Wednesday, 16 November 2022

Working around 74LVC165 supply problems for the MEGA65 Expansion Board

I mentioned in a previous blog post I mentioned that the expansion board was designed around using the 74LVC165 shift register for reading the IO lines. Unfortunately, they are a bit of Unobtainium right now, but the 74HC165 is available, and is 5V tolerant. 

However, if you run them on 3.3V VCC as I am doing to get 3.3V shift register output for input to the FPGA, then if the IO pins are at 5V, the voltage difference leads to current flowing continuously through.  

This can be solved by putting resistors in-line with the IO pins, to limit the current.  a 1K resistor will mean that with the 5V - 3.3V = 1.7V difference, that 1.7V / 1K = 1.7mA of current will flow -- well below the 50mA limit of the 74HC165s.

So the next step was to patch the boards to have these resistors in line.  This turned out to be a pain, so I instead just designed a little PCB that could be inserted into the 74LVC165 sockets, and have a socket on it for a 74HC165 and the resistors inline.  I made it a super simple 2-layer board, so that I could get it fabricated quickly by PCBWay. 

Those little boards arrived today, and I assembled three of them, and inserted them into the test expansion board I have been building:

So the next step will be to test this. But first, I want to finish working on the MK-II keyboard. I've got that mostly assembled now, and will do a blog post on my adventures and misadventures working on that.


  1. As I have _finally_ received my Meg65 I was wondering when these will be available?

    1. A good question :) As soon as I have them working fully, is probably the simple (but not especially helpful) answer. I'm currently working on the composite video output.