BPI-R4: issues with some WWAN modules in M.2 Key B

[Martin Schiller]

Hi Daniel,

I have discovered a problem with the M.2 Key B slot of the BPI-R4. The
32 kHz CLKOUT signal of the RTC is (according to the specs) connected to
PIN 68.

However, on all the m.2 WWAN modules I have tested, this PIN is either
not connected or is even used for a different function. For example,
the Telit MV32-W-A (aka Foxconn T99W368) is not working until I disable
the CLKOUT of the rtc. This module uses PIN 68 to enable
write-protection of the internal flash. (so, a 32kHz signal on this pin
isn't really a good idea)

Unfortunately, it happens from time to time that I still can't start
after disabling CLKOUT and the module has to be reset completely.

My idea for a workaround would be to deactivate the CLKOUT of the RTC in
the uboot (possibly using a preboot command) as soon as possible after
power-on.

However, this would also deactivate the signal at the M.2 Key M slot and
I don't know whether it is actually needed there.

Alternatively, only a hardware modification with removal of R62 would
remain.

What's your opinion?

Regards,
Martin