BELK-TN-012: Using XADC signal module

Info Box Applies to Bora Applies to BORA Xpress Applies to BORA Lite Applies to BORA Technical Notes Applies to BORA Xpress Technical Notes Applies to BORA Lite Technical Notes

This technical note was validated against specific versions of hardware and software. What is described here may not work with other versions.

History

Version	Date	BELK/BXELK version	Notes
1.0.0	Dec 2021	4.1.0	First release
1.1.0	Dec 2021	4.1.0	Add "Electrical notes" section

Introduction

This technical note provides some information for using the XADC module available on the Zynq-7000 family which is the SoC used in BORA, BORA Xpress and BORA Lite DAVE Embedded Systems' SOMs.

Reference

First of all, we suggest some references regarding the XADC module:

• Chapter 30 of the Xilinx Zynq Technical Reference Manual
• 7 Series FPGAs and Zynq-7000 SoC XADC Dual 12-Bit 1 MSPS Analog-to-Digital Converter User Guide UG480
• the Xilinx Atlassian's XADC Linux drivers illustrating how to access the XADC module from a software perspective

Electrical notes

From an electrical perspective, in regard to the XADC module BORA, BORA Xpress, and BORA Lite SoMs implement the same scheme, which is depicted in the following image.

As suggested in the UG480 page 16:

This pin can be tied to an external 1.25V accurate reference IC ±0.2%

the VREFP pad is internally connected to a 10ppm/°C 1.25V voltage reference.

For what concerns the input range, it is worth to remember that it is independent of the I/O voltage of bank 35 as illustrated in the Xilinx documentation:

Pins usage

The pins like IO_L1N_T0_AD0N / IO_L1P_T0_AD0P can be used as analog inputs: according to Xilinx nomenclature, these pins are indicated as “External Voltages” or “External Analog Inputs”.

Software configuration

For enabling the ADC channel, the device tree file has to include the proper entry, for example the ADC_0 channel is enabled the bora.dts

&adc {
	xlnx,channels {
		#address-cells = <1>;
		#size-cells = <0>;
		channel@0 {
			reg = <0>;
		};
	};
};

then, the related ADC channel can be read from sysfs with the analog value reported in the form raw/scale:

root@bora:~# cat /sys/devices/soc0/amba/f8007100.adc/iio\:device0/in_voltage0_vccint_raw
1348
root@bora:~# cat /sys/devices/soc0/amba/f8007100.adc/iio\:device0/in_voltage0_vccint_scale
0.732421875
root@bora:~#

that is the corresponding value:

1348 * 0.732421875 = 987.3046875 mV

The SoC temperature can be read too:

root@bora:~# cat /sys/devices/soc0/amba/f8007100.adc/iio\:device0/in_temp0_raw
2660
root@bora:~# cat /sys/devices/soc0/amba/f8007100.adc/iio\:device0/in_temp0_offset
-2219
root@bora:~# cat /sys/devices/soc0/amba/f8007100.adc/iio\:device0/in_temp0_scale
123.040771484
root@bora:~#

i.e.

((2660 - 2219)* 123.040771484)/1000 = 54.26°C