Connect FPGA pins directly to LED matrix

[ni9e]

Hi,

I want to control an 8x8 LED dot matrix using an FPGA. I know that there are chips like the MAX7219 that can do it for me, but I want to implement it myself as an exercise.

Can I connect the pins from the FPGA directly to the LEDs?

I am using the Cmod A7 board where the outputs are 3.3V. Here are the I/O characteristics taken from the datasheet:




These are the LED characteristics taken from the FND-5882W4SM00BW35 dot matrix datasheet:

[Ian.M]

Nope.  You need a LED matrix with a lower Vf to provide enough headroom for current limiting resistors.  That will probably be a red, green or yellow one.  If you try with your white matrix, the odds are it will be unusably dim, (compounded by the 1/8 duty cycle) but if any of its LEDs have an untypically low Vf, there's the possibility it may draw excessive current from the FPGA I/Os, which may damage them.

[ni9e]

OK, thanks.
Then I will use a buffer or linedriver like this one: http://www.ti.com/lit/ds/symlink/sn74lvc245a.pdf

[Ian.M]

Nope.  Still not suitable, as the problem is 3.3V logic doesn't have enough drive voltage for a LED with a Vf of up to to 3.5V.

Use  4x 74LVC2T45 chips run from 3.3V VccA and 5V VccB as level translators and high current buffers into 8 series resistors to set the current between them and the display columns as your display is column anode. 

To multiplex the rows use discrete MOSFETs with a max Vgs threshold of less than 1.6V.   When on, each has to handle the current for all lit LEDs in the row, and steady state that would be up to 0.2A, or pulsed it could be as much as 0.4A (the limit of what 4x 74LVC2T45 can output), so with only 3.3V on the gate, you are probably looking at 1A MOSFETs so they are on hard enough to handle the current without excessive Vds drop.