I'm building a device which will allow a teensy 3.6 to control a dot-matrix LED sign that I scavenged from a public transportation bus.
Here's a rough schematic I made to illustrate the way that the sign works. It's not completely filled in, but I think that for my question only the left two or three columns are very important:
http://extralife.xyz/articles/twinvision-sign-driver/twinvision_schematic.png
And here's a schematic for two PCBs I'm designing to control the sign:
http://extralife.xyz/articles/twinvision-sign-driver/controller_schematic.png
The driver board on the right is what I think is important here.
Here's my conundrum: BOTSIG and TOPSIG both go directly from the Teensy's ArduinoPin4 and AP5 to non-inverting inputs of some comparators. I use a voltage divider to feed about 1.5 volts to the inverting inputs of those comparators. By my understanding of things, that basically means that the comparators are giving out 5V when TOPSIG and BOTSIG are above 1.5V, and that they're giving 0V otherwise. But when I try this out on my breadboard, I don't get that result.
What happens instead is that the TOPSIG comparator behaves in the way I just described but the BOTSIG comparator is always sending out ground. In order to get the BOTSIG comparator to reflect the changes of the BOTSIG pin, I need to lower the bias voltage down to about .5V. What's even stranger is that if I move BOTSIG to AP8 (both physically and in the code), it works out perfectly and the comparators are back do doing their 3v3-5v level shifting.
What exactly is going on here? Does pin 4 on the Teensy 3.6 have a really slightly different output impedance than all the other pins?
(I was worried that it was just damaged hardware, so I tried swapping out the Teensy for another 3.6. Same result. I also tried swapping out the sign with another, identical one. Same result.)
If it matters, I'm programming this with Arduino.
Here's a rough schematic I made to illustrate the way that the sign works. It's not completely filled in, but I think that for my question only the left two or three columns are very important:
http://extralife.xyz/articles/twinvision-sign-driver/twinvision_schematic.png
And here's a schematic for two PCBs I'm designing to control the sign:
http://extralife.xyz/articles/twinvision-sign-driver/controller_schematic.png
The driver board on the right is what I think is important here.
Here's my conundrum: BOTSIG and TOPSIG both go directly from the Teensy's ArduinoPin4 and AP5 to non-inverting inputs of some comparators. I use a voltage divider to feed about 1.5 volts to the inverting inputs of those comparators. By my understanding of things, that basically means that the comparators are giving out 5V when TOPSIG and BOTSIG are above 1.5V, and that they're giving 0V otherwise. But when I try this out on my breadboard, I don't get that result.
What happens instead is that the TOPSIG comparator behaves in the way I just described but the BOTSIG comparator is always sending out ground. In order to get the BOTSIG comparator to reflect the changes of the BOTSIG pin, I need to lower the bias voltage down to about .5V. What's even stranger is that if I move BOTSIG to AP8 (both physically and in the code), it works out perfectly and the comparators are back do doing their 3v3-5v level shifting.
What exactly is going on here? Does pin 4 on the Teensy 3.6 have a really slightly different output impedance than all the other pins?
(I was worried that it was just damaged hardware, so I tried swapping out the Teensy for another 3.6. Same result. I also tried swapping out the sign with another, identical one. Same result.)
If it matters, I'm programming this with Arduino.