Rising Edge versus Falling Edge

[Digitalwave]

This drove me nuts yesterday, but I finally found out what the issue was. Chris and several others have said that the proper input capture for an M20 is rising edge. My car did not have the proper signal set to rising edge, and I couldn't figure out why. It had a long jump followed by a medium jump (similar to this). Switching it to falling edge fixed the signal, so it just had one long spike on the missing teeth (similar to this).

I first thought my VR signal wires were wired backwards (should be pin 47 +, pin 48 -, to VRIN+ and VRIN- on Microsquirt), but after a lot of research, I think I found the answer. The preferred method for MS2 and MS3 seems to be using VROUTINV (not VROUT), resulting in using 'rising edge' for an M20.

Microsquirt only has VRIN+ and VRIN-, and it inverts the signal before the processor (source below). So with Microsquirt-based setups, for an M20 you will need to use falling edge. DIYAutotune indeed recommends falling edge for their Microsquirt-based PNP kits. MS2 and MS3 setups using VROUTINV can use rising edge. Please note that which option you choose will alter your tooth #1 angle by ~3*, and you should verify your timing with a timing light.

I hope this helps someone, because it confused the hell out of me yesterday.

Source 1
Source 2

Note that the VR circuit used in your MicroSquirt® controllers performs an additional inversion on the VR signal before passing it to the processor. The net result is that you have to use the opposite edge to trigger off, compared to a MS-II controller with the V3 main board. So where MS-II uses 'rising edge', you would use 'falling edge' with MicroSquirt®, and vice-versa.

[Kershaw]

That is really good info, thank you.

[Bearmw]

Switching to falling edge also fixed the issue on my B35. It took a lot of trial and error and I didn't understand it as well as you explained it.

Thanks

[Digitalwave]

Glad it may help someone in the future! I wanted to add that after verifying timing with a timing light on my car, I still believe that with Microsquirt inverting the signal, it is still reading the rising edge of the tooth even though it's set in TunerStudio as "falling edge." I believe that because my 0 timing was still at 84* tooth #1 angle -- the expected setting for the rising edge on an MS2/MS3 board. I believe if it were reading the falling edge, it should be a few degrees different than that.