  We present a fixed energy sandpile (FES) model which, by increasing the
initial energy,undergoes, at the level of individual configurations, a
discontinuous transition.The model is obtained by modifying the toppling
procedure in the BTW rules: the energy transfer from a toppling site takes
place only to neighbouring sites with less energy (negative gradient
constraint) and with a time ordering (asynchronous). The model is minimal in
the sense that removing either of the two above mentioned constraints (negative
gradient or time ordering) the abrupt transition goes over to a continuous
transition as in the usual BTW case. Therefore the proposed model offers an
unique possibility to explore at the microscopic level the basic mechanisms
underlying discontinuous transitions.
