If you’re working (or playing) with digital electronics then you’ll probably want to display your data data on hex displays at some point. Normally this involves using some kind of decoder to turn your data into something that can drive a 7-segment display. Which means extra chips, extra wires and extra breadboard space.
I set off in search of an alternative. At first I looked for some kind of driver chip which could directly convert a 4-bit binary input into the needed outputs for a 7-segment display. These are available in decimal but a little searching showed there may be some ICs around which can do it but they’re all obscure, no longer manufactured and difficult to track down.
So that idea was a no go. But was there any type of display which could take a 4-bit input and display a hex output? After much searching and a fair few dead ends I managed to track down the Broadcom/Avago HDSP-0762 and it’s sisters. Still in production and widely available they run off a standard 5-volt TTL signal, take a 4-bit binary TTL input and have a couple of other features to boot. The only downside being a hefty price tag. But lets take a look at these displays and how they work.
How to Use
As you can see in the photo they are pleasingly different to the 7-segment LED displays we’re all used to, using an array of pixels to display each digit, with the result that the numbers and letters they display are a little more refined than we’re used to. The shapes of the numbers are a little more rounded and all of the letters are clearly uppercase with none of the ugly compromises you get from a 7-segment display.
The HDSP-0762 is the standard red hexadecimal version. There are also versions to display decimal and +/-1 indications, as well as high-intensity red, yellow and green.
Let’s look at the data sheet to see how to connect these up. The decimal and +/-1 versions behave slightly differently. For the hexadecimal version pins 7 and 6 are the 5 volt and ground connections respectively. Pins 1, 2, 3 and 8 are binary inputs. Input 1 (pin 8) is for the least significant bit, followed pins Input 2 (pin 1), 3 (pin 2) and 4 (pin 3). I’ll discuss pins 4 and 5 in a moment.
In my example circuit I’m using two of the displays, and you can see they nestle nicely next to each other. Both sets of binary inputs are connected to a DIP switch which either connects them to ground or leaves them floating. Obviously for a working circuit you’d want to drive them at 5V or use pull up resistors but I’m using the fact the inputs float high to save some wiring.
On the left hand display pins 4 and 5 are hard wired to ground. On the right hand display I’m using jumpers so you can see the behaviour of these pins.
You can’t see it in a photo but as I change the inputs the display updates to reflect the changes. These really are the prefect displays for showing binary numbers!
Blanking and Latching Features
Moving back to pins 4 and 5, pin 4 is called the ‘blanking control’ If it is low the display is illuminated, if high the display is off. (On the decimal versions of the display this pin controls the display of the decimal point).
Pin 5 is the ‘Latch enable’. The chip actually contains a memory, or latch, to store the input. Whilst the pin is connected low it stores the current input value into the latch, and updates the latch if the input value changes. When the pin is high the latch retains the stored value remains unchanged if the binary inputs change. This feature means we could, for instance, create an output to show the value on a bus by directly connecting a couple of these to the bus and simply pulsing the ‘latch enable’ input when we wanted to see the current bus value. Very simple.
One other thing to note is that the ‘latch enable’ and ‘blanking control’ operate entirely separately. If the blanking control is on and the latch enable is off you can change the input values but the display remembers the last latched value. With the display off (blanking control on) you can still update the value in the latch and only have it displayed when the blanking control returns to off. And even if the input value has changed the display remembers the value from when the ‘latch enable’ signal was active and displays this when the display is re-enabled rather than the current value on the input pins.
Pricing and Buying
I mentioned earlier that these displays are expensive. The HDSP-0762 model I’m using here costs over $30 for a single digit at the large electronics websites. I’ve seen prices on ebay which are more than double than, however I have tracked down an ebay seller (see below) selling two of then for about $30, and this is the seller from which I bought mine.
If purchasing bear in mind that there are a number of similar models so take care that you get the correct one.
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