  We set up a canonical Hamiltonian formulation for a theory of gravity based
on a Lagrangian density made up of the Hilbert-Palatini term and, instead of
the Holst term, the Nieh-Yan topological density. The resulting set of
constraints in the time gauge are shown to lead to a theory in terms of a real
SU(2) connection which is exactly the same as that of Barbero and Immirzi with
the coefficient of the Nieh-Yan term identified as the inverse of
Barbero-Immirzi parameter. This provides a topological interpretation for this
parameter. Matter coupling can then be introduced in the usual manner, {\em
without} changing the universal topological Nieh-Yan term.
