I installed a fresh version of the Tegra 21.4 kernel on my Jetson TK1, and I have a level shifting circuit to convert the PU lines from 1.8V up to 3.3 or 5V. I initially tested this on some earlier versions of the kernel and had no problems. When I switched to 21.4 I noticed that the PU0 line was acting funny. Initially I thought I had damaged my GTL2010 chip during reflow of the board. I then rebuilt a bread board of the circuit, and then tried a different circuit using the BSS138 transistor. In all cases I got the same behavior. When I put a volt meter to the PU0 line I can command it to go high and low and it does go from 1.8V to ~10mV. However, when I hook it up to the level shifting circuit it suddenly only goes from 1.8V to about 1.6V. The other PU I/O lines all work fine with the exact same circuit. They go from 1.8V down to a few millivolts still, and I can use them to drive external circuitry. I then switched back to an older Grinch kernel I originally tested with, and the PU0 on it worked fine. Also, the input works fine for PU0 and the other ones on 21.4. So I can use it as an input, but I cannot seem to drive anything with it. It is almost like it has another pull-up resistor in there that is keeping it from going down to ground or something. I compared the dts file for both the 21.4 and Grinch boards and the pinmux definition for both were the same, shown here:
pu0 {
nvidia,pins = "pu0";
nvidia,function = "rsvd4";
nvidia,pull = <0x0>;
nvidia,tristate = <0x0>;
nvidia,enable-input = <0x1>;
};
Tri-state and pull-up are both set to disabled, the same as the other pu lines. The only difference I see between the pu0 and the other lines is that it is the only one with a function of “rsvd4”. Can anyone explain what that function is, or where I can find an explanation for it? I tried looking in the TRM, but did not see anything obvious. Is there someplace else I can look to try and understand why the behavior of the two kernels are acting differently? I also looked to see if anyone else had reported anything similar, but did not see anything similar to this. Has anyone else run into this behavior?
Thanks,
David