Hex display

Project Homepage: Building the 8 bit computer

Once I started putting data out on the bus, I thought it might be good to have a hexadecimal display of the 2 bit values.

It would seem that it's straightforward to show 4 bit binary values in HEX using 0..9 and A..F on a normal seven segment display, but it turns out it's not that straightforward.

The 74LS246 /247 /228 range of chips provide BCD to 7 segment display functionality datasheet, but that's not quite what I want, because it only outputs valid values for 0..9 (binary 1001). Once you get to binary 1010 or what would be A .. F, it puts out a strange set of symbols which I guess made sense to someone back int the 80s...

74LS246 output

Anyway, to save you some googling, I found links like this and this

Now, I could have just used some logic chips to fix up the A..F outputs, or used an EEPROM like Ben does in his videos, but I wanted to try something on my own, without the aid of a safety net...

So, looking on mouser, I found some of these HDSP-076x Series (datasheet)

They're a bit pricey, but I wanted the challenge to prove to myself that I could go "off piste" with buying components by understanding the datasheet myself, and that I could wire it in. It didn't look too hard, and it turns out it wasn't.

I've wired it into a seperate board so that I can plug it into anywhere on the computer watch a HEX representation of my 4 or 8 bits. In the image below it's wired into the bus (which is getting its output from the ALU incrementing by 1 each time).