I'm building a USB-powered demo board with the Atmel SAM E70 controller, and a single hurdle I'm facing is the battery-powered operation. I intend to use a simple 3V coin cell battery as a backup power source, and want to preserve MCU's current date/time in a low-power mode.
I have a minor STM microcontroller background and kinda expected Atmel MCU to have similar capabilities, but E70 (and the whole line of Cortex-M7-based Atmel MCUs, it seems) lacks the VBAT input. E70 datasheet has a separate section (22.4.4 - "Using Backup Batteries/Backup Supply") on the topic, but it doesn't provide any ready answers - only a sketchy schematic and some power sequencing advice.
E70 has a sister chip - the V71, and I've found that V71's demo board from Atmel ("V71 Xplained Ultra") actually does provide a detailed schematic, which is far from being easy & compact:
It seems that Atmel expects system engineers to build a backup power system of their own; it's a bit of a letdown, as STM F4/F7 line has an embedded backup powering scheme.
Thus, 2 questions:
Is there any easier (than building a scheme with an opamp and 2 additional ICs) way or a tried-and-tested schematic to provide a backup battery power for Atmel S70/E70/V70?
Note: I was advised to use a separate RTC IC (like STM M41T62LC6F), but don't feel comfortable with adding an additional IC to the board.
(in case any Atmel engineer happens to visit this question) What was the reasoning behind leaving out the VBAT/low-power controller out of the MCU?