I have designed a base board for Gumstix OVERO FIRE board which is basically a battery powered device and supports USB charging. If a dedicated USB charger is connected, the device can be powered up only with VBUS power even if there is no battery.
Now the boot time for the device is ~9sec when powered only with battery but device takes ~12sec when VBUS (USB host/charger) is connected. I verified hardware and power up sequence and there is no difference in the sequences with or without VBUS connection.
I am wondering if issue is something related to OVERO COM here that I am not able to understand. OVERO COM uses TPS65950 to power OMAP3530. This PMIC has a ULPI transceiver built in and VBUS from by board is applied to PMIC. So, when I connect the USB host/charger, PMIC gets VBUS before input voltage (which is applied to PMIC only after I press a power on button on my board). I went through the TPS65950 technical manual and found that there is a bit called STARTON_VBUS in the register CFG_P2_TRANSITION which controls whether VBUS will affect the transition from OFF to ACTIVE state or not. I am not sure if this can make a difference of ~ 3sec in boot time. I am not very familiar with TPS65950 configuration and it will take some time for me to check whether this is the issue or not.
Can anyone here tell whether VBUS presence before input voltage for TPS65950 can be an issue be here?