Saturday, 29 October 2022

MEGA65 MK-II Keyboard PCB Design

Chipaggedon is still raging. The little FPGA we use in the MEGA65's keyboard has a lead time of 90 weeks. This shows just how crazy things have gotten. But we don't want to wait 90 weeks before we can deliver a 3rd batch of MEGA65s (the keyboards are already there for the 2nd batch, don't worry!).

So today I set about designing a new keyboard PCB for the MEGA65, that doesn't use an FPGA or any other components that are impossible to source at the moment.

To add to the challenge, I want the new keyboard to be able to work on existing MEGA65s, and vice-versa, so that there is minimal drama in making the switch.

Finally, I want it to be easy to build yourself, so that folks with a Nexys/Artix FPGA board can one day make a MEGA65 keyboard to connect to those boards.

My weapon of choice for PCB design is Kicad, because its open-source and really good. I've been learning to use it lately on a couple of projects that will be revealed in the fullness of time. But for now, we'll focus on the keyboard PCB.

To avoid the FPGA, and stick to the 3 wires available for communicating with the MEGA65, I have picked an I2C-based IO expander chip. These chips each have 16 IO lines, with internal pull-ups, so I don't need any resistors for that on the board. They also have 3 address straps, so that I can have up to 2^3 = 8 of them.  As it turns out, I need only 6: 1 for the RGB LEDs and lock key LEDs, and 5 for the 78 key switches.

These chips cost only about AU$3 each in small quantities, so we are talking AU$18 or less for them all -- which is about what the little FPGA costs in similar quantities. I expect in modest batch quantities these I2C IO expanders will be AU$2 or less each.  

One of the cost savings that using 6 of these chips is that we don't need to use key switches with internal diodes, or to supply diodes on the board. This also makes the design much less work to build yourself. There is about a AU$0.20 saving per key switch by getting the diode-free key switches, so over 80 keys, we are talking about AU$16 in savings -- i.e., the IO expanders quite literally just about pay for themselves.

A plus side of this non-traditional approach, instead of using a matrix with diodes, is that there is no limit to the number of keys that can be held down at once, as each of them has a separate wire.  The I2C protocol at 400KHz should allow sampling the keys and updating the LEDs at >1KHz, i.e, less than 1ms delay, which should be fine.

The code is all at https://github.com/mega65/mega65-kbd-pcb, if anyone would like to take a look.  Here are renders of the board from front and back:

I've tried to provide helpful information for assembly and sourcing on the board, make it as easy for people to build up as possible.

Next step for me is to get some PCBs made up, and try to assemble one.  This will reveal if I have any placement errors switches or mounting holes. I've very carefully measured it all, so hopefully it will be fine.

On that, because one of the goals of this is to make it easier for people to build their own low-cost MEGA65, I'll try using some Cherry MX clones, instead of real Cherry MX key switches. A set of those can be had for about AU$40, including postage.  So add AU$18 for the chips, and probably AU$10 for the PCBs in batches of 100, and about AU$10 for the remaining parts, and we have a keyboard that can be built in small quantities for AU$68, excluding the key caps.

But that's it for today, while I wait for the PCBs to arrive.


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