  We study the diagonalizability of the Atkin $U_t$-operator acting on Drinfeld
cusp forms for $\Gamma_1(t)$ and $\Gamma(t)$ using Teitelbaum's interpretation
as harmonic cocycles. For small weights $k\leqslant 2q$, we prove $U_t$ is
diagonalizable in odd characteristic and we point out that non
diagonalizability in even characteristic depends on antidiagonal blocks.
