DOC: Add notes about XADC read-out via OpenFPGALoader

doc/guide/fpga-update.md

@@ -95,10 +95,36 @@ If the `FPGA Config` LED is not coming on, this could indicate the bitstream was
 
 If the `FPGA Config` LED is on but the `Ibex Boot` LED is not, it may be that you have programmed (or selected) a different bitstream than one that runs the CHERIoT demo. Try reloading the bitstream, and try power cycling the device.
 
-### Device Rebooting During/After Programming
+### Device Rebooting During/After Programming (No Serial Activity)
 
 The Sonata board takes a fair amount of power (>500mA) from the USB interface, and should be connected via USB-C. Typically it is close enough to the USB 2.0 limits that it will work with the adapter most of the time, but if you are having reliability issues we recommend trying a different computer, ideally one with a USB-C port.
 
+If you use openFPGALoader built from the [xadc-vccs branch here](https://github.com/newaetech/openFPGALoader/tree/xadc-vccs) you can print the min/max VCCINT ranges. To do this, simply run:
+
+```sh
+openFPGALoader -c ft4232 -X
+```
+
+This will print several XADC values, pay careful attention to `minvccint`:
+
+```
+{"temp": 39.9061, 
+    "maxtemp": 40.3194, 
+    "mintemp": 25.9852, 
+"raw":  {"0": 40684, "1": 21949, "2": 39270, "3": 0, "4": 0, "5": 0, "6": 21948, "7": 0},
+"vccint": 1.00415, 
+   "maxvccint": 1.00635, 
+   "minvccint": 1.00195, 
+"vccaux": 1.79663, 
+   "maxvccaux": 1.79883, 
+   "minvccaux": 1.79517, 
+}
+```
+
+If `minvccint` is below 0.95V you may have had a brown-out.
+
+See note below about if you get permissions error running `openFPGALoader`.
+
 ### FPGA Programming via USB/JTAG
 
 If for some reason the mass storage programming isn't working, you can also use the built-in FTDI JTAG programming. This requires the setup described in [FPGA Programming](../dev/fpga-programming.md) to build openFPGALoader. Once built, you simply run: