ICM7218, dot matrix display design discussion


I have some questions about a dot matrix display that I am trying to develop. I am using an 8x8 dot matrix display. I have two ways to display the required text, one is by using ICM7218AIJI chip, the other is to write my own logic and design a circuit which does it. I have done both of them with obvious trade offs:

  1. By using 7218 I have a dedicated dot matrix refresh utility which makes it more bright, reduces the area of PCB (PCB design area and manufacturing cost are the most important factor in my design), reduces load on the microcontroller, but it is not more suitable for scrolling options than a dedicated logic. 2. While designing dedicated hardware, I have actually used decoders and ulns for grounding and a scanning function designed in the microcontroller. The number of chips is huge and ofcourse results in large area for PCB, not to mention since the scan time is very small for each row or column, the applied voltage increases drastically. Surprisngly however, the chips used are cheaper than a 7218 counterpart for each dot matrix.

My questions are as follows: 1. Are there cheap 7218 resellers in China and Taiwan etc. which can provide good quotations on bulk orders (say 100 pieces to start with?) 2. Does 7218 have an internal current limiter? I have given it

100mA, 150mA and 200mA, and each time it has resulted in increased brightness, does it even have a current limiter, or is it designers job to limit current on the chip?

Thanks in advance


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P9 on the maxim data sheet

50R limits the current to 30mA per segment


Reply to
martin griffith
7218's are about $3 in 100's, what price are you looking for? Why are 7218's not suitable for scrolling?

yes the discrete components to drive leds are cheap as dirt, but the board area is not free either

Reply to

dude where'd you get those prices from??? Farnell lists these chips for

6pounds or so...

I'll be pretty good with that price, can you direct me where I can buy these chips from?

Thanks, and yes, the discrete components are a good solution but very painful (increased area costs for the PCB and ofcourse troubleshooting... ugh)

Reply to

Out of general experience, is 30mA per segment sufficient for indoor applications? How much is generally required for outdoor applications where ambient temperature may soar upto 45 degrees Centigrade?

Is it better to stick to common Anode versions for more intensive light solutions? Further if I am using a dot matrix which supports both green/red LEDs, is it better to use two 7218s for separately lighting the chips or using some form of active switch or relays for each segment better?


Reply to

You can't fit two 18-pin SOICs and a couple of resistor networks into less than the space of a 28-pin (DIP?) 7218? And why do you say the brightness is any different? The 7218 can manage a few mA per segment average.

BTW, it would be a cold day in hell before I specified that ancient, huge and overpriced chip.

Best regards, Spehro Pefhany

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Spehro Pefhany

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