Hexadecimal Display



Note 25/12/09: Stand by, ordered a UV toothbrush steriliser to be retrofitted into a eprom erasing box, let's have a crack at finally finishing this eh?
All original design specifications have been successfully reverse engineered and/or recovered from original documents, schematics and other diagrams are now posted.
The only thing missing now is the data file for the EPROM.

This device is capable of taking an 8 bit binary number and displaying the Hexadecimal equivalent on it's dual 7 segment display.
It does this by using a few clever techniques, which I will explain.

Firstly, I use an EPROM as a kind of programmable logic circuit, the EPROM is hardwired to read mode and the binary number is supplied to the address lines, which causes it to output the data stored at that address. The data lines are connected to the dual 7 segment display, but there are not enough outputs for both displays to be used at the exact same time, in fact there are half as many as needed.

To solve this problem I bolted a square wave generator onto the side and ran it through a divider and a bunch of flip-flops, which flips an address line on the EPROM and two transistors, on and off that switch each display on and off in turn. Basically I implemented solid state multiplexing. I even included small gaps in switching the Output Enable line on the EPROM to allow it to retrieve the data and output it before the display is switched on, this makes for a flicker free display and not having to use more than one EPROM.

The square wave generator is based on a 32Khz watch crystal and a CMOS inverter, hence the divider.

I'll dig up the circuit diagram and timing diagrams for the flip flops later.

The only problem I had with my design is that I used a bunch of J/K flipflops configured in such a way that sometimes the transistors wouldn't get enough current to fully switch on or off, I solved this by throwing in a chain of two inverters on both lines, which acts as a sort of buffer to pull enough current.

This project has been revived after 3 years of inactivity, it will not be much longer before I can show it operating as I had intended it to.
Oh hold again, stupid old parallel port eprom programmer is shit, saving money for a new "willem" usb one, need to pay my damn bills first.

Pictures and Schematics

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