I have a custom PCB using the ATSAMB11-MR210 module. It's working fine and goes into ULP mode perfectly and wakes up with AO_GPIO_0 interrupts and AO_TIMER as expected. However, there's something very odd going on with ULP sleep current readings. I'm using a Fluke 45 bench DMM to take these readings, which are fairly accurate. Here's what I see at various VBAT/VDDIO voltages (as powered by different CR2032/LI2032 coin cells):
On a CR2032 battery at 3 VDC or less, ULP draws just 2 to 3 microamps, as it should and is very stable.
On a CR2032 battery at 3.3 VDC, ULP draws about 65 to 80 microamps, similar to what I see on Xplained Pro board using Data Visualizer and is somewhat unstable.
On a LI2032 battery at 3.8 VDC, ULP draws about 300 to 400 microamps! It's fluctuating a lot.
The only thing I'm changing out here is the battery. Both VBAT and VDDIO are tied to VCC3, a single power bus on the PCB. These voltages are well within the specified ranges for both VBAT and VDDIO for the AtSamb11 SoC and MR210 module.
I haven't been able to find a schematic of the ATSAMB11-MR210, so not sure what else may be going on here. This has me stumped. Here's a link to the MCU schematic.
Has anyone seen this behavior or know how to approach troubleshooting?
P.S. I originally thought this was a ASF power management issue and posted this.