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SAMA5D27 BSCR not retaining value

Posted: Fri Sep 13, 2019 8:48 pm
by TimH
I have two prototype boards using the A5D27. One is fine and I can set the BSCR to behave as needed - it's working great.

The other suffered a mishap (I applied an external 5V to the main 3V3 rail and popped the ACT8945 regulator and had to replace it).

It has a 3V lithium battery for VDD_BU and this is charged and good.

The DISABLE_BSCR fuse bit is not set. All fuses come back as 0.

The board can run code via JTAG debug and everything works OK (well, a few quirks compared to the other board, but nothing failed as best as I can tell so far).

This board will accept BUREG and BSCR programming while powered and I can read back the programmed values correctly using SAM-BA. But if I power cycle the board, the BSCR goes back to 0x00 and BUREG reverts to default.

This suggests a faulty backup supply but this is definitely OK - although I can't check the actual processor ball, obviously!

Before I write off the board (i.e. I damaged the processor when I applied 5V to the main 3V3 rail) is there something else I've missed that would cause the BSCR and BUREG values to be "non-sticky"?

Re: SAMA5D27 BSCR not retaining value

Posted: Mon Sep 16, 2019 3:21 am
by blue_z
Occam's razor would indicate that your battery is not supplying VDDBU when the board is not powered.
I can replicate your issue by removing JP6 (VDDBU) on an ATSAMA5D2C-XULT board when it is not powered.