Help interpreting datasheet of MAX7219 for a 512 LED DISPLAY - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: November 23, 2016, 12:39:23 am
content: Dear all.In order to run a 1,3m TEXT BAR of 10mm red diffused LED dots, I decided to chain 16 MAX7219 to drive a 8columns x 2rows (8x8 led each) display.According to the Maxim data sheet and to avoid any clock skews, I put all the MAX7219 onto the same PCB that contains the MCU (standalone ATMEGA328) and to connect the 64x16 LED display with flat cables (maximum distance from the board should not be more than 70cm.Now the MAX data sheet tells:Supply Bypassing and Wiring[…] The MAX7219/MAX7221 should be placed in close proximity to the LED display, and connections should be kept as short as possible tominimize the effects of wiring inductance and electromagnetic interference. Also, both GND pins must be connected to ground.Now the question. How can I bypass the "close proximity" problem? what kind of problems should the EM interference or wiring inductance bring keeping the length of the cable? should the single output of 7219 drive maybe a transistor that drives the LED or is it better to connect the MAX close to the LED display and then keep the CLOCK-DATA cable longer?Hope I was clear enough.Best regards for any help in advance.Steve

author: mariush
author_rank: Super Contributor
timestamp: November 23, 2016, 01:28:57 am
content: Why would you complicate your life with an expensive chip like max7219 first of all, and second of all why would you pick this chip that's meant to display up to 8 x (7 segment + dot) digits ?By design, the chip only turns on one digit at a time (8 leds), so you'd basically have 8x8 blocks and some flicker and a lot of pointless coding to convert your vertical columns of text into those 8 digits.There's plenty of led drivers out there with lots of channels, and led drivers which can be connected in series so that you just shift data and that goes into the displayFor example, TLC5955 has 48 channels each capable of up to 30mA , so you could control 48 leds at a time, or multiples of 48 leds if you multiplex. It's also only around 5.6$ if you buy at least 10 on Digikey, compared to MAX7219 which can cost upwards of 8-10$ each.Here's a link:http://www.digikey.com/product-detail/en/texas-instruments/TLC5955DCA/296-44424-5-ND/5176230If you want cheaper, you can get something like LED2472G which has 24 channels and costs only 2$ if you buy 100, or 2.5$ if you buy at least 10 :http://www.digikey.com/product-detail/en/stmicroelectronics/LED2472GBTR/497-14188-1-ND/4506339It behaves like a regular shift register, so you can chain several of these and then just shift the bits and hit the latch and all leds flip to new states at the same time. This particular chip even splits the channels into groups of red , green and blue channels, so you can enable or disable one group separately from the others groups if you want to, check the datasheet to see what I mean.There's even simpler chips like PCA9956B , still only about 2.5$ each if you buy more than 10:http://www.digikey.com/product-detail/en/nxp-usa-inc/PCA9956BTWY/568-12502-1-ND/5361204See datasheet, you can chain up to 125 on a i2c bus (can set unique address for each using three pins on them) and set data to each of them separately from the others using i2c, and they even have some special commands to make it easier for you to do text scrolling and stuff like that.

author: Ian.M
author_rank: Super Contributor
timestamp: November 23, 2016, 02:38:55 am
content: A MAX7219 in non-decoded mode can drive a single 8x8 LED matrix. If you are using an 8x8 font this is reasonably convenient but for any other display organisation its a PITA. Assuming controlled trace impedances, clock skew isn't an issue for daisychained MAX7219 chips less than 5m apart, so the most practical design would be to put the MAX7219 chips as close to each LED matrix as possible, I would suggest on the back of the board if using SMD LEDs.However I'd probably use 16x daisychainedTPIC6C595power shift registers as column drivers and a 74HC138 3 to 8 line decoder driving eight gate drivers controlling eight high side P-MOSFETs as row drivers, and handle the multiplexing in the MCU using a timer interrupt. That gives you a memory mapped display buffer + the option to use pulsed currents of up to 100mA per LED for a fraction of the cost of 'smarter' LED drivers. Again, I'd put the TPIC6C595 chips as close to the LEDs as possible.A limited number of greyscale levels could be handled using BAM modulation without lowering the refresh rate excessively.

author: woodchips
author_rank: Frequent Contributor
timestamp: November 23, 2016, 07:39:49 pm
content: Remember that the 7219 is now an old chip, early 1990's, and then LED displays were not very efficient so lots of current was need to get a bright display. This is now not the case with displays like the HDSP A103. So with the 7219 switching significant current then it did need to be close, try it with a cable and see what happens.The advantage of the 7219 is that it is designed to drive LEDs, so has the current limiting built in. I am sure it can be daisy chained but never needed that feature.It is a superb chip, but like other things there might be better devices now available.

author: Houseman
author_rank: Regular Contributor
timestamp: November 24, 2016, 09:37:30 pm
content: Hi thank You all so much for the fast and richness of content of the replies.I appreciate it a lot!The fact is that more often nowadays it's the business (power of $$) and the out-of-the-box-ready! that rules also in the DIY-product shelf. I was asked from my client that found online (search for: large-8x8-led-matrix-module-diy-kit) and bought 16 of them in order to get his big 16x2 module display. Simple SPI interface daisy-chained, ready! The fact is also that there are plenty of libraries online for Arduino that can easily handle scrolling text and images with just a few lines of code for that chip also for RGB. And more important of all I have not yet gained sufficient experience to route him in other directions.Wonderful forum.Thanks againRegardsSteve