36 seven segment display with pic.. need advise..

[dani]

Hello all,

I want to do make a pannel which has 7-segment displays on it. these displays display different values of weight and its units etc. Altogeather they are total 36 digits which i need to incoprate. I want to use PIC 16f877a. What type of muliplexing hardware and software technice i shoule follow .. i need advise for this from professionals..

Regards
Dani

[The Puma]

Look for the max7221
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339

It used only 3 wires to the uC
and your can cascades serveral ic

[John P]

It's a nice chip but expensive--actually Digi-Key doesn't seem to have the MAX 7221, but they have the similar MAX7219 and it's over $10 each in small quantities. The poor guy needs 5 of them! I don't know what the budget for the project is. Then again, I can't think of a better way to do it. Any kind of build-it-yourself replacement gets into a lot of hardware.

Philips has a part, the saa1064, which Digi-Key lists at less than $4, but it only drives 4 digits, so you'd need 9 of the things. That's a lot of space and wiring.

I dunno. With that many digits, I think you should consider an LCD panel.

[kender]

[fuzzy]

Sorry but from datasheet i can't understand in i can drive led up to 12V with SAA1064. should i use the same voltage to drive segments and for power input???

thanks

[Kenny]

Better to power the chip and the led segments with a 5V supply to keep the power dissipation down. The main concern is the maximum allowable total power dissipation, 500mW for the SO package and 1000mW for the DIL one.
High efficiency led displays need only about 3mA continuous (6mA multiplexed on the SAA1064) .
The scheme outlined by Kender would be be the way to go I think. Usually the displays would be on a front panel, so it is convenient to make a small pcb with the four-digit led display on the 'solder' side (and poking through the panel) and the SAA1064 and other components on the 'component' side.
The pcb height is not much more than the display height, so it's a compact module. If the pcb width is the same as the display, modules can be mounted side by side to display a variable of more than four digits.

[dani]

hi,
Thanks to all of you.

I have tried one MAX7219 on protues and it is working fine. Tell me will if be more sutiable then the other solutions described here or not. Becuase in local market i may not b able to find SAA1064. but max7219 would be available easily. 4 max7219 with one pic 16f877 will it work or there will be power issues...

regards

[John P]

I definitely think the MAX7219 is the simplest solution, and it's what I'd use if I thought the product could afford it ($10 apiece, ouch). They'd be cheaper if you bought a lot of them, of course. But 36 LED digits, or 32 if you can take a few out, would certainly be a "blaze of light". Still, it's your design and you have to decide how you want it to look.

As far as power is concerned, that's irrelevant to the PIC processor. Just make sure your power supply is up to the task!

Edited to say, if by "power" you actually mean "processing power", then it's nothing to worry about. If you run out of processing time, it won't be because of the display routine.

[fuzzy]

Unfortunately I've to drive segments with volage higher than 5V. I need almost 8V so i think to power up my device with 12 V.
If I use SAA1064 in multiplexed mode with 4 display, i'll connect leds anode to 12V and catode to chip output. is it better connecting vcc pin to 12V(as leds) or 5V as MCU? what voltage there will be at multiplexing outputs in both cases?? datasheet is not very clear.

Thanks

[kender]

[pasini]

Why don�t you use shift registers (SR) cascaded to drive the LEDS ? You only need data and clock lines for all the shift registers. The first SR receives the data and the following SR receive the last bit of the previous SR. Clock is the same for all. Update time can be near 5ms for all segments. Since you have 36 segments, clock period can be 5m/36segments or 138us. This leads to 7.2kHz which is not a problem at all. Standard 74HC logic gates can be as fast as 100MHz !!!!
For me it worked for 4digit 7segment displays.
Current consumption is an issue if you are connecting the LEDs directly to the SR ( displaying an '8' requires mor current the displaying an '-' or a '.' ). If you could put a MOS transistor to drive the LEDs power is not a problem, you can even drive 1amp�re LEDs depending on the MOS.
74HC logic gates are extremely cheap and can be found in every corner.

I hope this helps.
Pasini

[pasini]

Opssss....
Clock period is 5m/(36*7) = 19.8us --> f = 50kHz. Still not a problem.
Pasini