7 Segment Displays and MAX7219

[Leonardo]

Hello friends,

I've completed the test with the code and have not had any problems so far, will continue to test and if they notice something new.




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[QUOTE=Bill Legge;76621]I'm using a PIC18F8722 at 40 MHz to display data on 7-Segment displays.
There are 8 digits on each display
Each display is driven by a MAX7219 driver.
Initially, I had three displays, each on it's own port - all worked OK.
Then, to save ports, I daisy-chained the displays (Data_out of one MAX7219 to Data_in of the next). Clock and Data in parallel to all three MAX7219
Initially, all worked Ok with this chain and then:

1. After a few minutes, the third display (last in the chain) went blank.
2. Over the next half hour the second display dimmed and finally turned off.
3. Then the first display froze on '88888888'


The voltage without the displays connected in 4.97 and with all three displays connected it is 4.94, so i don't think suppy voltage is the problem?

Too high a load for the port?
Recompiling to run at 10 MHz makes no difference.
The connections between the PIC and displays is via 10-way IDC and ribbon cable.

Anyone experienced difficulties daisy-chaining displays/MAX7219?

[bearpawz]

Well, Since this is kind of what Im working with figure I will ask here. Im using the same MAX7219. I have a micro simply counting from 0 to 9999. So far no problems with the code or the MAX7219 IC. My problem is this: Im using displays LSD8151. Its spec sheet says I should be driving the segments at around 21mA, so looking at the handy dandy table in the 7219 spec sheet I chose a 28K resistor for Rset @ 20mA. Display works fine. Im using the internal osc on the pic, and just a small 10uf cap at the LM7805 5V reg IC. That is where my problem is: the LM7805 is getting quite toasty running this thing. I did put a heat sink on it and its down to the point where I can touch it now with out burning myself, but my question is this: Is there a more efficient way to supply the 7219 with its power needs? I just cant help thinking alot of power is being wasted being disipated by so much heat.. or is it just normal for this to run so hot in this configuration?

[Macgman2000]

If it's a one off that you are making, you can purchase a buck controller. Basically it PWM's to a desired voltage output and has active feedback to maintain that voltage. You will need filtering at the output with some caps / inductors depending on the current draw you have.

I have made my own using opamps (oscillator and feedback) to supply 5v. So I can run my motors from the main 12v battery and pwm down that voltage much more efficiently to my desired 5v for MCU, display...etc. Filtering caps and inductors are a must.

Nick

[bearpawz]

I added an UDN2981 and I am now pulling the current from the 12V supply and using current limiting resistors on each segment. Works like a charm now.