Setting a specific pin to high status when the device powered
(10-07-2016, 10:00 PM)MarkHaysHarris777 Wrote:
(10-07-2016, 06:46 PM)pfeerick Wrote:
(10-06-2016, 09:01 PM)MarkHaysHarris777 Wrote: Use a comparator on the PMIC output pin for DCDC1;  with an open collector, and pullup for 3v3 or 5v.

Set the threshold on the comparator for something reasonable so that the 'off' level ( .3 , whatever ) snaps the output to zero and holds it there. The LM339 is good in this kind of application.  These make great level detectors, operate at a wide voltage range, and have open collector outputs;  pullups are 'required'.

Is there any particular reason a LM311 wouldn't be more suitable - as it results in a smaller package size, unless you want the extra comparators. However, that is beside the point - you're basically agreeing that unless the threshold trigger voltage for the linbus transceiver the OP is wanting to use will accept 0.3v as off if using the 3.3v power pins on the RPi/euler bus, that there is no reliable way to get a pin to go high and stay high immediately from power on, without using external hardware?

I've never actually used the LM311. The LM339 is readily available, inexpensive, and very easily used; also, in my experience if you need one comparator|shifter you need more one (usually).

External hardware is probably required; reason being that the pins are not 'reliable' until the OS has control-- even the GPIO don't really reliably default to inputs until after the pine board is booted up. For instance, if I needed GPIO pins that 'would' be reliable inputs prior to full bootup I would expand the GPIO bus with the MCP23S17;  those pins really are going to default to inputs from power on.

I have found the LM339 ( I've used it many times ) to be reliable, affordable, and easy to use. The main error that folks forget about is using a pullup on the output (it is required). But that is the beauty of open collector circuits because they also double as level shifters (albeit uni-directional).
If operate on 5V, both LM311 and LM339 will work. If operate on 3.3V, better using LM339 due to LM311 minimum operating voltage is 3.5V.

Messages In This Thread
RE: Setting a specific pin to high status when the device powered - by tllim - 10-09-2016, 06:03 AM

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