In top-pair events where at least one of the tops decays semi-leptonically, the identification of the lepton charge allows to tag not only the top charge but also that of the subsequent  $b$-quark. In case the $b$ also decay semi-leptonically, the charge of the two leptons can be used to probe CP violation in $b$ and $c$ mixing and decays. Such a new measurement at the LHC, which is independent of other experiments, can already lead to some sensitivity to non-standard model sources of CP violation with the 2012 data. In addition, it opens a new window for doing precision measurements of CP violation in b and c quark physics via hight $p_T$ physics in ATLAS and CMS.