I'm accessing U-boot's console via serial connection and when u-boot prompt me to enter commands, it seems that I have limited time to do that. I want to enter several commands, but I need more time.
Does anyone experienced such an issue and how can I increase that time (if that is the problem)?
Without more details (such as platform, config and version), it's hard to say. Under normal circumstances the only timeout you have is to stop the automatic boot. If the board is resetting reliably after N seconds of being on it is likely that a watchdog is being triggered and U-Boot is not configured to know about and either disable or periodically pet the watchdog to keep the system from resettting.