Will I damage the MS3?

[billr]

I want to play around with a VSS, digital output, connecting it to PT4 via the MS3X. For this playing, it is convenient for me to use a BOB connected to the MS3X DB37. The VSS is a home-made unit based on an Allegro ATS667 Hall device. I want to power it with 12V, taking that from the "launch in" pin on the MS3X (11), and I want to pull-up the VSS/PT4 input (pin 31) to 5V at the "spare ACC" pin (21). Neither of those (launch in and spare ACC) are in use for anything else, but loading from the VSS may put those input in a "mid-state", not hard high or low. Is there any possibility this can hurt the MS3? The pull-up for the VSS is 47K. If there is a better way to get 5V and 12V from that MS3X DB37, let me know...

[jsmcortina]

Trying to source 5V and 12V that way is unlikely to work.

James

[elaw]

Do you have another BOB you can connect to the mainboard DB-37?

Neither voltage supply is available on the MS3C connector. But +12 of course is available on the mainboard connector, on the main supply pin. +5 is available too from Vref, although the current it can supply is limited. When I needed +5 to power an external COP buffer, I ran a wire from one of the +5 pads on the mainboard to a spare pin on the DB-37.

Oh one more thing: if you're making a DIY sensor, I'd advise putting a 0.1uF bypass cap between Vcc and Gnd right at the sensor. If you look at page 11 of the ATS667 datasheet you'll see they recommend that.

[billr]

James, I understand that may not work properly, but is there little-to-no chance I will damage the MS3 by having those inputs being somewhat "indeterminate"? No second BOB to use, and no cap at the 667 right now; I'll add that next time I have the VSS out. This powering scheme will not be permanent, just a convenient way to play with it right now.

[elaw]

I think what you're doing is very unlikely to damage the Megasquirt.

Probably the best thing to do is try it, and measure the voltage you're getting from your +5 and +12 "supplies". If the voltages are sufficient (and the ATS667 can operate down to 4 volts so you've got that in your favor) you should be okay.

[billr]

Thanks Eric, "very unlikely" is what I was hoping for and expecting. I'll post what voltages I get at those "supplies", just FYI.

[billr]

Yeah, that didn't work! The 12V at "launch in" dropped from 11 to 3.2 when powering the 667 (12.1V "battery voltage" in TS). Worse yet, the "spare ACC" pin has little-to-no voltage until applied externally. The schematic shows a 2.49K pull-up to VCC for that pin, but I'm guessing the other end of that line (AN13) has some kind of pull-down on the MS3 board. I experimented with the "ext MAP", which is in use and known to work, it acts the same; no voltage when the sensor is disconnected.

[elaw]

Well if you're feeling creative...

With the 2.2K pullup resistor (R25) on the MS3X board you got a 7.8-volt drop. Paralleling that resistor with another of the same value would cut the drop in half to 3.9 volts, yielding 7.1 volts with an 11-volt supply. 7.1 volts definitely should be enough to run the sensor. And the resulting 1.1K resistance of the two resistors in parallel would still give decent current limiting on the input pin.

Re the "spare ACC" pin... have you defined it as the VSS input in the tune? Some pins on the processor can be inputs or outputs, and if it's configured as an output for whatever reason, it might give that behavior.

[billr]

I'm still feeling creative, but adding a resistor in parallel with R25 would require removing and opening the MS, that's what I'm trying to avoid right now. I haven't configured "spare ACC" as anything, so that may be an issue. However, the "ext MAP" is certainly configured and acts much the same. I say "much" because the "spare ACC" sits at about 1.2V when the pin is open, whereas the "ext MAP" is more like .05V. I think your point there may be valid, the "spare ACC" pin is just floating, but I don't think I can get it to 5V except by programming it as an output that is "on". I'm thinking that would take special firmware, and absurd way to get the 5V power I need! The VSS input is set to "PT4" in TS, so I expect that pin is programmed properly as an input.

[billr]

Well, the mainboard RS232 DB9 is my savior here. I have lots of ribbon cable "pig-tails" with the mating connector to plug in there and grab 5V and ground, will power the 667 from 5V and forget about 12V for now.