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dani
Joined: 02 Jun 2004 Posts: 16 Location: Pakistan
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36 seven segment display with pic.. need advise.. |
Posted: Fri May 26, 2006 9:22 am |
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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 |
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The Puma
Joined: 23 Apr 2004 Posts: 227 Location: The Netherlands
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John P
Joined: 17 Sep 2003 Posts: 331
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Posted: Mon May 29, 2006 10:35 pm |
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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. |
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kender
Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley
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Posted: Mon May 29, 2006 11:20 pm |
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John P wrote: | 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. |
If there are 36 LED digits, there's probably quite a bit of board real estate. It could actually be easier to wire with the LEDs in goups of 4. And since the Philips chips are I2C, wiring shouldn't be too bad. SAA1064 can have only 4 slave addresses (via one address pin, oddly enough), so our friend would need three I2C busses to accomodate 9 of the SAA1064.
Last edited by kender on Thu Jun 01, 2006 12:35 pm; edited 1 time in total |
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fuzzy
Joined: 26 Feb 2005 Posts: 64
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Posted: Tue May 30, 2006 4:46 pm |
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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???
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Kenny
Joined: 07 Sep 2003 Posts: 173 Location: Australia
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Posted: Tue May 30, 2006 8:47 pm |
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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. |
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dani
Joined: 02 Jun 2004 Posts: 16 Location: Pakistan
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Posted: Wed May 31, 2006 1:08 am |
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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 |
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John P
Joined: 17 Sep 2003 Posts: 331
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Posted: Wed May 31, 2006 1:37 pm |
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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. |
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fuzzy
Joined: 26 Feb 2005 Posts: 64
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Posted: Thu Jun 01, 2006 11:33 am |
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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 |
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kender
Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley
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Posted: Thu Jun 01, 2006 12:46 pm |
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fuzzy wrote: | 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? |
Test your LEDs: see if they are off, when the voltage across them is +7V (12V minus 5V). |
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pasini
Joined: 12 Dec 2005 Posts: 50 Location: Curitiba - Brazil
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Posted: Fri Jun 02, 2006 6:20 am |
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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 1ampère LEDs depending on the MOS.
74HC logic gates are extremely cheap and can be found in every corner.
I hope this helps.
Pasini |
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pasini
Joined: 12 Dec 2005 Posts: 50 Location: Curitiba - Brazil
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Posted: Fri Jun 02, 2006 6:23 am |
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Opssss....
Clock period is 5m/(36*7) = 19.8us --> f = 50kHz. Still not a problem.
Pasini |
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