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IO11 / 12 High On Power Up


Hi All,

I’m hoping this is something possible and easy, at the moment, I have a couple of fez relays plugged into IO 11 and 12, however whenever the board turns on, these go high for a second or so until my code starts to set them to low.

This is an issue as it makes the stuff attached to those relays turn on when I dont want them to.

How can I set them to be low by default so when power is applied they stay low?


All pins are pulled high on power up. If you want them low then add 1K resistor from io to ground


I’m referring more to the ports on the Cobra, the middle two, which are “IO11 and IO12 are open drain pins with 2.2K pull up resistors” based on the brochure. Is there any way to make them act like the other 4 ports? I am using the Fez relays as sold on here - I’m using 5 of them, so at least one is currently in these ports…


I do not understand your question! Yes you can use these pins for about anything. What do you want to connect to these IOs?

The relays are not working for you?


I’m using the 6 JST connectors for connecting up FEZ relays.

The issue is that for JST ports IO11 and IO12 (in the brochure defined as “IO11* (Open Drain Pin) I2C SCL IO12* (Open Drain Pin) I2C SDA” the pins seem to start on/high on power up so the relays turn on during the time of power up until my code executing to set them to low. Sample timeline below:

0 - All power off, all relays off
1 - Power Up
- Relays attached to IO11 and IO12 turn on (i.e. ‘click’ and red status light on the FEZ relays activates)
2 - ~ 1 Second
3 - My code starts, immediately sets all JST IO’s to off / low
- Relays attached to IO11 / IO12 JST ports turn off

This means that between powering up the board, and my code starting to execute, the relays attached to JST IO11 / IO12 ports are turning on. I want these two ports to behave like the other 4 JST ports.


Oh I see, the relays work fine but the problem is that during reset, the pull up on these pins is activating the relays which is not favorable.

What about using the “normally closed” side of the relay instead of “normally open”?

If that is not a good option then I am afraid we would need to get some other IOs from the board.

Or use the 16-relay board we offer which connects to I2C…or create a board with only 4 relays…many options basically


Exactly right. Can’t use NC as that would make the appliances will turn on when/if the cobra powers down / resets etc.

I’ll just look at using a normal IO pin rather than JST for the last 2 relays then. I thought this was the likely outcome. Cheers.

I would love a 8-relay version, 16 is too big for a normal enclosure (for my use anyway). Also like to have a watchdog style interface on built into the relay board to cater for if the cobra has an issue. The relays would ‘timeout’ after a few seconds and auto turn off. (as a safety feature basically). Don’t like I2C for that reason at the moment (my system needs to fail closed, not open).


I suppose one could also just remove the 2.2k pullups off the board. They are located right next to the JST plugs.