T4 total digital pin load limit?

G

greg

Well-known member
Hi,

We need to drive a dozen or so opto-isolated solid state relays (i.e. PC817) to trigger external devices that basically just expect a "short" to turn on.

Using T4, and a series resistor, so the load on each pin ends up being around 6ma (I know its over 4ma recommended, but we only need to toggle it for 50ms or so, hope that's ok)

My question is, I can see the max recommended current per pin (4ma) (https://www.pjrc.com/store/teensy40.html), but is there a limit for a total power output across all pins? I.e. say I drive 20 digital pins at 6ma, the total power load on the chip will be 120ma. Is that OK?

Thanks!
 
You might take a look at <this> post discussing how I previously (successfully) used the 75HC595 to drive a large number of LEDs as indicators on an earlier version of my TeensyMIDIPolySynth. Using this shift register only consumes a couple of Teensy I/O pins (data, clock, and output enable) & more importantly, eliminates any concern for the current draw if/when using the Teensy I/O pins to drive your relays directly.

Hope that helps . . .

Mark J Culross
KD5RXT
 
So, basically you are suggesting using a shift register to serially drive opto-couplers? Makes sense. Complicates the setup, but will investigate. Thanks!

Still would love to know if there is a technical limit on the combined chip-wide digital pin current.
 
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