MAX9924 configuration and use cases

[Fred]

Hello all, but particularly Jean who I'm pretty sure will "just know" the answers to these questions...

MAX9924, as installed on Puma does not like reading 5v square waves. I removed the zeners from the "digi protect" circuits to allow a full 5v when it IS working. However still no luck while cranking. I bypassed them with wires and inverted the code and have signals now. But, we need a solution.

Is anyone familiar with the best way to setup the max chip such that it can read VR properly, AND read 5v square properly? Is that even possible, or shoudl their be a config zero ohm resistor to turn different things on and off? I'm willing to try and test anything anyone comes up with for this, as we need a final solution for spin 2 of the puma.

I've seen what looked like inconsistent results from it, but I think it could be the adaptive threshhold doing that. A static ground input gives 5v out, and a 5v in gives ground out, ie, it inverts the signal. However when plugged into the car it doesnt seem to function that way. I do not understand.

Fred.

[jharvey]

This fellow posts wave forms of the A2 config nearly the same as what puma is set to. A fellow named Jean appears to have run it in A2 with 0 - 5V input signals. The adaptive threshold seems to adjust to 2.5v as long as it's a signal, not a steady state.

http://www.miataturbo.net/showthread.php?t=47243

[Fred]

Yes, that fellow named Jean is the very same one I mention in the first post, though I wasn't aware of that board and this thread, I had seen him comment in an "I know about this" way on this part before, hence my hunch that he would know. And he assuredly does. If we can convince him to take a look at the schematic maybe he can spot the issue? Otherwise maybe he can help me diagnose my issues? This is important pre spin2, but not important to me now, as I've got another solution.

http://msextra.com/forums/viewtopic.php?f=67&t=35149

Fred.

[jharvey]

Hmmm, banned, but this forum will let me read it's content with out sucking.

http://forum.jbperf.com/viewtopic.php?f=6&t=78

[Fred]

From the photo his inputs look to be the same. I wonder what the resistor and capacitor values he uses on the input are, hopefully he'll be kind enough to share that with us shortly.

Fred.

[jharvey]

I would guess he used the 10k and 1nF noted in the MAX data sheet. However, Marcos mentioned upping the R values due to some heat issue. I'm not sure about the heat issue. 5V and 10K is small power. Can't a VR produce 100V if you get it really close? Even at 100v and 10K were still talking .01w.

[Fred]

VR can go higher than 100v, yes, I used 10k resistors (1/4w) based on what marcos said. I assume you're right about Jean following the spec, but perhaps not, and I'd like it from the horses mouth if he is OK with sharing.

[jharvey]

His picture on his web page is a bit blurry, these are very small parts and it's hard to get a clean picture. I've had good success with a flat bed scanner. Any how, it looks like the SMT resistors on the output note 10K, but that could be something different, as it's fairly foggy. I might be seeing what I'm looking for. I agree, horses mouth is the best way to go.

[jbelanger]

To read a 0-5V (or 0-12V) signal, all you need to do is connect the signal to VR+ and leave VR- floating. That will set the trigger point at the internal 2.5V value. Grounding VR- would set the trigger point at 0V which the signal would never cross.

And this is using the standard A2 config for the chip.

Jean

[jharvey]

Awesome, the horses mouth has done it. Thanks for filling us in on that bit of magic.