Not getting low current in Standby Sleep on ATSAML21J18B.

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I’m having trouble getting to low current in Standby Sleep in an otherwise working project on an ATSAML21J18B using the LDO regulator with PM PLCFG PL0, SUPC VREG RUNSTDBY=1, STDBYPL0=1 & LPEFF=1 & complying with all errata. I’m using the OSC16M at 4MHz, OSCULP32K & XOSC32K oscillators set On Demand & Run in Standby. GCLK0 is set On Demand & not Run in Standby & is used only by the CPU.

 

Other GCLKs & devices such SPI SERCOM5 are set On Demand & Run in Standby. In my Standby Sleep current test all devices except the RTC are disabled in their control registers so should not demand their clocks or oscillators so surely the Standby Sleep current should be very low with no oscillator running. Instead I measure ~130uA & ~400uA if I change OSC16M to 12MHz which is linear & leads me to believe that OSC16M is running in Standby Sleep & I don’t know why. Any suggestions appreciated.