Beaglebone PRU default GPIO values

Question

I am unable to set the default value of a GPIO for the PRU. I would like to have a “pull-down” default, but when I boot the signal is “high” > 3.3v.

My system:

Linux beaglebone 4.19.94-ti-r62 #1buster SMP PREEMPT Sat Apr 24 00:27:24 UTC 2021 armv7l GNU/Linux

Beaglebone Black Rev C

Booting with the UIO pruss

In /boot/uEnv.txt - uboot_overlay_pru=AM335X-PRU-UIO-00A0.dtbo

Steps I took:

1.  Went to https://kilobaser.com/beaglebone-black-device-tree-overlay-generator/ to get a dts
   

2.  Selected P9_27, fast Slew, Output, Pulldown, Mode5: pr1_pru0_pru_r30_5
   

3.  It generated:
   

/dts-v1/;

/plugin/;

/{

compatible = "ti,beaglebone", "ti,beaglebone-black";

part_number = "BS_PINMODE_P9_27_0x5";



exclusive-use =

    "P9.27",

    "pr1_pru0_pru_r30_5";



fragment@0 {

    target = <&am33xx_pinmux>;

    __overlay__ {

        bs_pinmode_P9_27_0x5: pinmux_bs_pinmode_P9_27_0x5 {

            pinctrl-single,pins = <0x1a4 0x5>;

        };

    };

};

 fragment@1 {

    target = <&ocp>;

    __overlay__ {

        bs_pinmode_P9_27_0x5_pinmux {

            compatible = "bone-pinmux-helper";

            status = "okay";

            pinctrl-names = "default";

            pinctrl-0 = <&bs_pinmode_P9_27_0x5>;

        };

    };

};

};

4.  Compiled and applied the device tree
   

5.  Reboot, on boot the o-scope on P9_27  goes high
   

I’ve tried on P9_25 as well, same result. (Update: I have tried ALL of the PRU gpios on P9 with the same result). I’ve configured it for Mode 7 (standard GPIO) and it works fine. Just Mode 5 does not seem to be responding as expected.

Can anyone verify my issue and offer a solution?

Additional clarification, the reason I believe I have configured P9_27 in the above example to be low on boot is this:

pinctrl-single,pins = <0x1a4 0x5>;

The 0x1a4 is the address offset for P9_27 0x5 = 0000 0101 - the top 5 bits are the mode in the table below. The 101 selects mode 5 (Bits 2-0) in table below. For a general GPIO, this functionality works perfectly, and there does not seem to be different rules for GPIOs associated with the PRUs

Bit Field   Reset   Description
6   conf_<module>_<pin>_slewctrl    X   Slew Control. Slew Rate: Fast is 0, Slow is 1
5   conf_<module>_<pin>_rxactive    1h  Receiver Active. Input Enable: Receiver Disable 0, Receiver Enable 1
4   conf_<module>_<pin>_putypesel   X   Pad Pullup/Pulldown Type. Pulldown is 0, Pullup is 1
3   conf_<module>_<pin>_puden   X   Pad Pullup/Pulldown enable. Enabled is 0, Disabled is 1
2-0 conf_<module>_<pin>_mmode   X   Mode. Pad functional mux select. A number between 0 and 7 i.e. 000 and 111. This depends on which mode we require.