myself at my endgame (for now, anyway lol): the Hillside 46.

TL;DR:

Please see "questions" section at the bottom regarding why, how, and if I should use the ESD protection on the right-half, left-half, or both halves of this split ergo-mech keyboard build.

Background:

In constructing this board, I came across a part of the circuit design that has confused my non-expert brain: the ESD chip and decoupling capacitors. At a theory level, I understand that it protects the board from electrostatic discharge (presumably, specifically, the microcontroller) and the damage it can cause. What is weird to me is that this is the only keyboard out of the several split-mech-ergo boards I've built that have featured this protection circuit, and even within the Hillside family of keyboards, the version with 46 keys that I built seems to be the only one with this protection circuit which makes the decision even more perplexing to me.

Given that this is a split-keyboard design with a reversible PCB, there are footprints for the SRV05-4 ESD chip (datasheet here) and decoupling capacitors on both sides of the PCB though they appear to be wired up differently depending on the side of the board you're using (schematic here). On the "top" of the PCB (left side of the keyboard), you would solder the ESD chip with pin 1 at the top-left position. I did this and everything works fine.

On the "bottom" of the PCB (right half of the keyboard), the connections to the pads seem to be mirrored from the "top" of the PCB, but it looks like that was done so in a way that would not allow me to invert the ESD chip, with pin 1 at the bottom-right of the footprint, and still have everything work. I definitely can't keep pin one at the top-left of the footprint on the "bottom" of the PCB, so I'm kind of stuck as to what to do.

Questions:

  1. Are the ESD chip and decoupling capacitors necessary or just nice to have?
  2. If they are necessary or very useful, do I really need them on both halves of the board?
  3. Looking at the Gerber file, it seems like I might be able to mount the ESD chip to the footprint on the underside of the right-side PCB and still have it functional; is that correct?
  4. What is this ESD circuit protecting against, exactly? I assume it's potential voltage spikes on lines that shouldn't have them that can occur if I were to unplug one end of the audio cable while the keyboard was still plugged into power/USB; is that correct?

Thanks in advance!

15

You are viewing a single comment
View all comments

With regards to the ESD chip:

The ESD chip is going to try to prevent transient overvoltages from persisting on the protected line (which is going to be a voltage-sensitive circuit). Since (if I understand the application and schematic correctly) this circuit has a microcontroller with a data pin directly exposed to an external connector, some form of ESD protection is going to be highly recommended, as modern microcontrollers are going to be extremely intolerant of surprisingly low overvoltages.

This could happen if, as you suggest, one end of the audio cable is unplugged and a transient voltage is applied (by touching the exposed connector end), while the other end is plugged in to an instance of the board. This applies whether or not the affected board is energized (and it looks like this ESD chip will function passively, as it claims to be effectively a pile of diodes).

You do in fact need protection on both halves of the board, since they can exist as physically separate devices (e.g. when stowed for transport).

The particular ESD chip you're using has VCC on pin 2 5, GND on pin 5 2, and four interchangeable-looking functional pins on 1 / 4 / 5 3 / 6. As long as you orient the VCC and GND pins correctly, you should end up with a valid circuit.

With regards to decoupling capacitors:

The main purpose of these is to filter out high-frequency noise on the voltage rails. These do serve a factor in ESD protection (which involves huge instantaneous voltages but not all that much charge), but also guard against other transient voltage variances that might affect operation or reliability of the circuit.

Because the decoupling capacitors you're looking at are part of the ESD solution (with regards specifically to voltage rails), I wouldn't suggest omitting them.

Thanks, that all makes a lot of sense.

It looks like pad 5/VCC is on the middle-left, pad 2/GND is on the middle-right, and pad 6/data is on the upper-left of the footprint when I open the hillside46.kicad_pcb file in the KiCad PCB Editor, click on 'View', and check "Flip Board View".

As a sanity check, given the info above: it looks like I could rotate that ESD chip 180 degrees (so that the ESD chip's pin 1 is on the lower-right pad of the footprint) and have everything work, correct?