Hi Frederik,
When you are using the "Download" programming method, the FPGA bitstream is programmed during the Linux boot up process. First the U-boot programs the FPGA bitstream, then it loads the device tree, as well as the Linux kernel.
Xilinx has added the xadc to the Zynq Linux device tree, so once the Linux is boot up, you can access xadc directly from Linux. (https://github.com/mathworks/xilinx-linux/blob/761490a77692f669202fde5e42cdc21cfa263331/arch/arm/boot/dts/zynq-7000.dtsi)
For example, you can try to run >iio_info command in serial console to get the xadc data. There are more details about xadc PS access in this document: https://www.xilinx.com/support/documentation/application_notes/xapp1172_zynq_ps_xadc.pdf
It may be that once Linux boot up, it took control of xadc from the PL. This may be the reason that you cannot get output from xadc any more. And when you are using JTAG, the Linux boot process is not triggered, so Linux does not take control.
I wonder whether it is possible to turn off Linux control using certain command.
Another option may be to take out the devicetree item related to the xadc.
adc: adc@f8007100 {
compatible = "xlnx,zynq-xadc-1.00.a";
};
HTH, Wang
