SCB V3.4 fail to boot at +5 degrees and below when plugged onto Backplane20sfp_v2.0
During the HALT test at IFIC Valencia it was found that the combination of the backplane20sfp_v2.0 + SCB v3.4 failed booting at temperatures below +5 degrees.
This issue is due to a combination of circumstances:*
- Backplane20sfp has a CP2015 UART  which powers-up in "suspend" (see , paragraph 5) when no USB cable is attached. IO is configured in GPIO mode which measn that the TXD output is configured as input and the ARM DBG_RXD line is floating.
- The ARM apparently can't handle the floating RxD (see also , paragraph 10.5). Either a defined '1' (non-asserted RS232) or a '0' will satisfy the ARM.
Put a pull-up (4K7) at the CP2105 TXD outputs to avoid floating lines during "suspend" and signal "non-asserted RS232".
- The SCB V3.4 should have taken care of a defined DBG_RXD input, i.e. in a future version there should be a pull-up on the SCB board.
- The SCB v3.4 works fine in combination with the miniBackplane_SFP_V3_3  since on the miniBackplane CP2102 devices are used at 3V3 and their outputs are made 2V5 (ARM VDDIOP0) compliant using a resistor divider, hence effectively having a pull-down when the CP2102 powers-up in "suspend" mode. Therefor the ARM DBG_RXD is defined and the SCB never fails boot when plugged onto a miniBackplane.
- Apparently temperature influences the leakage current on the ARM and/or CP2105 thus influencing the floating voltage.