Posts: 16
Joined: Tue May 03, 2016 7:55 pm

Pull downs on GPIO 28,29 if I2S?

Fri May 05, 2017 5:20 pm

Hi there, I'm using GPIO 28 and 29 for their i2s PCM_CLK and _FS functions. The datasheet specifies these should be pulled down, but I'm wondering if that's still the case if I'm using them for this particular ALT function.

Posts: 2071
Joined: Tue Jun 30, 2015 1:35 pm

Re: Pull downs on GPIO 28,29 if I2S?

Sat May 06, 2017 9:10 am

Pull-Downs are there to prevent floating pins because these pins don't have internal resistors.
You configure them for I2S by overlay/it device tree; external HW is connected --> no need for pull-downs.

Posts: 1
Joined: Sat Apr 29, 2017 12:38 pm

Re: Pull downs on GPIO 28,29 if I2S?

Tue Jul 18, 2017 8:36 pm

Note that, without external pulldowns, at reset and before the device tree stuff is loaded and the pin direction is configured, those pins will be floating inputs. That condition might persist for a second or two at startup, depending on several factors.

If having those pins floating briefly is going to be a problem for the hardware attached, then you would still want to add pullups or pulldowns as appropriate... weak ones though, given that you're going to make them outputs during startup.

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