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serial to 7-seg driver

Posted: Fri Nov 14, 2008 11:30 am
by mct75
Greetings,

In my various projects, I've always wanted to use 7-segment displays to show numbers. They're cheap, and easy to read at a glance. I've not been able to implement them, however, due to the number of pins it takes to drive them. For 4 decimal digits, It would take ~32 i/o pins, not to mention the processing time to derive which pins to drive. I've seen some bcd to 7-seg driver chips, but this would only cut the number of pins in half, and still require processing time. I know 'scanning' implementations exist to cut the pins further, but without more dedicated chips, this would load the microcontroller down significantly.

What I would love to see SFE carry, is a module that takes TTL serial bytes and displays the decimal equivalent. That's only one i/o pin to take care of that task. I've looked for a chip that offers this, and i've yet to find one, but I would have to imagine it exists.

If not, I'd love to see SFE develop one. It could even take the form of a "backpack" like they use for the 44780 lcd's, with a board to hold the 7-seg displays, and all the drive hardware on the back of the board. The only wires that would be needed are ground, power, and signal. My idea assumes only 4 digits, but in reality, any number could be used. Since there would always be numbers that could be sent, but are too large to display(10000-32767 for 4 digit setups), you could reserve certain large numbers for configuration options. For example, to toggle individual segments, set the number of display digits(if the end user wants less than what the unit is capable of), set baud rate, toggle separate general-purpose outputs, or to set brightness. I have a feeling that a product such as this, assuming it does not already exist, would fill a large niche of easy to implement, low cost displays.

Thanks for you time.

Posted: Fri Nov 14, 2008 11:48 am
by macegr
Here's an example of something similar, I2C though: http://cubloc.com/product/02_03.php

Lots more on the Korean site, some RS232/RS485 ones too: http://www.comfile.co.kr/product/06_05.htm http://www.comfile.co.kr/product/06_09.htm

Here's one from Make that uses USB, could be repurposed to RS232: http://www.makershed.com/ProductDetails ... Code=MKFL1

Here's one from Futurlec that uses the MAX7219, easy to use, they also have RS232 displays: http://www.futurlec.com/8x7_Segment.shtml

Posted: Fri Nov 14, 2008 12:01 pm
by mct75
macegr, Thanks for that list. While I would prefer something cheaper(individual 7-segs are just pennies apiece) i'd guess the r&d would add to the cost.

If SFE would carry some of those, I would likely pick a few up.

Posted: Fri Nov 14, 2008 3:02 pm
by FartingMonkey92
Driving 4 single digit, seven segment displays (including the decimal point)would only use a maximum of
12 I/O pins, or 8 I/O pins using an 8-bit shift register and 4 MOSFETs - would be simple to set up... :wink:

Or

You could drive 8 single digit, seven segment displays (including the decimal point) with just 5 I/O pins
using 2 8-bit shift registers...

(I think the more LEDs, the better... :wink: )

Posted: Sat Nov 15, 2008 5:55 pm
by lyndon
Those prices are pretty good. The people making them do have to eat, you know. And if Sparkfun were to resell them the price probably wouldn't be any less.
mct75 wrote:macegr, Thanks for that list. While I would prefer something cheaper(individual 7-segs are just pennies apiece) i'd guess the r&d would add to the cost.

If SFE would carry some of those, I would likely pick a few up.