Forums › SM Series Discussions › GPIO Drive Capabilities
- This topic has 3 replies, 2 voices, and was last updated 3 years, 3 months ago by kaiser.
- kaiser September 19, 2019 at 7:01 am
We’ve been using the GPIO on the SM200 to drive some switches, creating interrupts for microcontrollers, etc. It’s been working great.
We have an upcoming situation where we need to drive/control 60+ devices. There are 8 GPIO ports, so to make math easy we can say they each control 8 devices. Off the top of my head, I don’t think that will be an issue. But it is a good number of devices on that one GPIO port.
But, just to double-check are there any specs on the drive current or capabilities of the GPIO, ping and/or port level limits?
Justin Crooks September 20, 2019 at 10:52 am
- This reply was modified 3 years, 8 months ago by Justin Crooks.
The GPIO is driven with two, SN74AVC4T774 chips. 10k pulldown, 100 ohm series resistor. The 100 ohm resistor will limit your switching rate as you add capacitive loading, and you might have to worry about ground bounce, etc. with long cables. But this should give you enough to model it if you have concerns.
A fan-out buffer board could always be the fall-back position. I’d use something like a Max 10, so that you could get creative if you needed to drive all 60+ devices independently (e.g. one nibble is data, the other nibble is address for 64 independent outputs)kaiser February 21, 2020 at 11:31 am
Thanks Justin, we’ve had this going and working for a while. We’ve been playing with sensitivity some, and we’re only triggered our device by the 15mV 🙂
If I wanted to up the drive voltage out, do I supply a new Vdd in? Is there some circuitry already setting that somewhere? It would seem so since there’s obviously some drive voltage out already 🙂
kaiser February 21, 2020 at 2:49 pm
- This reply was modified 3 years, 3 months ago by kaiser.
Looping pins 3 and 11 together did bump the output signal to 3.3V.
It was crazy — I have a system that’s supposed to trigger at 1.4V, and I could see the ~1.6V SH level going into it, and it just wasn’t triggering. If I slowed it down, it would sometimes trigger. Very odd. I hooked up a function generator and exactly matched the SH output signal, and it triggered perfectly!
So, something about the pull down and series resistor or amount of current available to source coupled with my digital IO card was causing more voltage loss inside the card. The bump to 3.3V in solved it all.
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