Yes, the smell of marijuana may create probable cause, particularly if it is very strong or there is some indication that its use is ongoing (like smoke or the sound of a bong). However, there are a number of legal issues raised by your explanation outside of the specific question that you asked. You also need to be aware that the officer may have evidence beyond smell alone. You need an attorney to check on that and give you a more detailed analysis than I can give you in this format. Good luck and check out NORML's website for detailed info on your legal rights as they relate to the Mary Jane.
This answer does not constitute or imply an attorney-client relationship, nor should it be construed as legal advice.
Generally, the purported smell of marijuana in a home is NOT sufficient to allow an officer to bust in without a warrant. However, if he claims that the door was opened to him and that he saw paraphenalia in plain site (and yes, officers lie because they know what it takes to give them the right to get in), then the officer could enter. If an officer was just walking by the house & said he smelled marijuana, he could not bust in - he would have to get a warrant, and that little smell alone would not likely sustain a warrant (or at least it would not sustain a challenge in court.)
When an officer knocks on the door, you do not have to answer. So, my question is why would the officer be knocking your door unless he had something else was at hand.
You should hire a lawyer to review the offense report and advise you on what the officer's claims are.