Arduino UNO R4 test-code to demonstrate:

1. Wake-From-Interrupt
2. Change System-Clock divisors 1/1, 1/16, and 1/64 settings
3. Switching between 48MHz HOCO and 8MHz MOCO internal oscillators (and PLL with XTAL if fitted)
4. USB Module Disable (with code initiated Reset after timeout delay)

Unlike a linear regulator which adds to the current, the onboard ISL854102FRZ DC/DC converter trades Volts for Current and therefor the readings when powered from an external 9.0V supply to Vin show a lower power-consumption than would be measured on the +5V rail input itself (I don't have a USB DVM module).

Standard R4 Minima - LED series resistors all 330 ohm

Board running with +9.0V VIN at 48 MHz HOCO clock with default IDE settings:

1. Without sleep, USB active = c. 21.5 mA
2. With WFI and USB active = c. 13.3 mA
3. With WFI but USB disabled = c. 11.2 mA

To properly get low-power operation some board mods are needed, as supplied the ON LED takes 8 to 9 mA from the +5V rail just by itself! It is likely that low-power modes are mostly of interest for stand-alone setups which are not connected to a host USB - but turning off the internal USB 3v3 regulator also removes the +3v3 from the board's Power-Header's pin.

Board changes for Low_Power

1. ON LED series resistor changed to 2.7K, and LED VCC connection changed to VUSB.
2. L LED series resistor changed to 1.7K (which = 1.8mA if 100% on).

Board running with +9.0V VIN at 48 MHz HOCO clock with default IDE settings:

1. Without sleep, USB active = c. 15.5 mA
2. With WFI and USB active = c. 7.5 mA
3. With WFI but USB disabled = c. 5.3 mA

Board running with +9.0V VIN at 8 MHz MOCO clock with default IDE settings:

1. Without sleep, USB active = c. 6.2 mA - HOCO still running for USBFS Module
2. With WFI and USB active = c. 4.1 mA
3. With WFI but USB disabled = c. 2.8 mA - HOCO stopped

Further measurments with 1/16 and 1/64 clock divisors in C code file.

I have also added an external 12MHz XTAL.