Cannot get DAC to work on custom teensy 3.6 board with 144LQFP chip version

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johnrom

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Hey Folks!

I have been dabbling creating Teensies in my own form factor (modules kind of like Sparkfun's "microMod" system, see pics below). I have made teensy 3.2's and now teensy 3.6's. For the teensy 3.6 I used the 144LQFP (MK66FX1M0VLQ18) version of the processor (Digikey part # 568-13334-ND) because I do not have the experience or tools to solder the BGA part used in the actual teensy 3.6. So far I have tested all of the regular digital I/O, SPI bus, CAN bus, USB Host, etc... and everything has worked! However I have now tried to test the two DACs, and I just cannot get any voltage at all out of my teeny 3.6 DAC's. After looking at the forum for advice I have tested the following code (Arduino 1.8.2 Teensyduino 1.52) which should produce a simple low frequency square wave on the following three systems:

//<BEGIN CODE>
#include <arduino.h>
void setup()
{
analogWriteResolution(12);
while(true)
{
analogWrite(A21, 2000);
analogWrite(A22, 2000);
delay(1000);
analogWrite(A21, 1000);
analogWrite(A22, 1000);
delay(1000);
}
}

void loop()
{
delay(1);
}
//<END CODE>

System 1. PJRC Teensy 3.6 DAC -> WORKS!
System 2. Custom teensy 3.2 in carrier board (A14) -> WORKS!
System 3. Custom teensy 3.6 in carrier board (A21 || A22) -> Doesnt work =(

Since the two custom teensies (3.2 and 3.6) fit into the same carrier board, and the test code (Using A14 for teensy 3.2, and A21 and A22 for the T3.6's) works for the PJRC teensy 3.6, and my custom 3.2 works but doesn't work for my custom 3.6 doesn't, to me that points to some problem with either my hardware or to some firmware or software issue involving the different between the 144LQFP and BGA versions of the chips. According to the MK66 datasheet, the two DAC pins in the LQFP version are DAC0 -> L3 -> pin 38 and DAC1 -> L4 -> pin 39, which I have connected according to the the schematic attached as well as a screenshot of the layout with DAC0 trace highlighted.

Has anyone else used the DAC's on a custom teensy 3.6 with the LQFP version, did they work?

Let me know if anyone can shed any light on this or wants more information.

Thanks!
-John

layout-DAC0.jpg
experiment.jpg
schematic_T36_Custom.jpg
Screen Shot 2021-01-12 at 4.52.14 PM.jpg
 
Thanks for checking so fast! Any idea what could be wrong, or further ways to narrow down any possible causes?
 
Nope, no idea why. I tried to glance at your schematic, but the max image size the forum allows is too small. Maybe try attaching a PDF to your message? Click "Go Advanced" if you're in the "Quick Reply" editor, to get to the editor which lets you attach files.

But I must admit, I can't spend long on this, and I have a hard time reading schematics where the connections are mostly net names rather than wires. Maybe someone else will view it and notice something?
 
Thanks so much for your help! I actually just figured out what was wrong, I had made an error copying over the schematic and connected the ferrite that is supposed to bridge VDDA and 3.3V to GND instead, so the DACs weren't getting any power... I.e. I forgot to check if it was plugged in... haha. I fixed it and now DACs work as expected. =). With a rather sketchy soldering job I can just rotate the ferrite to bridge between the 3.3V rail (Pic attached in case anyone is curious). Also I have attached a pdf of the fixed design just in case anyone else is looking for examples for their own custom teensy 3.6 project, which for me has been a project well worth the effort.
View attachment SNAP_teensy36_SMD_V1_2.pdf
fix.jpg
 
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